Record 2017/04 | eCat 100945
Bynoe Harbour Marine Survey 2016: GA4452/SOL6432 Post-survey report
Siwabessy, P.J.W., Smit, N., Atkinson, I., Dando, N., Harries, S., Howard, F.J.F., Li, J., Nicholas, W.A., Picard, K., Radke, L.C., Tran, M., Williams, D. and Whiteway, T.
APPLYING GEOSCIENCE TO AUSTRALIA’S MOST IMPORTANT CHALLENGES
www.ga.gov.au
Bynoe Harbour Marine Survey 2016: GA4452/SOL6432 Post-survey report GEOSCIENCE AUSTRALIA RECORD 2017/04
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Siwabessy, P.J.W. , Smit, N. , Atkinson, I. , Dando, N. , Harries, S. , Howard, F.J.F. , Li, J. , 1 1 1 1 3 1 Nicholas, W.A. , Picard, K. , Radke, L.C. , Tran, M. , Williams, D. and Whiteway, T.
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Geoscience Australia, GPO Box 378, Canberra, ACT 2601 Department of Environment and Natural Resources, Northern Territory Government, PO Box 496, Palmerston, NT 0831 Australian Institute of Marine Science, PMB 3, Townsville MC, Qld 4810
Department of Industry, Innovation and Science Minister for Resources, Energy and Northern Australia: The Hon Josh Frydenberg MP Assistant Minister for Science: The Hon Karen Andrews MP Secretary: Ms Glenys Beauchamp PSM Geoscience Australia Chief Executive Officer: Dr Chris Pigram This paper is published with the permission of the CEO, Geoscience Australia
© Commonwealth of Australia (Geoscience Australia) 2016 With the exception of the Commonwealth Coat of Arms and where otherwise noted, this product is provided under a Creative Commons Attribution 4.0 International Licence. (http://creativecommons.org/licenses/by/4.0/legalcode) Geoscience Australia has tried to make the information in this product as accurate as possible. However, it does not guarantee that the information is totally accurate or complete. Therefore, you should not solely rely on this information when making a commercial decision. Geoscience Australia is committed to providing web accessible content wherever possible. If you are having difficulties with accessing this document please email
[email protected]. ISSN 2201-702X (PDF) ISBN 978-1-925297-41-6 (PDF) eCat 100945 Bibliographic reference: Siwabessy, P.J.W., Smit, N., Atkinson, I., Dando, N., Harries, S., Howard, F.J.F., Li, J., Nicholas W.A., Picard, K., Radke, L.C., Tran, M., Williams, D. and Whiteway, T. 2016. Bynoe Harbour Marine Survey 2016: GA4452/SOL6432 – Post-survey report. Record 2017/04. Geoscience Australia, Canberra. http://dx.doi.org/10.11636/Record.2017.004
Contents
1 Introduction ............................................................................................................................................1 1.1 Background and survey aims ..........................................................................................................1 1.2 Study area ........................................................................................................................................2 1.3 Seabed mapping ..............................................................................................................................4 1.3.1 Scientific rationale ......................................................................................................................4 1.3.2 Seabed mapping approach, purpose and desired outputs ........................................................5 1.3.3 Survey aims ................................................................................................................................6 2 Methods .................................................................................................................................................7 2.1 Survey overview ...............................................................................................................................7 2.2 Sampling overview ...........................................................................................................................7 2.3 Geophysical data .............................................................................................................................8 2.3.1 Seabed mapping ........................................................................................................................8 2.3.2 Sub-bottom profiler ...................................................................................................................10 2.4 Physical sampling ..........................................................................................................................10 2.4.1 Smith-McIntyre grab .................................................................................................................10 2.4.2 Underwater photography and video .........................................................................................12 2.5 Oceanographic data.......................................................................................................................12 2.5.1 Tide...........................................................................................................................................12 2.5.2 Sound Velocity Profiles (SVP) ..................................................................................................14 3 Data summary .....................................................................................................................................15 3.1 Survey summary ............................................................................................................................15 3.2 Geophysical data ...........................................................................................................................15 3.2.1 Multibeam echosounder ...........................................................................................................15 3.2.2 Sub-bottom profiler ...................................................................................................................28 3.3 Physical Data .................................................................................................................................29 3.3.1 Sedimentology ..........................................................................................................................29 3.3.2 Geochemistry ...........................................................................................................................32 3.3.3 Biology ......................................................................................................................................33 3.4 Oceanographic data.......................................................................................................................37 3.4.1 Tide...........................................................................................................................................37 3.4.2 Sound Velocity Profile (SVP)....................................................................................................38 4 Summary and concluding remarks ......................................................................................................40 Appendix A Scientific and technical crew ...............................................................................................45 Appendix B Spatial coordinates of waypoints used to guide station selection .......................................46 Appendix C Technical report on multibeam sonar operations ...............................................................49 Appendix D Survey details for sub-bottom profiles ................................................................................56 Appendix E Survey details for Smith-McIntyre grab (GR) samples .......................................................57 Appendix F List of Smith-McIntyre grab sub-samples ............................................................................64 Appendix G Survey details for photo and video .....................................................................................70 Appendix H Survey leader daily log 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Executive Summary
A benthic sediment sampling survey (GA4452/SOL6432) was undertaken in Bynoe Harbour in the period from 2 to 27 May 2016. Partners involved in the survey included Geoscience Australia (GA), the Australian Institute of Marine Science (AIMS) and the Department of Environment and Natural Resources within the Northern Territory Government (NT DENR) (formerly the Department of Land and Resource Management (DLRM)). This survey forms part of a four year (2014-2018) science program aimed at improving knowledge about the marine environments in the Darwin and Bynoe Harbour region. The program involves the collection and collation of baseline data to create thematic habitat maps that underpin marine resource management decisions. The program was made possible through environmental offset funds provided by the INPEX-led Ichthys LNG Project (http://www.inpex.com.au/, hereafter referred to as INPEX) to NT DENR and co-investment from GA and AIMS. Under Condition 11(a) of the approval, INPEX must deliver habitat mapping for the region. The program builds upon an NT Government project (2011-2018) which will see the collection of baseline data (multibeam echosounder data, sediment samples and video transects) from inner Darwin Harbour (Siwabessy et al., 2015). The AIMS vessel RV Solander was used to obtain high-resolution multibeam sonar data to map 2 bathymetry, backscatter and water column data over 698 km of Bynoe Harbour. Physical data (including underwater video and stills, grabs, tide gauge and Sound Velocity Profiles (SVPs)) were obtained from 102 stations to characterise the seabed sediments, guide interpretations of geomorphology and seabed hardness, and improve knowledge of the water column. Initial interpretations of the newly acquired data show that the interaction of tidal currents with river outflow and storm-generated currents has created a seabed characterised by a complex network of geomorphic features and associated habitats, including:
Complex hardgrounds (rock with shallow pits and hollows), which indicate there have been periods of subaerial exposure and erosion when sea-level was lower than the current seabed. They occur mostly in shallow water and provide suitable habitat for benthic communities;
Plains which have variable backscatter results (indicating that hardness is variable). This likely indicates that some regions have a thin sediment veneer while others have thicker accumulations of sediment over rock. Areas of veneer correspond to patchy rocky outcrops supporting locally abundant patches of octocorals and sponges.
Channels, which are typically harder-scour features with gravel sediments;
Sandwaves and ripples, of linear, dendritic and sinuous types occurred predominantly over plains and channels.
The baseline environmental data acquired in this habitat mapping program is providing new insights into the marine environment of Bynoe Harbour. The data will underpin future environmental assessments and consequently support management decisions. This report provides details of the Bynoe Harbour survey, including the area surveyed, methods used and initial results. Data from this survey will be made publically available via the Geoscience Australia website (http://www.ga.gov.au/).
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1 Introduction
This report provides the details of the activities undertaken by Geoscience Australia (GA), the Australian Institute of Marine Science (AIMS) and the Northern Territory Government Department of Environment and Natural Resources (DENR) during a seabed mapping survey (GA04452/SOL6432) of Bynoe Harbour. This project was made possible through offset funds provided by the INPEX-led Ichthys LNG Project to the Northern Territory Government DENR, and co-investment from Geoscience Australia and Australian Institute of Marine Science. This survey was completed between 3 May and 27 May 2016. Scientific results of the survey will be detailed in a final habitat mapping report that will synthesise all data collected across the habitat mapping program.
