Sea Bottom Types of a Coral Reef Marine Protected ...

Sea Bottom Types of a Coral Reef Marine Protected Area revealed by Side Scan Survey

Ian S. F. C. Fortes, Jhone C. Araujo, Beatriz S. Britto Pereira and Jose Carlos S. Seoane Departamento de Geologia - Instituto de Geociências - Universidade Federal do Rio de Janeiro (IGEO-UFRJ) Rio de Janeiro, RJ, Brazil [email protected], [email protected], [email protected], [email protected]

Abstract— Besides being one of the most biodiverse ecosystems in the world, coral reefs are also very interesting from a geological point of view. The “StructureScan”, present in modern commercially available chartplotters, such as Simrad’s NSS7, provides echosounding with SideScan and DownScan options, and is an imaging echosounder operating at 455 kHz and 800 kHz frequencies in two side beams and one downscan beam. When combined with seafloor samples, it is able to provide an understanding of the differences in material and texture of the seabed. The different textural responses from sediments and objects on the seafloor make it possible to differentiate and precisely locate them. These objects can be natural, such as reefs and isolated stones, or manmade, such as shipwrecks. This work consisted on mapping the sedimentary structures and different bottom types of the reefs from the Parque Municipal Marinho da Coroa Alta/BA, by using the aforementioned side scan mounted permanently on Iamany, a 7 m-long, fiberglass-hull, twin-engine boat. Simultaneously, bathymetric data was acquired in order to generate a complete bathymetric map of the area and generate a digital elevation model of the seabed. Later, the results from the reef scanning will be compared with satellite images of the seabed and granulometric /compositional analysis from the sediments, in order to check how accurate this remote method of mapping is for coral reef areas. Several different bottom types are easily recognised, including coral and algal reefs, flat areas of hard bottom sediments, sand waves and sand sheets covering both reef tops and flat areas, and, eventually, man-made objects, such as a possible shipwreck. Keywords— side scan sonar, echosounder, sea bottom, GPS, MPA (Marine Protected Area)

I.

INTRODUCTION

Coral reefs are biodiversity hotspots of the Oceans [1], and their structure is also of great geological interest, due to their complexity as well as for being modern-day analogues to a class of carbonate reservoirs for the oil industry, including the Brazilian pre-salt system [2]. In order to understand the carbonate bio-construction, one needs to detail their reef structure and environment, as that of the surrounding siliciclastic-carbonatic mixed shelf. Such studies are usually carried using seismic methods, capable of observing the shelf at depth [3]. Mapping natural resources is also fundamental for Marine Protected Areas management plans, as themes as varied

Projeto Coral Vivo is sponsored by Petrobras Ambiental.

as fish distribution, bottom cover types, and water quality and circulation depend on the substrate. However, seismic surveys are costly and require specialized software and hardware. Also, due to the very shallow nature of the structures, traditional survey vessels cannot operate in modern coastal coral reefs. Therefore, availability of surveys over modern Brazilian corals are scarce to non-existent, hindering MPA management plans. Shallow-penetrating sound waves are used in this work to illustrate the modern-day coral reefs of the Cumuruxatiba basin in South Bahia, where the Parque Municipal Marinho da Coroa Alta (PMMCA), a city-sanctioned MPA, is located in the coastal coral/algal reefs of Santa Cruz de Cabralia (Fig. 1). II.

MATERIALS AND METHOD

A. Equipment The “StructureScan”, present in modern commercially available chartplotters, such as Simrad’s NSS7, provides echosounding with SideScan and DownScan options, operating at 455 kHz and 800 kHz frequencies in two side beams and one downscan beam [4]. Equipment was mounted permanently on Iamany, a 7 m-long, twin-fiberglass-hull, twin-engine boat, which hull only dips 35cm below the waterline, an essential feature for safe navigation over the reefs (Fig. 2). Moreover, an integrated 12-channel GPS equipped with an external GPS antenna. The unit recorded latitude, longitude, and left and right sidescan and downscan channels to removable media (a SD flash card), which was subsequentially downloaded to a computer. The unit produced pulses of sound with a particular frequency whose beam lobes were approximated by a cone for the depth sounder, and that are fanshaped sideways for the sidescan and aimed down for the downscan (Fig. 3).

unique image mosaic from the area, however this is beyond the scope of this project. Free DeepView FV 3.0 software [6] was used to visualize the data, while commercially-available Sonar TRX [7] was used to convert .slg data to .csv, in order to plot on Arc-Gis and create a shape-file of the covered area, in any computer on a workstation. Downscan data is best visualized as profiles, and penetrate at least 5-10m on soft bottom types, allowing to observe burrowed structures right beneath the transducer. Transducer depth was also corrected and in the case of very shallow areas, like the reef top and sand-dunes, this correction is important. Boat pitch and roll motion, GPS error and other more sophisticated corrections that demand specialized software were not applied.