1.1 Background and program objective Darwin is Australia’s most northern capital city and the only major population centre on 5,600 km of comparatively undeveloped coastline between Perth and Cairns. It lies on Darwin Harbour, which is located on the southern shore of the Beagle Gulf in the Timor Sea (Figure 1.1) and is one of Australia’s largest deep water harbours. With ongoing development in Northern Australia and increasing coastal development in Darwin Harbour, robust baseline data and a strong scientific understanding of ecological processes are required to inform planning and management for sustainable development and multiple use of this region. The Darwin Harbour Regional Plan of Management (Darwin Harbour Advisory Committee, 2003) has identified that there is limited information on the local environmental values and attributes which could potentially be affected by coastal development. Furthermore, the plan identified that there is a lack of data on broad scale habitat and biodiversity distributions and of processes which maintain ecosystem health. This has been highlighted in environmental assessments associated with recent large development proposals. In response, the Northern Territory Government announced in December 2010 that $500 000 over two years would be allocated for a baseline habitat mapping program to ensure that the best scientific data would be available to inform sustainable development. Under this program, DENR collaborated with GA, AIMS and the Darwin Port Authority to undertake multibeam and backscatter data acquisition for inner Darwin Harbour in 2011. The survey data has been processed, analysed and interpreted to produce a series of mapping products describing the seabed characteristics to support seascape analysis of inner Darwin Harbour. In 2011, INPEX Corporation (http://www.inpex.com.au/, hereafter referred to as INPEX) and the Northern Territory Government entered into a voluntary agreement where INPEX would fund and/or deliver several environmental and social offset programs. The environmental offset programs were incorporated into conditions for the approval decision for INPEX’s Ichthys Liquefied Natural Gas proposal [2008/4208] by the Department of the Environment (DOE). Under Condition 11(a) of the approval, INPEX must deliver habitat mapping for the Darwin Harbour region (including Bynoe Harbour). The intent of this offset program is to improve knowledge of the marine habitats in the Darwin and Bynoe Harbour region through collaboration and collation of baseline data that will allow
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creation of thematic habitat maps to underpin marine resource management decisions. The four year (2014-2018) program will:
improve knowledge of marine habitats in the Darwin Harbour region;
produce habitat maps of Darwin Harbour region and Bynoe Harbour;
improve understanding of how new future use may impact on Darwin Harbour; and
improve environmental management decisions for Darwin Harbour.
To deliver the Offset Habitat Mapping program, DENR built on previously developed relationships and established a working group in 2013, with joint partners AIMS and GA, to deliver the program’s objectives. The working group has identified four core tasks for the program: 1. Conduct seabed mapping to obtain high resolution bathymetric and backscatter data within Darwin and Bynoe harbours; 2. Examine the abiotic patterns important to benthic communities within Darwin and Bynoe harbours; 3. Characterise the seafloor fauna and flora (i.e. benthos) within Darwin and Bynoe harbours; and 4. Model species/community distribution within Darwin and Bynoe Harbours.
1.2 Study area The Darwin – Bynoe Harbour region lies in the eastern part of the Beagle Gulf, which is located between the southern shores of Melville and Bathurst Islands and the Australian mainland (Figure 1.1). This region includes Port Darwin, outer Darwin Harbour (waters between Gunn Point and Charles Point and includes Shoal Bay) and Bynoe Harbour, which is bounded by Cox Peninsula in the east 2 2 and five islands in the west. The total area of interest is about 2000 km of which 1300 km are subtidal environments. Similar to Darwin Harbour, Bynoe Harbour is a tropical estuary formed within a drowned river valley (a ria), located on the northwestern coast of the greater Joseph Bonaparte Gulf, northern Australia. Bynoe Harbour is positioned approximately 12.5° south of the equator, facing Cape Fourcroy on Bathurst Island and the Timor sea, across the intervening Beagle Gulf (Woodroffe et al., 1988; Nott, 2003). The climate is monsoon tropical. Nearby Darwin Airport receives about 1726 mm of rainfall annually, with the majority falling in the monsoon (wet) season between November and March. Riverine discharge is low as the Darwin and Bynoe harbour catchments are relatively small. Prevailing winds are seasonal with north-west monsoons in November to March giving way to southeast trade winds from May to September. During the wet season, the monsoonal conditions generate turbulent wave action and high turbidity along this coast. The annual range in sea surface temperature in the adjacent Darwin Harbour is from 23 - 34°C. Beagle Gulf is dominated by tidal currents that flow in an east to west direction, turning to northwest– southeast near the entrance to Bynoe Harbour. The tides amplify as they transit across the wide continental shelf and the currents become bidirectional as they approach the coast. Darwin and Bynoe harbours have a large tidal range (macrotidal) with a 7.9 m maximum tidal range (5.5 m mean spring range and 1.8 m mean neap range) (Woodroffe et al., 1988; Andutta et al., 2013). The tidal cycle is semidiurnal (two tidal cycles are experienced every 24 hours). Tidal currents in this region can reach up to 2 m/s (7.2 km/h) during a maximum spring tide (Andutta et al., 2013; Williams et al., 2006). As a result of the tidal energy, the harbour is considered to be a reasonably well-mixed homogenous water body that is naturally turbid but carries low nutrient loads. Turbidity values range from 5 Nephelometric
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Turbidity Units (NTU) in the dry season to in excess of 400 NTU during wet season storms. Bynoe Harbour contains one main channel. Near the harbour entrance this channel bifurcates around a single island, Indian island. Unlike Darwin Harbour, comparatively little information was available on the sediments and geomorphology of the seabed for Bynoe Harbour prior to this study. However, the near shore geology is reasonably well known. The Bynoe region is underlain by the western domain of the Pine Creek Orogen, the Litchfield Province (Pietsch and Edgoose, 1988). Deformed and intrusive Archean to Proterozoic basement rocks are overlain to the north by Cretaceous marine sediments of the Bonaparte Basin, extending offshore onto the shelf. To the south and north of Bynoe Harbour lies the Welltree Metamorphics, comprising amphibolite facies gneisses, schist, and quartzite (Pietsch, 1986, Carson et al., 2008). The Welltree Metamorphics is intruded by the post-orogenic Two Sisters Granite (Pietsch and Edgood, 1988). At the eastern end of Bynoe Harbour the Burrell Creek Formation outcrops. The metasediment is comprised of phyllite, siltstone, conglomerate and greywacke and is incised by the Anna and Charlotte rivers as they travel west into Bynoe Harbour. The Burrell Creek Formation is intruded by the Two Sisters Granite and numerous pegmatite bodies. A large number of cassiterite-rich pegmatites are located between Bynoe Harbour and Darwin Harbour th to the north and were mined for tin from the late 19 century (Ahmad, 1995). This area is commonly referred to as the "Bynoe pegmatite field", or "Bynoe Field" (Jiang and Sun, 2015). During the Last Glacial Maximum (23-19 ka BP) Bynoe and Darwin Harbours and the adjacent Beagle Gulf were subject to subaerial erosion and weathering. While Holocene sea level and sedimentary processes are well documented (Woodroffe and Grime, 1999; Woodroffe et al., 1993), less is known about the post-Cretaceous to Holocene history of this area. Several lines of evidence suggest that the greater Joseph Bonaparte Gulf region was affected by global low sea levels, similar to the last ice age, since at least the end of the mid-Pleistocene Transition (~ 650 ka BP) with buried river channels present just outside Beagle Gulf (Bourget et al., 2013; Saqab and Bourget, 2015; Nicholas et al., 2014). For the majority of this time, sea level was too low for Bynoe Harbour to be an estuary (Figure 1.1). Thus, Bynoe Harbour is a drowned river valley which has been exposed to erosion and weathering for a large proportion of the past 650 000 years. The harbour has only been intermittently connected to the sea over that time. Estuarine conditions similar to present were established some 7 to 8 thousand years ago, just as rising sea levels stabilised after the last ice age (Lewis et al., 2013).
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Figure 1.1 Location map of the area surveyed during the Bynoe Harbour Marine Survey (red outline) along with the location of areas previously surveyed and areas to be surveyed in future field seasons.
1.3 Seabed mapping 1.3.1 Scientific rationale Historically, benthic habitat maps produced from taxonomic lists and biodiversity values for a site/region were based on point sampling methods (Eleftheriou and McIntyre 2005 and references therein). However, the outputs from this approach, such as community patterns, can be limited due to insufficient knowledge about the spatial extent of physical characteristics that underpin community patterns (e.g. substrate type), and gradual shifts in species composition along environmental gradients
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(e.g. Gray 2002; Ellis and Schneider 2008; Gray and Elliot 2009).Recent advances have overcome many of these constraints. Underwater acoustic remote sensing technologies such as multibeam echo sounder (MBES) systems (Rattray et al., 2009; Huang et al., 2011; Huang et al., 2013), Geographic Information Systems (GIS) and geospatial analysis (Lund and Wilbur, 2007; Zajac, 2008; Bishop et al., 2012) provide high-resolution data to describe the physical seafloor. In addition, developments in numerical modelling of hydrodynamics (Williams et al., 2006), surface processes (Tenore et al., 2006; Pitcher et al., 2004) and biological surrogacy (Pitcher et al., 2007a) have transformed benthic habitat mapping into a complex multi-disciplinary task enabling full-coverage predictive mapping products (Pitcher et al., 2007a; Haywood et al., 2008). There are three main approaches to benthic habitat mapping (Brown et al., 2011): 1. abiotic habitat mapping; 2. benthic community mapping; 3. single species habitat mapping. Abiotic habitat mapping uses physical and chemical characteristics as surrogates for biodiversity with the underlying premise that species distribution can be predicted based on environmental data (Post et al., 2006; Gray and Elliot, 2009, ). Pitcher et al. (2007b) identified grain size distribution, carbonate, irradiance, bottom sheer stress, bathymetry, bottom-water physics, nutrients, turbidity as surrogates for mapping and characterisation of biotic and physical attributes of the Torres Strait Ecosystem. McArthur et al. (2010) concluded that sediment variables such as grain size, rugosity and compaction hold the strongest independent predictive power as a surrogate for biodiversity. Analysis of these data coupled with data derived from MBES (e.g. depth, backscatter strength, slope, aspect, benthic position index, rugosity), allows for seascape analysis and mapping of areas with similar environmental characteristics, which is of value for use in many aspects of marine spatial planning (e.g. Ierodiaconou et al., 2007; Rattray et al., 2009; Heap and Harris, 2011; Huang et al., 2011; Berkström et al., 2012; Anderson et al., 2013; Lucieer et al., 2013; Rattray et al., 2013). Further, the outputs are commonly used in gap analysis and sampling design for additional benthic community and single species sampling programs.
1.3.2 Seabed mapping approach, purpose and desired outputs The seabed mapping survey will build on data collected in 2015 also under the current seabedmapping program, along with data from a previous Port Darwin seabed-mapping program, conducted in 2011 (Figure 1.1). The purpose of this task is to obtain and collate detailed full coverage acoustic datasets representing depth, seabed morphology, acoustic backscatter and sediment characteristics (e.g. grainsize and sediment chemistry, seabed mapping). The collected data will contribute towards the creation of thematic habitat maps that will, a) support the design of cost effective sampling of benthic fauna and flora; and b) make inferences about species/assemblages beyond sampling locations alone (community and species mapping). Data and GIS layers include:
Bathymetry, acoustic backscatter and derived data products, e.g. slope, aspect, curvature, rugosity, hardness, inferred substrate types (e.g. mud, sands) and benthic position index.
Sediment characteristics (e.g. % gravel, % sand, % mud, Folk classification, percentage carbonate, sediment origin (marine vs terrestrial)).
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Sediment chemistry characteristics (e.g. TOC, TN, C isotopes, N isotopes, pore water, oxygen demand, contaminants (heavy metals)).
1.3.3 Survey aims The aim of the survey was to collect bathymetry and sediment chemistry data for Bynoe Harbour in waters deeper than 5m (LAT). The specific objectives of the survey were to: 1. Obtain high resolution geophysical (bathymetry) data for Bynoe Harbour including Port Patterson; 2. Characterise substrates (acoustic backscatter properties, sub-bottom profiles, grainsize, sediment chemistry) for Bynoe Harbour, including Port Patterson; 3. Collect GPS tidal data for the survey area.
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2 Methods
2.1 Survey overview Survey GA4452/SOL6432 was undertaken in two operational legs: Leg 1 (02/05/2016 to 16/05/2016) and Leg 2 (17/06/2016 to 27/05/2016). Operations included geophysical data acquisition (using multibeam echosounders), seabed sampling (grabs), underwater imaging and sound velocity profiling (SVP). In addition, a GPS tide gauge was deployed for the survey period.
2.2 Sampling overview We collected approximately 100 samples in water depths greater than 5m (LAT). However, prior to the survey, a total of 200 random sample locations were identified to accommodate for areas that could not be mapped once on site due to factors such as, shallow depths, difficulty in reaching the area, and lack of time (Appendix B). The following sampling design method was used and implemented in R (R Development Core Team, 2012), with sp (Pebesma and Bivand, 2005), raster (Hijmans, 2014) and spcosa (Walvoort et al., 2015) used to analyse and plot the data: 1. each area was spatially divided into 25 strata based on latitude and longitude; 2. eight sampling points were randomly selected within each stratum (random waypoints, Figure 2.1). 3. the distribution of sampling locations within each area was visually examined. The sampling design was deemed acceptable because all sampling locations were spread relatively evenly over the survey area (Figure 2.1). The sub-selection of the 100 samples was done during the survey as areas were mapped. The total initial area was divided into smaller polygons and samples were sub-selected proportionally to the size of the new area (Figure 2.1).
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Figure 2.1 The spatial distribution of 200 random waypoints. The waypoints are shown overlaid on 2.5 m resolution SPOT satellite imagery. The spatial coordinates of the 200 waypoints are summarised in Appendix B.
The water column and seabed of each waypoint was sampled, when time allowed, using the methods described below. Once a waypoint was sampled, the location was assigned a station number.
2.3 Geophysical data 2.3.1 Seabed mapping 2.3.1.1 Multibeam echosounder Continuous bathymetric data were collected within the study area and while transiting using a Kongsberg EM2040C multibeam echosounder system, mounted in the moon pool of the RV Solander in dual head configuration (Figure 2.2). Technical details of the multibeam operations can be found in Appendix C.
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The grid was divided into 13 cells which were each individually mapped to ensure that the data acquisition was undertaken in swaths that ran parallel to depth contours. However, in areas with abundant ship traffic, safety considerations meant that mapping could not follow the isobaths.
Figure 2.2 Kongsberg EM2040C multibeam sonar system installed on RV Solander, showing: (a) POS MV motion reference unit, data acquisition and processing units, and; (b) dual head mounting deployed in the moon pool.
Bathymetry The multibeam bathymetry data were processed using CARIS HIPS/SIPS v8.1.13 software. The processing included: i) applying algorithms that corrected for tide and vessel pitch, roll and heave; and ii) applying filters and visually inspecting each swath line to remove any remaining artefacts and noisy data (e.g. nadir noise and outliers). During the survey, a co-tidal solution was adopted to minimise tidal bursts. This solution used average tides from multiple tide stations weighted inversely to distance between the tide station and the point of interest available in CARIS. Study areas were divided into closed zones with designated primary tide stations of known locations, all defined in a Zone Definition File (*.zdf) in CARIS. The predicted tides were calculated for predefined locations (Section 2.5.1.1). Final high-resolution bathymetric surfaces at 1 m horizontal resolution were created within CARIS and then exported as surface grid bathymetric maps for display and analysis.
Backscatter strength Along with bathymetric data, the Kongsberg EM2040C multibeam system co-registered backscatter data. These data were processed using the multibeam backscatter CMST-GA MB Process v15.04.04.0 (64) toolbox software co-developed by the Centre for Marine Science and Technology (CMST) at Curtin University of Technology and GA (described in Gavrilov et al., 2005; Parnum and Gavrilov, 2011). The process within the toolbox involved removing system transmission loss, removing the system model, calculating the incidence angle, correcting the beam pattern, calculating the angular backscatter response within a sliding window of 100 pings with a 50% overlap in a 1° bin,
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removing the angular dependence, and restoring the backscatter strength at an angle of 25° (Daniell et al., 2010). The final processed backscatter data were then gridded to 1 m horizontal resolution and exported for display and analysis. Removing the angular dependence from the backscatter is necessary for consistent backscatter strength across the swath in various incidence angles (for a homogeneous seafloor). Here the angular backscatter response was calculated in a 1° bin of incidence angle and averaged within the sliding window. This angular backscatter response is an intrinsic property of the seafloor, illustrating that the backscatter strength changes with the angle of incidence and is dependent on substrate type.
2.3.2 Sub-bottom profiler Sub-bottom profiles were collected using an Edgetech SB-216S chirp profiler. The profiler was deployed from the stern of the RV Solander and towed about 40 m away from the stern for a total of about 53 m from the GPS antenna (Figure 2.3). The chirp signal was set up to sweep between 2 and 15 kHz and used a 20 ms pulse length. Profiles were collected mainly across the axes of the channels (Appendix D) and where multibeam backscatter suggested soft substrate (low acoustic return).
(A)
(B)
Figure 2.3 A) Edgetech SB-216S sub-bottom profiler deployed off the stern of the ship, and B) Acquisition system
2.4 Physical sampling 2.4.1 Smith-McIntyre grab 2
Unconsolidated seabed sediments were collected using a Smith-McIntyre (SM) grab (10 L, 0.1 m opening). The grab sampler was deployed from the starboard side winch of the RV Solander. The SM grab sampler is mounted on a sturdy, weighted, steel frame, with springs to force the two-jaw bucket into the sediment substrate when released. Tripping pads, positioned below the square-based frame on which the bucket is suspended, first make contact with the seabed and are pushed upward to release two latches holding the spring-loaded bucket jaws. Tension from the winch wires keeps the grab closed during retrieval. Each SM grab was subsampled for both sedimentology and geochemistry. A summary of SM grab sampling is found in Table 2.1 and Appendix Table E.1.
2.4.1.1 Sedimentology The principal aim of the sedimentology component of the survey was to determine the texture and composition of the surface sediments for seabed characterisation. Up to 100 g of bulk sediment was
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sub-sampled from the top 2 cm of the grabs. Samples were submitted to the GA sedimentology laboratory for grain-size analysis using: (i) sieve separation to determine the particle size distribution and summary statistics of the Wentworth grain-size fractions; and (ii) laser diffraction to determine the size distribution of the mud fraction using a Malvern Mastersizer 2000. Separate sample splits were used to measure carbonate content using the carbonate digestion method (Müller and Gastner, 1971).
2.4.1.2 Environmental geochemistry The principal aims of the environmental geochemistry component of the survey were: (i) to determine the geochemical composition of surface sediment for seabed characterisation; and (ii) to quantify the levels of reactive organic matter in the sediments that were due to pigments and organic matter. Sediments from the grabs were sub-sampled into seven separate containers (Appendix F) and processed aboard according to the methods described in Table 2.1. Table 2.1 Details of shipboard laboratory processing used to prepare and analyse the sub-samples. Sub-sample codes (_A1, etc.) correspond to the file extension assigned to the sub-sample types in the shipboard database. See Appendix F for a summary of all sub-samples taken during the survey. Subsample
Parameters to be measured
Shipboard processing
_A1
Grain size
Approximately the top 2 cm of sediment was spooned into a labelled bag
_B1
Porosity; Total chlorins and chlorin indices.