Fig.1: Location and main access to Parque Municipal Marinho da Coroa Alta (PMMCA). A: Location in southern Bahia; B: Main access route, through the town of Santa Cruz de Cabrália; C- PMMCA reefs. (Data sources: IBGE 2013, ESRI basemap, 2014 Wordview II Imagery from Digital Globe).[5]

The processed raster files are visualized and interpreted on the computer screen, and interesting features are annotated and georeferenced, the resulting raster are mosaicked and imported into ArcGIS software to be integrated with satellite imagery and bathymetry, that will be used to generate the 3-D views (with a 5× vertical exaggeration) using ArcScene. III.

B. General Guidelines for surveys As the area was simultaneously surveyed for bathymetry [5], transects were designed perpendicular to the coastline, since the bathymetric lines usually run parallel to the coast and concentric from islands, cays and reefs. Thus, transects will collect the information in the direction where higher variability is expected. Separation between transects was selected according to the desired horizontal accuracy. However the needs of the bathymetric surveys, the characteristics of the working area (such as seabed variability, bottom morphology and the sheer shallowness of the reef areas), and time and cost logistics were taken into account in order to decide the number of transects. Cross-shelf, East-West lines were spaced 100m apart (Fig. 4) over 30 days of 4-6 hours of navigation around the highest tide (ranging from 1.3 to 2.0 m above mean sealevel). Survey dates were carefully chosen to coincide with the highest tides, a strategy that was essential for navigating safely over the reefs, which are exposed at low tides. Sand cays were avoided. Design of surveys is greatly enhanced by previous knowledge of an area’s main features, especially when aided by high resolution imagery, which is increasingly available for MPAs. A 0.4m pixel WorldView II scene (acquired April 04th, 2014 @ 13:12:17 UTC) was used as the main tool in planning, and the return time (calibrated for temperature and salinity) was used to calculate the depth. Boat speed was kept to below 7 km/h to improve image acquisition quality. C. Data Processing and Bottom Mapping Data were downloaded to a computer on Simrad’s proprietary .slg (echosounder data) and .xtf (StructureScan) formats. Recorded beam width is a function of depth, and usually was kept at 25m, but noise prevails at the outer part of the images, especially at shallow depths over reefs, due to beam angle (Fig. 3). By adjusting the transect lines separation and the recorded beam width, it is possible to construct a

RESULTS AND DISCUSSION

From the 62 km2 of surveyed coral reef area, with depths varying from 0 to 20m, maps and illustrations have been prepared to show the different bottom types.

Fig.2: Iamany, the boat used in the PMMCA survey. A: Twin hull construction, general view; B: top view of cockpit, with Simrad NSS-7 echosounder and StrataScan unit; C: twin hull, twin engine in action over very shallow reef. Note echosounder/stratascan transducer near the hull bottom at right. Together with seafloor samples, the sidescan and downscan views are able to provide an understanding of the differences in material and texture of the seabed. Including the real geometry and depth of features that can be seen on the satellite image

such as reef lagoons and holes on the reef structure (Fig. 5), which are not taken into account by the bathymetric modeling. The different textural responses from sediments and objects on the seafloor make it possible to distinguish and precisely locate them. Several different bottom types are easily recognised in both side and downscan images, including coral and algal reefs (Figs. 6 and 7), flat areas of soft and hard bottom sediments (Fig. 8), sand waves and sand sheets covering both reef tops and flat areas (Fig. 9), and, eventually, man-made objects, such as a possible shipwreck (Fig. 10).

ACKNOWLEDGMENT The authors would like to thank Petrobras Ambiental for continuously supporting the Coral Vivo Project (www.coralvivo.org). Our enthusiastic survey boat crews and captains are also thanked, along with the most excellent team of volunteers and researchers.

These figures can then be integrated in the GIS with satellite imagery and compared to the DEM generated and to their 3-D views (Fig. 11). These visualizations allow efficient management planning of an MPA. Future work includes comparing the reef scanning with granulometric /compositional analysis from the sediment samples, in order to check the accuracy of this method of remotely mapping coral reef areas.

Fig. 4: Geostatistical Analyst screen capture depicting bathymetry data points along E-W survey lines, classified by depth, from shallow (earthy tones and yellow) to intermediate (green hues) to deep (progressively darker blue hues). Depths vary form above msl to 25m. The World View II image serves as a background. Stratascan data was acquired for the full survey REFERENCES [1]

[2]

[3]

[4]

Fig.3: Echosounder and StrataScan beams. A: Echosounder data captures in either a in a 200 kHz, 12◦ cone angle 150 m or in a 50 kHz, 37◦ cone angle. B: StrataScan operates at 455 kHz and 800 kHz and produce images of both the water column and the surface of the sea bottom.