6.5 mL of surface sediment (0-2 cm) was syringed into pre-weighed plastic container. The samples were wrapped in Al-foil and frozen.
_C1
Sediment oxygen demand
Bulk sub-sample (6.5 ml) of surface sediment (0-2 cm) incubated in filtered (0.5μm) seawater in Biological Oxygen Demand (BOD) bottles for ~24 hrs in the dark at approximately sea surface temperatures (SST). Dissolved oxygen concentrations (and saturation values) were measured at the start and finish of the incubations.
_C2
Total sediment metabolism
Salinity, temperature and pH were measured on pore waters extracted from sub-samples C_D1. These pore waters were then filtered (0.45 µm) into 3 ml gas-tight vials (pre-charged with 25 μL HgCl2) within 1 hr of collection (T=0). The procedure was repeated on pore waters from an additional bulk sample collected as per C_D1 and incubated for ~24 hrs at SST (T=1). All samples were refrigerated prior to laboratory analysis for dissolved inorganic carbon (DIC). Samples taken at core incubation stations only.
_C3
CO2 production and consumption rates; total oxygen uptake rates
Two 5 cm diameter cores of surface sediment overlain by 0.153L of filtered (0.5μm) and UVtreated seawater was incubated in the dark for ~6 o hours at ~25 C after a ~3 hour pre-incubation period. Dissolved oxygen levels were measured at ~30 minute intervals. Samples for DIC analysis were taken at the start and finish of each of the incubations. Blank cores (seawater only) were run intermittently throughout the survey period. Core incubations were undertaken at 46 stations only. At 10 of these stations additional samples were taken for N2 and dissolved nutrients at the start and finish of the incubations.
_D1
Major, minor, trace and rare earth elements; Bulk
Surface sediment (0-2 cm) was syringed into falcon vials. Pore waters were removed within 20 minutes
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Subsample
Parameters to be measured
Shipboard processing
carbonate of collection (see C_C2). Residual sediment was Specific surface area; TOC, TN and C & N isotopes frozen for transport to the laboratory. _S1
Major, minor, and trace and rare earth elements
Notation used to denote solid specimens from grabs. The specimens were placed in plastic bags and refrigerated
2.4.2 Underwater photography and video Epifauna and broad-scale sedimentology and topography of the seabed were surveyed by underwater video cameras (GoPro ©) attached to the Smith-McIntyre grab. Underwater video and still cameras captured the deployment of the grab to the seafloor. High-resolution still images (12 MP and 1080p video resolution, captured every 1 second during the grab deployment) were used in conjunction with the video footage to assist identification of the biota. Upon retrieval of the cameras, the still images were downloaded and renamed by station and sequential image number. These images were then sorted into usable images for seafloor classification. Images of interesting or representative habitat and biota were singled out for further analysis and description.
2.5 Oceanographic data 2.5.1 Tide Tidal data acquisition was over 30 days, a full spring – neap tidal cycle. The tides for the period recorded at the national tide reference station at Fort Hill wharf are shown in Figure 1.1. AIMS services two moorings for the Integrated Marine Observing System (IMOS) network and these are located at the entrance to Darwin Harbour and in the middle of the Beagle Gulf. Another mooring had been deployed to the east of the survey area and collected data for a period of 24 days during the survey. A two-dimensional finite element hydrodynamic model was developed for the area to simulate tidal elevations and currents. The boundary conditions for the model are harmonics derived from observations from Tapa Bay to the west and Cape Hotham to the east. These harmonics are available from the Australian Hydrographic Office national electronic tide predictions (AHO, 2015). The model was run to include all periods of mooring observations. The model calibrates well to observed tides. An example of modelled output and observed data is shown in Figure 2.4. The model performs equally well against all moored data.
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Figure 2.4 Observed (blue line) versus modelled (orange line) tidal elevation for Beagle Gulf.
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2.5.2 Sound Velocity Profiles (SVP) Sound velocity profiles (SVP) are required to correct for refraction on multibeam depth readings due to changing sound speed within the water column. Local oceanographic conditions dictate how often the SVP measurement is required. At each SVP station, a Valeport Mini SVP was deployed by hand with rope concurrently with the sediment sampling (Figure 2.5). After being submerged at approximately 1 m below the sea surface for approximately 1 minute, the Valeport Mini SVP was lowered through the water column, and then raised back to the ship. The data was downloaded immediately using a serial connection to a dedicated laptop running the Valeport software. Using the SVP editor available in the multibeam acquisition software called Seafloor Information System (SIS), the data were examined and formatted to the required format and then applied to the SIS. The downcast data was always used.
Figure 2.5 The Valeport Mini SVP.
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3 Data summary
2
The study area, covering 698 km of seabed across water depths ranging from 3 to 51 m, was mapped using the multibeam sonar (Figure 3.1; Figure 3.2; Table 3.1). The final mapped area also included a patch of data 28 km west of Bathurst Island to investigate and map a potential unidentified shipwreck for the Northern Territory Department of Lands, Planning and the Environment Heritage Branch (Figure 3.3). Multibeam water column data were also collected over the entire surveyed area. A total of 102 Smith-McIntyre grabs, 104 underwater camera drops, 29 sub-bottom profile lines and 34 sound velocity profiles were collected during the survey (Table 3.1). A summary of the data collected and preliminary observations for each dataset are presented in this section.
3.1 Survey summary Table 3.1 Summary of data acquired during GA4452/SOL6432 survey. Data Type
Sample Total
Units
Multibeam
698
km
Smith McIntyre grab
102
no.
Underwater camera drops
104
no.
Sub-bottom profiles
112
km
Sound velocity profiles
34
no.
GPS tide
1
no.
2
3.2 Geophysical data 3.2.1 Multibeam echosounder Bathymetry The high-resolution bathymetry mapping revealed a seabed dominated by a large drowned braided river-valley, and (potential) lake with adjacent shoreline platforms (Figure 3.1; Figure 3.2). Along the western boundary of the outer harbour a large number of rocky reefs were mapped in detail. Depths in the survey area ranged from 3 m near the coast and offshore islands to up to 50 m in the deepest sections of the channel. While large parts of the mapped area and channel are exposed bedrock, unconsolidated sediment dominates other areas and is found in a variety of morphological features, including ripples, flat plains, sediment lobes, hummocks, furrows and various types of dunes. The interaction of strong tidal currents with river outflow and storm-generated currents has given rise to this diversity of sedimentary depositional features, examples of which are described below (Figure 3.2; Table 3.1).
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Figure 3.1 Hillshaded bathymetry gridded at 1 m resolution along with the location of the Smith-McIntyre grab sampling stations. For more information on grab locations see Appendix Table E.1
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Figure 3.2 Location of seabed features of interest identified from multibeam data and summarised in Table 3.2
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Table 3.2 Description and representative examples of seabed features identified from multibeam data. For the location of these representative examples see Figure 3.2. Name
Bathymetry
Backscatter
Representative Profile
Description
Channel
This channel morphology is characterised by generally enclosed, long and narrow bathymetric lows on the seafloor indicated by laterally converging contours of increasing depth and relatively steep flanks. Channels typically have a higher backscatter response as they are generally scoured features with finer/softer material being transported elsewhere.
Complex bedform
This morphology is characterised by widely spaced (600-750 m) large bedforms that are up to 2 m high. Smaller bedforms and dunes are superimposed on these larger features (15-30 m spacing, variable height but up to 2 m) The backscatter indicated that these bedforms are composed of coarser material than the surrounding seabed. The smaller bedforms are likely the consequence of tidal reworking of the underlying (relict) bedform features.
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Name
Bathymetry
Backscatter
Representative Profile
Description
Complex hardground
The broad shallow pits in the seabed are most likely solution hollows, and indicative of periods of subaerial exposure and erosion when sea-level was lower than the modern seabed, the most recent interval being either during the last glacial (18-23 ka BP), or slightly later (Lewis, et al., 2013).
Dune (sinuous)
This dune morphology is characterised by slightly asymmetric, sinuous, crests. The dunes have wave heights of 3-5 m and a period of 700-800 m. The slight asymmetry of these sandwaves suggests that the likely flow direction, for these dunes, at the seabed is from west to east. Backscatter values indicate that these dunes are composed of coarser material than the surrounding seafloor.
Dune (trochoidal)
This morphology is characterised by fields of dunes that are trochoidal in cross-section. Dunes are up to 5 m high and 100 - 350 m wavelength. Trochoidal dunes typically indicate bidirectional flow, with one direction slightly preferred to the other. The slight asymmetry of these dunes suggests that the flow direction, for these features, at the seabed is towards the south-southeast (i.e. inshore).
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Name
Bathymetry
Backscatter
Representative Profile
Description
Straightcrested Dunes with secondary ripples
This morphology is characterised by straight crested, slightly asymmetric transverse sandwaves. Crests are oriented east-west and are overprinted by rippled seabed. The slight asymmetry of sandwaves suggests that the dominant flow direction at the seabed is from southeast to northwest here (i.e. offshore). The large dunes would have formed in moderately high energy conditions, while the smaller ripples would form in lower energy environments.
Furrows with ripples
This morphology is characterised by linear, sub-parallel 20 m wide channels up to 1 m deep. The channels are up to 1.5 km in length. Backscatter values indicate that sediment within the channels is softer than the surrounding seabed. Commonly the base of the furrow has ripples present, aligned orthogonal to the channel axis. Ripples have wave heights of ~20 cm wavelengths of ~7 m.
Hummock
Numerous mounds are present on the seabed where sediment texture is gravellymuddy-Sand (gmS). The mounds commonly have diameters of 10-25 m, rising 15-45 cm above the surrounding seafloor. The distance between mounds is approximately 30 m or more, giving the seabed a dimpled appearance. Many mounds appear to have a linear to sub-linear crest, suggesting formation by traction. Backscatter values indicate that the mounds have a slightly weaker backscatter response than the surrounding seabed. These are probable storm deposits.
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Name
Bathymetry
Backscatter
Representative Profile
Description
Ledge (cf. small terraces)
This morphology is characterised by a flat or gently sloping area with a steep rise on one margin and a steep drop on another margin. The rise and drops are between 0.5-3 m in elevation difference. These features occur on hard seabed and possibly represent bedding planes, and/or different layers in the partly lithified sediment.
Plain
This morphology is largely flat and smooth with depth gradually increasing away from the present coastline. Despite having a simple morphology and appearance in the bathymetry, plains can have quite variable backscatter values (Figure 3.5) which likely indicates that some regions consist only of a thin sediment veneer while others are composed of thicker accumulations of sediment.
Reef
This morphology is characterised by exposed hard substrate, raised above the surrounding seabed. In this example the reef rises approximately 7-10 m above the adjacent and comparatively flat plain.
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Name
Bathymetry
Backscatter
Representative Profile
Description
Sandwaves
This morphology is characterised by slightly sinuous sandwaves with heights of 0.25-1 m and wavelengths of 10-15 m. The slight asymmetry of the sandwaves indicates a direction of sediment transport at this location, at the seabed, is from north to south (i.e. onshore).
Sandbank/S ediment lobe
This sandbank is up to 12 m thick, approximately 4 km long and oriented eastnorth east to south-southwest. The asymmetry of the sandbank, and the morphology of dunes on its surface indicates a direction of sediment transport at the seabed towards the west. Backscatter values indicate the middle of the sediment lobe appears composed of softer material than at its margin.
Weathered bedrock
At this location a very thin veneer of sediment overlies cemented or sedimentary rock, which has a distinctive surface typical of subaerial exposure and weathering. The most recent interval of time when significant weathering may have occurred here is the Last Glacial Maximum (last ice age, 18-23 ka BP) or intervals of time immediately before or after when sea levels were below the level of the seabed here (Lewis, et al., 2013). Backscatter values support this interpretation as do the presence of hummocks overlying the rugose morphology.
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Tiwi Wreck At 8:18 pm on 26 May 2016 the RV Solander mapped an unidentified wreck, referred to as the Tiwi wreck, at -11.7694° 129.7636° approximately 28 kilometres west of Bathurst Island (Figure 3.3a). Mapping was undertaken at the request of an officer at the Northern Territory Department of Land, Planning and Environment, who provided coordinates and a sidescan sonar image of a potential unidentified wreck. Mapping was completed by 9.56 pm, 2 May 2016. The wreck was mapped using both 300 kHz and 400 kHz frequency settings. The soundings were then converted, examined, manually cleaned and gridded at 30 cm resolution using Caris v8 software.
Figure 3.3 Unidentified wreck, referred to as the Tiwi wreck.
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The Tiwi wreck can be seen lying on its starboard side in 60-62- meters of water on generally flat and featureless seabed (Figure 3.3b and c). The wreck is 27 m long, 7.5 m wide and is shallowest at 58 m – rising ~5 m above the surrounding seafloor (Table 3.3). Off the port side in the middle of the wreck there is potential evidence of a mast, rigging or some other part of the vessel that has fallen off (Figure 3.3c). However, a lot of noise and ringing of soundings off the hull was noted (Figure 3.4) and this feature could just be produced by noisy soundings. The ringing soundings likely indicate the vessel is composed of a highly reflective material such as steel. Table 3.3 Summary characteristics of the Tiwi wreck. Length (m)
27
Width (m)
7.5
Shallowest point (m)
58
Deepest point (m)
63
Figure 3.4 Image of all soundings collected around the Tiwi Wreck. Ringing and noisy soundings are clearly visible. Noisy soundings were manually excluded from the final product (Figure 3.3).
Backscatter intensity Backscatter strength varies between seabed features (Table 3.2). High backscatter strength is typically associated with elevated features including reefs due to hard substrate, while low backscatter is associated with soft sediments e.g. within plains (Figure 3.5). Among the seabed features listed in Table 3.2 weathered bedrock and reef are among those having the highest backscatter values. Theoretically, backscatter strength increases with the sediment grain size. However, this was not always the case here. In some cases, the same grain sizes observed on the same plain (Table 3.2) are associated with low and high backscatter values (bimodal) (Figure 3.6).
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The difference in backscatter strength between areas with high backscatter values and areas with low backscatter values within the same plain along the profile line A-B (Table 3.2) is 4 dB (Figure 3.6). The higher backscatter values of the plain are at the same level of the backscatter value of the weathered bedrock (Figure 3.6). This suggests the presence of a sand veneer over cemented sediment or other harder seabed materials in the area. The complex bedform (Table 3.2) also has a bimodal distribution of backscatter values (Figure 3.6). The higher backscatter values of the complex bedform peak at -21.5 dB, associated with the dune crest whereas the lower backscatter values of the complex bedform peak at -31 dB, associated with flat areas between dunes. This difference may indicate the difference of grain size of the sediment between the dune crest and the areas in between. In addition, higher backscatter values of the complex bedform are comparable to the lower backscatter values of the plain (Figure 3.6). Many sandwave/dune type seabeds show a pattern of alternating high and low backscatter values which coincide with the crests and troughs. Some have troughs associated with higher backscatter values whereas crests associated with lower backscatter values. In contrast, others such as the one observed in the furrows with ripples (Figure 3.7) have troughs associated with lower backscatter values and crests associated with higher backscatter values. The hummock (Table 3.2) is another interesting morphological feature showing an interesting backscatter pattern associated with mounds and their surrounding seabed. The hummock is characterised by lots of rounded mounds (10-25 m in diameter) rising 15-45 cm above the surrounding seafloor. These rounded mounds have backscatter values about 0.25 to 1 dB lower than the surrounding (Figure 3.7).
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Figure 3.5 Hillshaded backscatter data gridded at 1 m resolution and the location of sub-bottom profiler (SBP) lines. For more information of SBP lines see Appendix D.
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0.35 Complex bedform Plain Weathered bedrock
0.3
-17.5 dB
Probability
0.25
0.2 -21 dB-17.5 dB
0.15
-31 dB
0.1
-21.5 dB
0.05
0 -50
-45
-40
-35
-30 -25 -20 Backscatter [dB]
-15
-10
-5
0
Figure 3.6 Histograms of complex bedform, plain and weathered bedrock along corresponding profile line A-B shown in Table 3.2.
A
B
Depth (m)
(A) -16
-18
-20
-22
0
100
200
300
400
500
600
0
100
200
300 Distance (m)
400
500
600
Backscatter (dB)
(B) -24
-26
-28
-30
Figure 3.7 (A) Water depth and (B) backscatter intensity for a cross section of furrows with ripples A-B (see Table 3.2 for location) showing a pattern of alternating high-low backscatter associated with crests and troughs.
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A
B
Depth (m)
(A) -13
-14
-15
-16
0
100
200
300
400
500
600
0
100
200
300 Distance (m)
400
500
600
Backscatter (dB)
(B) -16 -18 -20 -22 -24
Figure 3.8 (A) Water depth and (B) backscatter intensity for a cross section of hummock A-B (see Table 3.2 for location) showing mounds characterised by a subtly weaker backscatter response than the surrounding seabed.
3.2.2 Sub-bottom profiler Sub-bottom profiles reveal up to 20 m penetration and at least 7 distinct acoustic facies (Table 3.4). The facies characterise features such as bedforms (AF1), channel infills (AF3), sediment lobes (AF5), and anomalies such as, gas masking (AA1). Table 3.4 Representative examples and characteristics of the different acoustic facies observed in the sub-bottom profiler data. Acoustic Facies
Reflection Characteristics
AF1
Reflection-free facies with distinct continuous sharp top reflector and absence of bottom reflector
AF2
Reflection-free facies with distinct continuous sharp top reflector, and discontinuous, weak bottom reflector
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Example
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Acoustic Facies
Reflection Characteristics
AF3
Sigmoidal and parallel reflections with distinct continuous sharp top reflector and often overlaying reflection free with diffractions facies. The facies infills depressions
AF4
Parallel and continuous reflections, in places, dipping and truncated by the distinct continuous sharp top. The facies drape the bottom reflector.
AF5
Discontinuous sigmoidal reflections with distinct continuous sharp top reflector and discontinuous bottom reflector
AF6
Discontinuous parallel reflections with distinct continuous sharp top reflector and discontinuous parallel bottom reflector
AA1
Acoustic anomaly masking the stratified facies AF4. The upper reflector bounding the mask area is strong and mainly continuous
Example
3.3 Physical Data 3.3.1 Sedimentology The sampled seabed sediment is composed of material similar to that of the adjacent Darwin Harbour (Siwabessy et al., 2015) with sand and gravel in Bynoe Harbour composed to a large degree of shell grit and iron-rich sediment grains. In particular, the iron-rich sediment grains commonly occur as nodules of coffee-coloured sedimentary rock ("Coffee rock"; cf. pisoliths, but without typical pisolitic banding). Coffee rock is pre-existing sediment that is commonly cemented by hydrated oxides of Fe and sometimes Al which originate from weathering of soils and/or igneous and metamorphic rock, giving coffee rock the dark brown colour. At least one sample of this lithology was recovered from an area of rugose seabed, either having been broken from either a much larger rock, or more likely broken off from in-situ bedrock.