[5]

[6] [7]

Seoane, J.C.S., Arantes, R.C.M., Castro, C.B., Tedesco. E., Barbosa, C.F., Pires, D.O., 2010a. A GIS-ready database for coral reef faunal distribution patterns Recife de Fora, Bahia, Brazil. Eos Trans. AGU, 91(26), Meet. Am. Suppl., Abstract B33E1- JA10. AGU Meeting of the Americas at Foz do Iguaçú. Thompson, DL, Stilwell, JD & Hall, M 'Lacustrine carbonate reservoirs from Early Cretaceous rift lakes of Western Gondwana: Pre-Salt coquinas of Brazil and West Africa', Gondwana Research, no. 0. Simrad NSS-7 manual: http://www.simrad-yachting.com/enUS/Products/NSS-Touchscreen-Navigation/NSS7-Chartplotter-enus.aspx. Last Accessed April 18th, 2015. Jhone C. de Araújo, Ian S. F. C. Fortes, Fernando C. Duarte, Beatriz S. Britto Pereira, Elisa Elena de S. Santos and José Carlos S. Seoane. 2015. Low-cost Bathymetric Survey for Marine Protected Areas: coral reefs and coastal islands. Unpubliched. Britto Pereira, B.S., 2015. Geoprocessamento aplicado ao mapeamento e caracterização sedimentar do Recife do Araripe, BA. Trabalho de Conclusão de Curso – IGEO, Universidade Federal do Rio de Janeiro. Deep Vision WEB SUPPORT Sonar TRX WEB SUPPORT

Fig. 5: Reef lagoon seen at the left-side. Height of lagoon wall calculated using its shade and beam-angle.

Fig.6: Sidescan segments from StructuralScan survey in screen captures of DeepView software. A: Top left. A seaward facing coral-algal reef and the sand bottom. A small isolated pinnacle is clearly distinguished. A sand sheet is discernible over the reef top. Water depth varies from 5m at pinnacle and reef top to 10m deep at the sand bottom. B: Top right: Gentle seaward facing coralalgal reef slope (top of image) is contrasted to its landward steep slope (bottom of image). Water depth varies from 5m at the reef top to 15m deep at the sand bottom. C: Bottom. Several pinnacles are visible in this backreef image. Depth varies from 4 to 12m. Scale bars in meters.

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Fig.7: Downscan segments from StructuralScan survey. A: Top. Screen capture of Sonar TRX software showing image manipulation controls to the left and a coral-algal reef surrounded by sand in the main screen. Scene is 35m across. B: Middle: Screen capture of Sonar TRX software showing the navigation/georeferencing control tab to the left and a discontinuity in the coral-algal reef (possibly a fracture-fault or the boundary between two huge coral colonies) in the main screen. Scene is 24m across. C: Bottom. Mosaicked and georeferenced image showing a coral reef structure.

Fig.8: Sidescan segments from StructuralScan survey in screen captures of DeepView software. Flat areas. A: Left. Shallow sand plain over reef top, averaging 2m depth. B: Right: Gentle slope, with depths varying between 8 to 10m. Hard bottom sediments with a possible outcrop. Scale bars in meters.

Fig. 9: Sidescan segments from StructuralScan survey in screen captures of DeepView software. Sand waves over shallow reef tops. A: Left. Very pronounced sand waves surrounding the reef pool limits. Average depth of pool bottom is 5 m, top reef is less than a meter deep. B: Right: sand sheet over the flat reef top, with ripples visible. Reef top is about a meter deep. Scale bars in meters.

Fig. 10: Sidescan segments from StructuralScan survey in screen captures of DeepView software. Possible remains of a shipwreck covered in sand is visible at the left side of the side scan Ripples are more pronounced at the reef limits. Average depth of sand bottom is 5 m, reef top is about a meter deep. Scale bars in meters..

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Fig. 1. Sidescan segments from StructuralScan survey in screen captures of DeepView software. Possible remains of a shipwreck covered in sand is visible at the left side of the side scan Ripples are more pronounced at the reef limits. Average depth of sand bottom is 5 m, reef top is about a meter deep. Scale bars in meters..

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Fig 11: Data integration in GIS. A; Top: bathymetry DEM of the heart-shaped reef in the central area of the MPA displayed in 3-D. B: Middle data integration with the acquisition line (in red) over the WorldView II imagery, and corresponding downscan and sidescan products. Red arrows on the downscan indicate buried reef, suggesting the continuity of the reef under the sand . Side scan image of the same area shows the tip of the outcropping reef. C: Bottom: Comparing the sidescan image (left) to the survey line (center of right image, color coded for depth) over the WorldView II imagery confirms the two reef pools.