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Of the samples analysed, thirty-six percent are composed of gravelly muddy Sand (gmS, having a sand to mud ratio ranging between 1 and 9, (Folk, 1980) (Figure 3.9). Most of the samples with gmS textures were recovered from the seabed where mounds are present, and indicate mixed sediment. Sixteen percent of samples have muddy sandy Gravel (msG) textures (gravel = 30-80 % of total). Of the remainder, five samples have a Mud (M) texture, and one sample is Sand (S). These data indicate that the sediments sampled are largely mixed, both in grain size and in composition. In general, Mud and mud-rich sediment appears to be primarily located away from rugose seabed and away from high energy shallower areas (Figure 3.10). If, as is the case onshore, that the underlying geology is exposed and partly consolidated at the seabed, it is likely that a small proportion of these samples may represent some in-situ lithologies. However most samples have textures that suggest mixed composition and size ranges, indicative of reworking. A single sample of iron-indurated sediment was retrieved from station 71, broken from either a larger piece of rock, or more probably from in-situ rock. This is the only gravel texture (G) in these samples.
Figure 3.9 Folk textural classes for samples recovered from Bynoe Harbour on a ternary diagram with separate symbols for each textural class. Sediment proportions based on weight percent are dominated by sand, with 5-50 % of the remainder of each sample composed of gravel.
Sampling stations with muddy sediment had a lower average backscatter value (-29.04 dB, n = 13) than either sand (-19.09 dB, n = 60) or gravel (-16.57 dB, n = 24). The range of backscatter values was found to overlap between sandy, muddy and gravelly sampling stations (Figure 3.11).
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Figure 3.10 Sediment textures derived from visual inspection of seabed samples from survey SOL6187. Samples are dominated by sand and mud, with five samples dominated by gravel. Sandy samples were generally recovered from seabed with prominent sedimentary structures; while mud dominated sediment was recovered seabed with low relief.
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Figure 3.11 Preliminary box plots of backscatter data for mud, sand and gravel-dominated sediments for survey station point locations (Figure 3.10). The red line indicates the median, values in the box indicate the first and third quartile, and the tails of the distribution indicate the minimum and maximum.
3.3.2 Geochemistry -2
-1
In this study, total oxygen uptake (TOU) rates were higher in Bynoe Harbour (-19.5 ±8.7 mmol m d ) -2 -1 than in Outer Darwin Harbour (-12.2 ±6.1 mmol m d ) (Siwabessy et al., 2016). There was a broad decline in TOU in a southwest to northwest direction although low rates were also observed in the innermost part of the Harbour (Figure 3.12).
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Figure 3.12 The spatial predictions of total oxygen uptake (TOU) using inverse distance squared (IDS). IDS was preferred than ordinary kriging (OK) because no reliable variogram could be produced for OK. The sites designated as "enhanced" had samples taken to calculate nutrient fluxes as well as TOU.
3.3.3 Biology 3.3.3.1 Underwater photography and video Seabed habitats and benthic macro-organisms were surveyed at 101 stations using underwater videos and still imagery. In total 2:49 hours of useful video was collected from 31 stations and 475 still images from 47 sites across water depths of 9m to 35m (Appendix G). Benthic habitats throughout the surveyed areas were broadly classified where possible, based on video and still imagery observations into three main categories (see Appendix G; Przeslawski et al., 2011; Carroll et al., 2012):
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1. Barren sediments: Sediment is flat with little evidence of infaunal (bioturbation) or epifaunal activity (20 individuals over the transect; up to 15 individuals per 15 seconds estimated over the course of the video). 3. Mixed patches (octocorals and sponges): Patchy rocky outcrops supporting locally abundant patches of octocorals and sponges (that occupy a proportion of at least 20% of the transect - up to 25 individuals per 15 seconds), interspersed with areas of soft sediment and low epifaunal cover. Rocky outcrops may be covered with a thin veneer of sediment; however, epibenthic growth of sessile organisms indicates that a hard substratum is present. Preliminary classifications of benthic habitats consist of predominantly barren habitats (12 stations), bioturbated habitats (11 stations), mixed patches (9 stations) and 69 stations which were unable to be classified. Observed benthic epifauna generally filter feeders from the subclass Octocorallia (sea whips, hydroids) and Tunicates (Stalked solitary ascidians), and Phylum porifera (sponges) (Figure 3.13). Only 32% of the sites were able to be broadly described and it is likely that some seabed habitats were not adequately represented due to turbidity and/or lack of lighting in deeper stations (Figure 3.14). The lack of useable video/images is caused by the fact that the video and stills were taken opportunistically, with no consideration for tidal, turbidity and light conditions. A more appropriately designed epibethic survey will be scheduled for the second half of 2017.
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Figure 3.13 Still underwater photographs showing benthic habitats (a-f). a) Mixed patches with occurrences of sponges and ascidians, SOL6452_074CAM074, b) Mixed patches with sponges and bioturbation, SOL6452_065CAM065, c) Bioturbated, SOL6452_005CAM005, d) Bioturbated, SOL6452_061CAM061, e) Barren muddy sandy Gravel sediment SOL6452_058CAM058, f) Barren muddy sand SOL6452_057CAM057.
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Figure 3.14 Benthic habitat classifications derived from underwater video and still imagery from survey SOL6452 (Appendix Table G.1).
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3.4 Oceanographic data 3.4.1 Tide The tidal model, described in Section 2.5.1, was run for the entire survey period to produce a network of tidal nodes for the tide elevation corrections required for the bathymetric survey. These modelled nodes are summarised in Table 3.5. Modelling showed some differences in tides between all nodes used in this survey (Figure 3.15). The amplitude and phase differences are observed in SW-NE direction, e.g. on 05/05/2016 20:24 the difference is 0.5 m in amplitude and 15.8 minutes in phase. Table 3.5 Modelled nodes for tidal elevation corrections. Node
Easting
Northing
Latitude
Longitude
12658
682910.4
8631014.8
-12.378513
130.682412
12823
671805.8
8634311.2
-12.349327
130.580121
12829
657107.9
8632462.2
-12.366793
130.445066
12491
643026.4
8632082.9
-12.370879
130.315588
12623
654405.5
8623880.5
-12.444505
130.420635
12621
660853.6
8623358.9
-12.448902
130.479975
12722
664504.3
8626440.8
-12.420856
130.513395
12572
668486.9
8622742.6
-12.45408
130.550223
12479
668866.3
8616484.1
-12.510635
130.55405
12397
664883.6
8614113.5
-12.532274
130.517533
12439
661043.2
8616436.8
-12.511469
130.482075
12485
650422.8
8618760
-12.490987
130.384245
12366
646866.9
8615583.4
-12.519871
130.351678
12180
650944.3
8601264.7
-12.649125
130.389893
12235
654689.9
8604583.7
-12.618939
130.424209
12234
657439.8
8604963
-12.615374
130.449502
12328
659288.9
8609656.8
-12.572849
130.466281
12204
660663.9
8602734.6
-12.635356
130.479294
12158
666400.8
8602592.3
-12.636344
130.532111
12227
666969.8
8608044.8
-12.587025
130.537054
12086
671047.3
8596665.7
-12.689668
130.57521
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4 3
Elevation (m)
2 1 0 -1 -2 -3 -4
125
130
135 Julian Day
140
145
Figure 3.15 Predicted tide at nodes given in Table 3.5.
3.4.2 Sound Velocity Profile (SVP) Thirty-four casts were conducted over the entire survey duration, (Table 2.1) with a subset of 23 applied in SIS during acquisition. Between 1 and 3 casts were conducted per day. The sound speed variation between all SVP casts is around 3 m/sec (Figure 3.16). Depending on the intermediate result during the post-processing process, sound velocity corrections using all available SVP data may or may not be re-applied.
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Sound speed [m/s] 1540 0
1541
1542
1543
1544
1545
1546
1547
1548
1549
1550
5
Depth [m]
10
15
20
25
30
35
Figure 3.16 All SVP data applied to the multibeam acquisition system during the survey.
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4 Summary and concluding remarks
The primary objectives of this survey were to characterise the seabed and associated biota in order to provide baseline information and underpin marine resource management decisions. The survey provided insight to the Bynoe Harbour region, where limited seabed information was previously 2 available. This marine survey mapped a total of 698 km using a high-resolution Kongsberg EM2040C multibeam echosounder in dual head configuration. The mapping included bathymetry, backscatter and water column data. Investigations at pre-defined waypoints were undertaken with physical and visual sampling (including underwater video and still, grab, tide gauge, and SVP) at a total of 102 stations. Key preliminary observations from the survey include:
The seabed of Bynoe Harbour is characterised by a complex network of geomorphic features. Sandwaves and dunes observed in the area may help understand the hydrodynamic conditions of the area. Numerous complex hardgrounds located in mostly shallow waters provide support for benthic communities.
Gravelly muddy sand textures, indicating mixed composition, as well as particularly common in the gravel fraction, are dominant. At four stations mud was the sole component, all other samples consisted of gravel, sand and mud. At this stage, there is no obvious relationship between geomorphology and sediment texture.
Useable imagery from drop down cameras and video show that barren and bioturbated sediments dominate the area. However, in comparison to Darwin Harbour a larger number of mixed patches of suspension feeders (e.g. sponges, octorcorals, ascidians) were found in the survey area, which may reflect the more diverse habitats sampled within outer and inner Bynoe Harbour.
There was not a strong spatial pattern of total oxygen uptake, though the parameter declined approximately from west to east.
In areas where soft sediment is present at the seabed, sediment thickness reaches up to 20 m. Channel migration and gas masking are observed in the sub-surface throughout the area.
Integration of this newly acquired data with existing data will provide new insights into the geology and benthic habitat of the Bynoe Harbour area. This work will contribute to the Northern Territory Government’s Darwin Harbour Habitat Mapping Program by providing key seabed environmental and geological data and producing thematic habitat maps to underpin marine resource management decisions. Datasets, imagery, and related research products from this survey will be made available via the Geoscience Australia website (http://www.ga.gov.au/).
40
GA4452/SOL6432 – Post-survey report
Acknowledgements
The Bynoe Harbour Marine Survey was made possible through offset funds provided by INPEX-led Ichthys LNG Project to Northern Territory Government Department of Environment and Natural Resources (DENR), and co-investment from Geoscience Australia (GA) and Australian Institute of Marine Science (AIMS). The authors wish to thank the Master and crew of the RV Solander for the highly professional conduct of the survey. We also thank the Engineering and Applied Scientific Services (EASS) staff at GA for their logistical support and contributions, including Matt Carey, Craig Wintle and Andrew Hislop. Thanks to Matt Carey and Craig Wintle from the EASS for design of new core incubation set-up. We’d like to acknowledge those from the NT Government, AIMS and GA who were involved in the survey planning and advice during the survey. We would like to thank Dan Rawson from the Production and Promotions team for producing this record. We gratefully acknowledge Dr Rachel Przeslawski and Dr Stephanie McLennan at GA for their valuable reviews of this report. This record is published with the permission of the Chief Executive Officer of Geoscience Australia and the Australian Institute of Marine Science.
GA4452/SOL6432 – Post-survey report
41
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Pitcher CR, Doherty P, Arnold P, Hooper J, Gribble N, Bartlett C, Browne M, Campbell N, Cannard T, Cappo M, Carini G, Chalmers S, Cheers S, Chetwynd D, Colefax A, Coles R, Cook S, Davie P, De'ath G, Devereux D, Done B, Donovan T, Ehrke B, Ellis N, Ericson G, Fellegara I, Forcey K, Furey M, Gledhill D, Good N, Gordon S, Haywood M, Hendriks P, Jacobsen I, Johnson J, Jones M, Kinninmoth S, Kistle S, Last P, Leite A, Marks S, McLeod I, Oczkowicz S, Robinson M, Rose C, Seabright D, Sheils J, Sherlock M, Skelton P, Smith D, Smith G, Speare P, Stowar M, Strickland C, Van der Geest C, Venables W, Walsh C, Wassenberg T, Welna A, Yearsley GK. 2007a. Seabed Biodiversity on the Continental Shelf of the Great Barrier Reef World Heritage Area. CSIRO Marine Research, Cleveland, Qld, Auatralia. Pitcher CR, Haywood M, Hooper JNA, Coles R, Bartlett C, Browne M, Cannard T, Carini G, Carter A, Cheers S, Chetwynd D, Colefax A, Cook S, Davie P, Ellis N, Fellegara I, Forcey K, Furey M, Gledhill D, Hendriks P, Jacobsen I, Johnson J, Jones M, Last P, Marks S, McLeod I, Sheils J, Sheppard J, Smith G, Strickland C, Van der Geest C, Venables W, Wassenberg T, Yearsley GK. 2007b. Mapping and characterisation of key biotic and physical attributes of the Torres Strait ecosystem. CRC-TS task number: T2.1 Final Report. CSIRO Marine and Atmospheric Research, Cleveland, Qld 4163, Australia. Post AL. 2006. Physical Surrogates for Benthic Organisms in the Southern Gulf of Carpentaria, Australia: Testing and Application to the Northern Planning Area. Record 2006/009. Geoscience Australia, Canberra. Przeslawski R, Daniell J, Nichol S, Anderson T, Barrie JV. 2011. Seabed Habitats and Hazards of the Joseph Bonaparte Gulf and Timor Sea, Northern Australia. Record 2011/040. Geoscience Australia, Canberra. R Development Core Team. 2012. A Language and Environment for Statistical Computing. Vienna: R Foundation for Statistical Computing. Rattray A, Ierodiaconou D, Laurenson L, Burq S, Reston M. 2009. Hydro-acoustic remote sensing of benthic biological communities on the shallow South East Australian continental shelf. Estuarine, Coastal and Shelf Science 84: 237-245. Rattray A, Ierodiaconou D, Monk J, Versace VL, Laurenson LJB. 2013. Detecting patterns of change in benthic habitats by acoustic remote sensing. Marine Ecology Progress Series 477: 1-13. Tenore KR, Zajac RN, Terwin J, Andrade F, Blanton J, Boynton W, Carey D, Diaz R, Holland AF, López-Jamar E, Montagna P, Nichols F, Rosenberg R, Queiroga H, Sprung M, Whitlatch RB. 2006. Characterizing the role benthos plays in large coastal seas and estuaries: A modular approach. Journal of Experimental Marine Biology and Ecology 330: 392-402. Saqab MM, Bourget J. 2015. Controls on the distribution and growth of isolated carbonate build-ups in the Timor Sea (NW Australia) during the Quaternary. Marine and Petroleum Geology, 62, 123-143. Siwabessy PJW, Tran M, Huang Z, Nichol SL, Atkinson I. 2015. Mapping and Classification of Darwin Harbour Seabed. Record 2015/018. Geoscience Australia, Canberra. http://dx.doi.org/10.11636/Record.2015.018 Walvoort, D.J. Brus, D.J.J., de Gruijter, J.J. 2015. Spatial Coverage Sampling and Random Sampling from Compact Geographical Strata. R package version 0.3-6. http://CRAN.Rproject.org/package=spcosa. Williams D, Wolanski E, Spagnol S. 2006. Hydrodynamics of Darwin Harbour The Environment in Asia Pacific Harbours. Springer Netherlands, pp 461-476. Woodroffe C D, Bardsley KN, Ward PJ, Hanley JR. 1988. Production of mangrove litter in a macrotidal embayment, Darwin Harbour, N.T., Australia. Estuarine, Coastal and Shelf Science, 26, 581-598. Woodroffe C D, Grime D. 1999. Storm impact and evolution of a mangrove-fringed chenier plain, Shoal Bay, Darwin, Australia. Marine Geology, 159, 303-321. Woodroffe CD, Mulrennan ME, Chappell J. 1993. Estuarine infill and coastal progradation, southern van Diemen Gulf, northern Australia. Sedimentary Geology, 83, 257-275. Zajac RN. 2008. Macrobenthic biodiversity and sea floor landscape structure. Journal of Experimental Marine Biology and Ecology 366: 198-203
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GA4452/SOL6432 – Post-survey report
Appendix A Scientific and technical crew
Appendix Table A.1 Role and affiliation of personnel on leg 1 of survey GA4452/SOL6432, 3–15 May 2015. Name
Principal Role
Agency
Responsibility
David Williams
AIMS voyage leader, Oceanographer
AIMS
AIMS voyage leader, tide gauge and other oceanographic device operations, and multibeam acquisition support
Neil Smit
NT Government scientist, Marine ecologist
DENR
NT Gov. science representative, sediment sample acquisition, benthic ecology sampling, underwater camera operations and shipboard database
Justy Siwabessy
GA chief Scientist, Seabed acoustician
GA
Science leader, multibeam acquisition support, processing, data management and GA daily report
Lynda Radke
Geochemist
GA
Geochemical sample acquisition and analysis, and shipboard database
Ian Atkinson
Senior multibeam specialist
GA
Multibeam operations and all technical support
Nick Dando
Multibeam Specialist
GA
Multibeam operations and all technical support
Kim Picard
Seabed acoustician
GA
Multibeam operations and all technical support, sub-bottom profile operations
Appendix Table A.2 Role and affiliation of personnel on leg 2 of survey GA4452/SOL6432, 16–28 May 2016. Name
Principal Role
Agency
Responsibility
Simon Harries
AIMS voyage leader
AIMS
AIMS voyage leader, Multibeam acquisition support
Neil Smit
NT Government scientist, Marine ecologist
DENR
NT Gov. science representative, sediment sample acquisition, benthic ecology sampling, underwater camera operations and shipboard database
Justy Siwabessy
GA chief Scientist, Seabed acoustician
GA
Science leader, multibeam acquisition support, processing, data management and GA daily report
Lynda Radke
GA Geochemist
GA
Geochemical sample acquisition and analysis, and shipboard database
Nick Dando
Multibeam Specialist
GA
Multibeam operations and all technical support
Floyd Howard
Multibeam processor
GA
Multibeam acquisition support and processing
GA4452/SOL6432 – Post-survey report
45
Appendix B Spatial coordinates of waypoints used to guide station selection
Appendix Table B.1 The spatial coordinates of 200 waypoints. The coordinate system is UTM Zone 52 South. No.
Easting
Northing
No.
Easting
Northing
1
637421
8615470
101
654479
8626117
2
637517
8626068
102
654777
8604287
3
637759
8616098
103
654903
8629570
4
638151
8620694
104
654964
8620050
5
638868
8616121
105
654997
8606453
6
638926
8623510
106
655034
8635832
7
639034
8615385
107
655139
8628108
8
639106
8610712
108
655714
8633048
9
639203
8607292
109
655983
8627802
10
639240
8611235
110
656008
8631179
11
639456
8618321
111
656081
8634261
12
639497
8628378
112
656212
8608734
13
639548
8613472
113
656484
8624659
14
639710
8626090
114
656754
8626524
15
640175
8630992
115
656818
8631412
16
640212
8612669
116
657296
8623950
17
640291
8620022
117
657615
8606737
18
640592
8614769
118
657846
8634961
19
640608
8634500
119
658120
8633897
20
640653
8619782
120
658140
8623920
21
640719
8628629
121
658324
8622206
22
640817
8607902
122
658632
8608899
23
640863
8623149
123
658941
8631094
24
640880
8636741
124
659096
8619211
25
641108
8635210
125
659115
8620010
26
641202
8627715
126
659195
8610428
27
641443
8622818
127
659310
8616618
28
641496
8629245
128
659767
8623004
29
641715
8613225
129
659928
8625231
30
641839
8621788
130
659941
8627418
31
641853
8625271
131
660083
8608936
32
641899
8635897
132
660143
8625378
46
GA4452/SOL6432 – Post-survey report
No.
Easting
Northing
No.
Easting
Northing
33
642041
8604395
133
660215
8613787
34
642153
8632574
134
660229
8602513
35
642229
8632405
135
660283
8635176
36
642249
8631429
136
660409
8618798
37
642537
8621431
137
660655
8604008
38
642782
8630585
138
661072
8628866
39
642917
8634698
139
661133
8617946
40
642926
8602326
140
661388
8636679
41
643302
8603309
141
661445
8629171
42
643828
8633863
142
661638
8632887
43
644119
8626913
143
661693
8632346
44
644185
8601743
144
661739
8611264
45
644285
8628668
145
661781
8627600
46
644427
8636077
146
661822
8602466
47
644473
8630075
147
662062
8615844
48
644800
8626126
148
662159
8615346
49
644938
8625145
149
663302
8619042
50
645009
8624691
150
663394
8635694
51
645135
8623366
151
663645
8619156
52
645240
8631125
152
663684
8619473
53
645569
8629657
153
663739
8624152
54
645774
8613544
154
664142
8628120
55
646014
8616396
155
664248
8624958
56
646140
8629257
156
664574
8629210
57
646166
8627338
157
664610
8616235
58
646284
8614387
158
664773
8615500
59
646476
8632460
159
664773
8634330
60
646483
8617441
160
664826
8631052
61
646537
8619063
161
664920
8632164
62
646648
8624028
162
664963
8635803
63
646701
8635148
163
665008
8597497
64
646823
8621647
164
665190
8612203
65
647039
8617135
165
665222
8615196
66
647094
8626715
166
665880
8609298
67
647362
8625926
167
666134
8602554
68
647533
8621637
168
666331
8607011
69
647667
8627168
169
666420
8623121
70
647721
8628612
170
666492
8634596
GA4452/SOL6432 – Post-survey report
47
No.
Easting
Northing
No.
Easting
Northing
71
647772
8620569
171
666816
8609666
72
647795
8616217
172
667116
8635039
73
647806
8631731
173
667162
8634943
74
647919
8627450
174
667377
8610932
75
648660
8622946
175
667717
8611133
76
648676
8616632
176
668156
8623293
77
648723
8634107
177
668310
8600025
78
648740
8622779
178
668362
8630911
79
648981
8623763
179
668385
8611159
80
648993
8623011
180
668496
8614909
81
649259
8600208
181
668538
8607897
82
649858
8636035
182
668651
8625127
83
650701
8636299
183
668702
8620430
84
650822
8634715
184
668828
8623541
85
650872
8632959
185
668849
8615277
86
650932
8621406
186
668927
8598817
87
651157
8597137
187
668932
8616978
88
651470
8631553
188
669060
8623213
89
651846
8628256
189
669584
8619239
90
652117
8624726
190
669765
8621130
91
652286
8635233
191
669818
8623178
92
652298
8602728
192
669825
8619776
93
652722
8632509
193
670164
8617902
94
653213
8603139
194
673393
8632317
95
653245
8625005
195
673593
8631352
96
653733
8630316
196
674392
8631169
97
653997
8634632
197
674623
8595658
98
654005
8622057
198
675903
8595246
99
654026
8602974
199
677246
8596081
100
654112
8627451
200
678991
8595699
48
GA4452/SOL6432 – Post-survey report
Appendix C Technical report on multibeam sonar operations
C.1 Introduction C.1.1 System overview Vessel – The survey was conducted on the Australian Institute of Marine Science Vessel RV Solander. The RV Solander is 35 m long and equipped with a moon pool through which the multibeam sonar heads could be raised and lowered on a mounting system. Multibeam unit - The multibeam sonar was a dual head Konsberg EM 2040C with a frequency range of 200 - 400kHz. Multibeam data was acquired at 300 kHz for comparison with the inner harbour survey backscatter data. Outer beams (> 70°) on the port head were detrimentally affected by the ship’s echo sounder (over various frequencies) which was turned off for all multibeam collection. Motion referencing unit - Applanix PosMV 320 V5 using the integrated Marinestar (a.k.a Omnistar) correction for GPS and GLONASS navigation systems. Sound Velocity Profiler – Sound velocities were measured using a Valeport Mini SVS at the sonar heads to aid in beam forming, and profile casts using a Valeport Mini SVP (sound velocity profiler) which was battery powered and deployed on rope. Locations of the casts are presented in Appendix Table B.2 below. Time and position All times are UTC. 1PPS from the PosMV V5 was used to sync the Mutlibeam Sonar. All data acquisition PC’s were also synchronised to UTC. All positions are derived from the Global Navigational Satellite System and referenced to the World Geodetic System 1984 (WGS84)
C.2 Technical Notes C.2.1 Multibeam System Components C.2.1.1 Dual sonar head Transducers with Dual CBMF Card slimline PU EM2040C Port Head Serial No: 1106 EM2040C Starboard Head Serial No: 1338 Looking up angle set during operation varied between 75-83°. "Auto" coverage mode was used with maximum swath width in metres or degrees, in an attempt to keep to keep swath widths consistent. EM2040C heads were run in high density equidistant mode, 400 beams per head (800 total). The Ping rate varied between 7-14Hz.
GA4452/SOL6432 – Post-survey report
49
Seafloor Information Software (SIS) system display, control and logging: 4.3.0 Processing Unit and sonar head software versions as follows: PU - serial 1106:
CPU: 2.1.7 151119DDS: 4.60 140106DSV: 3.1.8 141125VxWorks 6.9 SMP Build 1.09.01 Nov 23 2015 13:05:13CBMF: 1.08 13.09.18
Sonar Head (Port) - serial 1106
TX: 1.01 Feb 3 2014 RX: 1.00 Sep 28 2012
Sonar Head (Stb) - serial 1338
TX: 1.01 Feb 3 2014 RX: 1.00 Sep 28 2012
Installation Components of the multibeam (Sonar Heads, IMU, GNSS antennas) were installed in their specified mounting locations. The offset between the measurement points of the components was calculated using results from a vessel survey conducted in July 2014. The sonar heads were installed on GA’s dual head 40° mounting system on the moon pool trolley and lowered into place. A stern facing Valeport sound velocity sensor is mounted immediately above the plates to provide real time sound velocity for beam-forming accuracy. The system was setup in a moon pool on a trolley system and winched down the moon pool shaft and locked into position; the locked position being precise, extremely rigid and secure. The IMU was installed with a mounting plate on a stainless plate forward and above the moon pool well. The primary (port) and secondary (starboard) antennas for the PosMV System were installed on dedicated brackets on the rigid awning on the roof deck. Antenna and equipment cables were routed from these locations to the Dry lab were the PU, PosMV, HWS and networking equipment are located. Offsets to the measurement centres of the GNSS antennas, IMU and Sonar heads are entered into the PosMV system, which provides positioning values to SIS/PU at midpoints between the sonar heads. Roll, pitch and yaw angular offsets calculated from installation and calibration are entered into the Kongsberg SIS software.
C.2.1.2 Motion Referencing System: Applanix PosMV 320 V5 using the integrated Marinestar (a.k.a. OmniStar) correction for GPS and GLONASS positioning systems:
Inertial Measurement Unit (IMU)
50
Model: IMU type 27 200Hz
Processing unit; incorporating two GNSS receivers connected to two separate Applanix system GNSS antennas with a 2 m spacing.
GNSS boards BD982, v.00490, Omnistar license 1487797
Model: MV V5
Serial: 5810
GA4452/SOL6432 – Post-survey report
Firmware: POS MV320 V5 8.46
Manufacturers Specifications:
Roll, Pitch accuracy:
Heave accuracy:
Heading accuracy:
Position accuracy: