Dec 17, 1995 - Abstraet Microresistivity imaging and X-radiography of ... of the Ocean Drilling Program (Jackson et al. ..... University of Miami, Miami, Florida.
Geo-Marine Letters(1996) 16:219 225
© Springer-Verlag 1996
Peter D. Jackson • Kevin B. Briggs • Robert C. Flint
Evaluation of sediment heterogeneity using microresistivity imaging and X-radiography
Received: 17 December 1995
Microresistivity imaging and X-radiography of 3-cm-thick sediment core slabs collected by divers from Eckernförde Bay, Germany, indicate subtle delineations and features created by hydrodynamic and biological processes within the top 20 cm of the sediment. Variations in the images of both techniques are controlled by the physical properties of the sediment. The X-radiographs record attenuation by solids throughout the 3-cm thickness of the sediment slab, while microresistivity images depend on pore water in terms of amount, salinity, and distribution. The microresistivity method is shown to detect layered structures and the presence of shells within the highporosity sediment. Abstraet
Introduction Measurement of sediment inhomogeneities and subsequent statistical characterization of the sediment heterogeneity is essential to successful acoustic modeling. Typically, acoustic models have been developed to describe bottom scattering in terms of two components: interface scattering from bottom roughness and volume scattering from sediment inhomogeneities. Although the measurement of interface roughness has been accomplished recently by established methods of stereo photogrammetry and close-range high-frequency sonars (Briggs 1989), characterization of inhomogeneities buried within the sediments has remained an elusive goal. Characterization of subsurface inhomogeneities is especially important in fine-grained sediments where the
P. D. Jackson . R. C. Flint British GeologicalSurvey,EngineeringGeologyand Geophysics Group, Keyworth,NottinghamNG12 5GG, UK K. B. Briggs([~:~) Naval Research Laboratory,SeafloorSciencesBranch, Stennis Space Center, Mississippi 39529-5004, USA Approved for public release; distributionunlimited
impedance difference between the overlying water and the high-porosity sediment is not appreciable. It is in these fine-grained sediments that acoustic energy is scarcely influenced by interface roughness but is likely to penetrate and be influenced by subsurface structures (Jackson and Briggs 1992). Subsurface structures capable of scattering sound are typically shell lag layers or burrows but can be any discontinuity in the sediment fabric. In addition, gas bubbles may be present in a variety of coastal sediments (Schubel and Schiemer 1973) and may be a source of sediment volume scattering. All these features affect porosity, and as described by Archie's Law, the electrical resistivity of the sediment. Furthermore, sediment porosity is theoretically and empirically related to sediment density, which is one factor (sediment sound velocity is the other) in determining sediment acoustic impedance (Hamilton 1980). Differences in acoustic impedance within the sediment are responsible for scattering of acoustic energy by the sediment volume. Thus, from this premise we proposed to use measurements of microresistivity to document inhomogeneities theoretically capable of creating sediment volume scattering. We then confirmed the presence of electrical detection of these inhomogeneities with X-radiography. The development of microresistivity imaging as a tool for quantifying two-dimensional sediment heterogeneity would be superior to existing methods of determining porosity fluctuations on cores, which provide data solely in the vertical dimension (Briggs 1994).
Eckernförde Bay muds High-porosity muds are found in the deeper parts of Eckernförde Bay in the Baltic Sea and are subject to bioturbation and to deposition of thin, coarser-grained, storm laminae (Milkert et al. 1995). The presence of gas bubbles or shells within these high-porosity muds represents barriers to the flow of electric currents and should therefore result in anomalously high values of measured resistivity. These sediments have very high water contents
220 and low rigidity, making them extremely weak and deformable. Typically, the uppermost 20-30 mm of sediment consists of brown oxidized mud overlying a soft anoxic black sediment. Biological mixing is mainly through tube dwellers and is thought to be restricted to the upper 10 mm or less, with larger bivalves and crustaceans in burrows that can extend down to 50-100 mm depth. High sedimentation rates coupled with restricted biological activity result in very high-porosity sediments with low shear strength. Density variätions within the topmost 200 mm are thought to be due to bioturbation (burrows and shells) and storm lamina rather than the presence of biogenic methane gas bubbles.
Electrical microresistivity imaging of sediment cores Sediment heterogeneity has been characterized using microresistivity imaging of split cores during Leg 133 of the Ocean Drilling Program (Jackson et al. 1991). Electrical resistivity measurements on cores of unconsolidated sea-floor sediments, made at frequencies less than 1000 Hz, are scale-independent, being controlled by the spacing of the measuring electrode array. As such they are an ideal tool with which to investigate a wide variety of sedimentological structures. Thin dipping and horizontal layers (given a vertical borehole) were particularly well defined by the technique; laminae less than 5 mm thick were seen in resistivity images, obtained from turbidites, that were not apparent to the naked eye hut were indicative of greater density near the base. Other types of heterogeneity detected included debris flows and the onset of cementation in a carbonate ooze. Artificial samplinginduced heterogeneity was successfully imaged, demonstrating the potential of microresistivity imaging to characterize sediment heterogeneity (Jackson et al. 1992). Because current flows in three dimensions during microresistivity imaging, the technique has the potential to be extended to three dimensions (Lovell et al. 1994). Novel techniques have been developed to enhance the response of microresistivity imaging to otherwise undetected threedimensional (3-D) heterogeneities using perturbation to the electrical current flow as an independent parameter (Jackson and McCann 1993). The controls of sedimentary processes on electrical resistivity can be evaluated using basic research results initiated by the oil industry to predict oil reserves (Archie 1942). Electrical currents are known to flow preferentially through the pore fluids of unconsolidated marine sediments, and the Archie relationships have been shown to be applicable to them (Kermabon et al. 1969; Taylor Smith 1971; Jackson et al. 1978). Consequently, electrical resistivity is related to the fraction of the sediment that is pore-fluid and to the way these fluids are distributed through the sediment. The relationship between these parameters, which Archie put forward over 50 years ago, is still relevant (Archie 1942): F = 1/n"
where F is the formation factor or ratio of the formation resistivity to that of the pore fluid, n is the fractional porosity, and the exponent rn can be used as an indicator of the style of porosity. Archie showed that m increased from 1.5 to 2.0 between unconsolidated sand and a cemented sandstone. It has been shown that the exponent rn is dependent on particle shape for unconsolidated sediments, ranging from 1.2 for spheres, 1.4 for rounded sand, 1.6 for platey sand, and 1.8 2.0 for shell fragments and clays (Atkins and Smith 1961; Jackson et al. 1978). For cemented rocks the exponent m increases from 2.0 for rocks with interparticle porosity to 3.5 for rocks with moldic porosity where large pores are interconnected by relatively smatl-diameter, long, tortuous channels (Focke and Munn 1987). For both interparticle and moldic styles of porosity, there is a large body of evidence confirming that the Archie equation is applicable to both consolidated and unconsolidated sediments. Jackson et al. (1978) have shown that marine sediments of different particle shape, each of which individually obeys the Archie equation, can be described by a general envelope with major trends that could be described by the Winsauer equation (Winsauer et al. 1952): F = a/n"
The electrical tortuosity ~ of an Archie sediment can be readily computed (Schopper 1966) as z = 1/n(l-m) To quantitatively assess the heterogeneity in the porosity of unconsolidated high-porosity sediments, it is beneficial to determine the exponent m. In this study the exponent has been taken to be the value that produces agreement between porosities derived from the Archie equation and those obtained from measurements made on subsamples from cores that had been previously been imaged (microresistivity imaging and X-radiography).
Experimental procedure Microresistivity imaging was accomplished on 30-mmthick sediment slabs collected in rectangular cores by divers. The diver cores were constructed of 6.4-mm-thick acrylic plastic and were 360 mm wide, 440 mm long, and 30 mm thick. Orte face of the core was sealed with silicone sealant and held together with steel machine screws; the other face was held tight against a rubber neoprene gasket with steel machine screws and could be removed for access to the sediment slab. Two 19-mm holes in the top of the core were used for displacement of watet during core insertion by divers. The holes were closed by means of neoprene rubber bungs after sediment collection. Bottom edges of the cores were beveled to minimize disturbance of the sediment slab during insertion. The bottoms of the cores were sealed with rectangular acrylic caps lined with ceUular neoprene. Rubber straps held the bottom caps fast to the cores.
221 The diver cores were X-ray imaged by placing a sheet of Kodak AA industrial X-ray film (354 mm x 430 mm) on the back of the core and exposing the core and film to 50 kV at 20 mA for 20 s with a Kramex PX-20N portable X-ray unit (Rhoads et al. 1977). Cores were exposed to X-rays within a lead-lined box and were subject to microresistivity imaging within 12-36 h. After development and fixing, the X-radiographs were used as negative images to produce X-radiographic positives for the published figures. The principles of the two-dimensional (2-D) microresistivity imaging technique have been published elsewhere (Jackson et al. 1990, 1994). We modified the technique to image the sediment slabs by extending the spatial coverage to a square aspect ratio rather than the elongate array used to investigate split cores. The arrangement of the electrode arrays is shown in Fig. 1, where the imaged portion of the sample is delineated by the dotted square. The elongate rectangles within this square represent one pad position consisting of 576 electrodes spaced on a 5mm grid. A total of 2880 electrode positions is required to produce one microresistivity image covering the dotted outline in Fig. 1. The major obstacle to acquiring the data was the tendency of the sediment to slump when the core was placed on its side (Fig. 2). It was necessary to place the sediment slab on its side to provide access for electrodes (Fig. 1) and
Plan view of core sample prepared for imaging process. Dotted areas denote five electrode pad positions,
7 curren electrod~ Sea wat(
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Above:-Slump resulting from unsupported sample/water interface. Below:-Interface supported with saturated foam to maintain sample integrity.
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Fig. 2 Disturbance of the weak, high-porosity sediments was inevitable if any nonhorizontal surface was left without support
to ensure that the pore fluid remained in the sediment during the measurement process. The problem was overcome by placing saturated porous foam immediately adjacent to the sediment surface. Initially, saturated foam was introduced immediately after removal of the side of the core. Subsequent experimentation, however, showed that introducing foam while the core box was intact and in the vertical position minimized disturbance to the sedimentwater interface. This saturated porous foam supported the weak sediment while allowing electric current to pass through uniformly. This was necessary for 2-D imaging as the current is constrained to flow uniformly through the sample, in this case in what was the vertical direction when the sediment was in situ. The measurement sequence comprised removal of one side of the diver core box to enable access to the sediment slab (Fig. 1). Once access to the core was gained, electrodes were fitted as shown in Fig. 1 and the electrode pad was placed in the first of five positions. The potential difference between the potential dipoles was logged by a computer while a current of constant magnitude, but alternating direction, was passed through the sample. All aspects of this operation were under computer control, enabling the electrode arrays and acquisition parameters to be reconfigured if necessary. The measured parameters were converted to resistances at each electrode coordinate, forming the fundamental data set from which formation factors and porosities were calculated. The coordinates of the electrodes were calculated knowing the position of the corners of the electrode pad.
Sedimen
Results 7 current electrodes
Fig. 1 The sediment slab and positions ofelectrodes used for microresistivity imaging
Examples of data obtained from the X-radiography and microresistivity imaging of sediment slabs can be seen in Figs. 3 to 5. Cores were exposed to X-rays within 5 h of being collected, whereas the resistivity data was obtained
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Fig. 3 Core 222_1. Microresistivity (upper) image compared to X-radiograph 0ower) for sediment from Eckernf6rde Bay. The
sediment-water interface is at 350 mm (y position), and shells can be seen at 250 mm (y position) 70 mm below the sediment-water interface
Fig. 4 Core 195_1. Microresistivity (upper) image compared to X-radiograph (lower) for sediment from Eckernf6rde Bay. The sediment-water interface is at 350 mm (y position) in the resistivity image
within 12--36 h. Consequently the X-radiographs are less affected by handling disturbance than the resistivity images. Sampling disturbance can be evaluated from the X-radiographs and is thought to be significant in Fig. 3. This is an extreme case, however, and we believe the results Figs. 4 and 5 are more typical. In Figs. 3-5, the dark areas in the X-radiographs show greater attenuation of the X-rays, which is indicative of increased density and, hence, lower porosity and higher resistivity. Porosities have been calculated from Archie's equation assuming an exponent of 2 based on independent porosity data (described above) and a measured porewater resistivity of 0.51 ohm-re. In Fig. 3 the lighter areas of the X-radiograph correspond to lower resistivities and higher porosities, in particular, the light/dark/lighter (top to bottom) sequence at 270 mm (x position). A small cluster of shells can be seen between 100 and 140 (x position) in the X-radiograph, which corresponds to a zone of anomalously high values in the resistivity image. Figure 4 is an example where there was partial disturbance of the sediment during the insertion of the foam barrier, which resulted in generally lower values on the left-hand side of the resistivity image (60-200 mm x position). The right-hand side of the sediment slab was less disturbed and shows good correspondence between X-radiograph and resistivities, particularly the general trends
and the lighter zone beneath the dark layer between 300 and 360 mm x position and 300 and 320 mm y position. The X-radiograph in Fig. 5 depicts a burrow (145 mm x position) 130 mm deep that has been infilled with shells near its base. As a result, a lighter zone (lower density) is seen to intersect a darker layer (higher density) within a sequence where the topmost 30 mm is far lighter than the lower t00 ram. The resistivity image depicts the burrow as a zone of lower resistivity over the range from 290 to 350 mm (y position) with a zone of anomalously high resistivity at 270 mm (y position) where shells can be seen in the X-radiograph. Subhorizontal layering (terminated by the burrow) is more evident in the resistivity data than in the X-radiograph. Porosity data were derived from the microresistivity images, as described above, and are presented in Fig. 6. The features seen in the corresponding microresistivity images are seen as inverse trends with the areas of lower porosity (higher resistivity) being prominent The three porosity images exhibit markedly different characters: 222_1 displays evidence of cyclic changes with a "wavelength" of 50-60 mm, 195_1 displays laminae with thicknesses of 5-10 ram, 196_2 displays three layers of 5 mm thickness (180-280 x position) having low porosity values that are not prominent in the corresponding X-radiograph (whereas the burrow feature and shells are prominent in both).
223 Porosity
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Fig. $ Core 196_2. Microresistivity (upper) image compared to Xradiograph (lower)for sediment from Eckernf6rde Bay. Layering can be seen that is intersected by a burrow containing shells in its lower part
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Because electrical currents seldom flow exactly in straight lines as X-rays do, it is therefore likely that there are fundamental differences in the way the resistivity and X-ray data portray the same heterogeneity. For our purposes, X-ray attenuation can be considered to be a linear process where the exact parcel of sediment characterized by X-rays can be identified, and averaging processes can be used to predict attenuation. Resistivity measurements on the other hand cannot be treated in this way and should be treated as a "volume" measurement. A shell within the sediment slab represents a 3-D problem to be simulated to predict the response we may expect. As the electrodes are applied only to one surface of the sediment, we would expect that a heterogeneity such as a shell would be well resolved if it were close to this surface, but less well resolved if it were closer to the opposite surface where there are no electrodes. As all the sediment slab carries a uniform electric current, we expect that heterogeneities wilt be resolved by the electrode array even when they are situated near the opposite surface. This hypothesis was tested using a 3-D numerical model that has been verified by comparison with physical models (Jackson and McCann 1993; Jackson et al. 1994). The results in Fig. 7 show the simulated response to a thin, dipping layer that occupies the half of a sediment slab
260 240 60 80 t00120140 t60180200220240260280300320340 x position (ram)
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Fig. 6 Porosity images derived from the microresistivity images for core samples 222_1, 195_t, and 196_2 displayed in Figs. 3 5, respectively
next to the surface where electrodes are attached (25 mm out of a total "depth" of 45 mm). The measurements can be seen to be sensitive to this layer and produce a response that faithfully depicts its lateral extent, although it does not extend across the whole depth of the core stab. The small halo where the layer is terminated within the sample is due to current gathering; the image can be improved using a correction developed to counteract this effect (Jackson and McCann 1993; Jackson et al. t994). The simulated seabed interface (y position 205) is faithfully depicted and the simulated response of an isolated shell (25 x position, 175 y position) can b e seen, although it does not intersect the surface where the electrodes make
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gations demonstrated hefe have been shown to respond to 3-D heterogeneity and offer the potential for development in this direction. Investigations could be made on different sizes and shapes of sample, including remote measurements on the seabed. The porosity fluctuations depicted with microresistivity imaging can be used statistically for calculating correlation lengths at 5-mm resolution in both the vertical and horizontal dimensions. We plan to adapt the image processing techniques of Hoyler et al. (1995) to these data to obtain correlation functions and correlation lengths of porosity fluctuations. The microresistivity imaging method is successful in detecting discrete layers and anomalous shells within the relatively homogeneous sediment of Eckernförde Bay. The hext challenge will be to image microresistivity patterns within sediments comprised of dense concentrations of volume inhomogeneities, such as shells and shell fragments, and to combine fabric studies, formation factor measurements, and porosities derived from X-radiography to assess electrical tortuosity and its relevance to acoustic modeling.
200-
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Acknowledgmeùts The research was undertaken as part of the ONR Coastal Benthic Boundary Layer (CBBL) Program managed by the Naval Research Laboratory, Dr. Michael D. Richardson, chief scientist. P.D.J. and R.C.F. publish with the permission of the Director, BGS (NERC). The NRL contribution number is NRL/JA/7431-950045.
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Fig. 7 Numerical simulation of a thin dipping layer and an isolated shell, which does not intersect the surface where the electrodes are attached
contact with the sediment and it is thinner than the dipping layer (20 mm).
Conclusions Microresistivity imaging has been compared to X-radiographs for sediment slabs of high porosity and very low rigidity from Eckernförde Bay in the Baltic Sea. Microresistivity measurements have been shown to be sensitive to several classes of sediment heterogeneity, layers of differing density, bioturbation, shells, and sample disturbance. Thus, the method offers an independent means that could be used to identify sediment heterogeneity in the study of acoustic propagation scattering and attenuation. Microresistivity imaging is scale-independent and is controlled by the 3-D structure of sediments; the 2-D investi-
Archie GE (1942) The electrical resistivity log as an aid in determining some reservoir characteristics. Transactions of the Mining and Metallurgical Engineers 146:54-62 Atkins ER and Smith GH (1961) The significance of particle shape in formation factor porosity relationships. Journal of Petroleum Technology 13:285 291 Briggs KB (1989) Microtopographical roughness of shallow water continental shelves. IEEE Journal of Oceanic Engineering 13: 360-367 Briggs KB (1994) High-frequency acoustic scattering from sediment interface roughness and volume inhomogeneities. Unpublished thesis. University of Miami, Miami, Florida. 143 pp Focke JW and Munn D (1987) Cementation exponents in Middle Eastern carbonate reservoirs. Society of Petroleum Engineers, Formation Evaluation 2:155-161 Hamilton EL (1980) Geoacoustic modelling of the sea floor. Journal of the Acoustical Society of America 68:1313 - 1340 Hoyler RJ, Young DK, Sandidge JR, and Briggs KB (1995) Sediment density structure inferred by textural analysis of cross-sectional X-radiographs. Geo-Marine Letters 16: 204-211 Jackson DR and Briggs KB (1992) High-frequency bottom backscattering: Roughness vs. sediment volume scattering. Journal of the Acoustical Society of America 92:1313-1340 Jackson PD and McCann DM (1993) Non-destructive geophysical site investigation--an aid to the re-development of sites in urban areas with groundwater problems. In: International Conference on Groundwater Problems in Urban Areas, The Institution of Civil Engineers. London: Thomas Telford. pp 122 133 Jackson PD, Taylor Smitb D, and Stanford PN (1978) Resistivityporosity-particle shape relationships for marine sands. Geophysics 43 : 1250-1268
225 Jackson PD, Lovell MA, Pitcher C, Green CA, Evans CJ, Flint RC, and Forster A (1990) Electrical resistivity imaging of core samples. In: Worthington PF (Ed.), Advances in Core Evaluation, Accuracy and Precision in Reserves Estimation. Proceedings of the European Core Analysis Symposium (Eurocas I), Society of Core Analysts. London: Gordon & Breach Science. 555 pp Jackson PD, and Shipboard Party of Leg 133 of the Ocean Drilling Program (1991) Electrical resistivity core scanning: a new aid to the evaluation of fine scale sedimentary structure in sedimentary cores. Scientific Drilling 2:41-54 Jackson PD, Lovell MA, Harvey PK, Ball JK, Williams C, Ashu P, Flint RC, Meldrum PI, Reece G, and Zheng G (1992) Electrical resistivity core imaging: Theoretical and practical experiments as an aid to reservoir characterisation. In: Society of Professional Well Log Analysts 33rd Annual Logging Symposium, Oklahoma City, Oklahoma 2: Paper VV. 7 pp Jackson PD, Lovell MA, Harvey PK, Ball JK, Williams C, Flint RC, Ashu P, and Meldrum PI (1994) Advances in resistivity core imaging, In: Society of Professional Well Log Analysts 35th Annual Logging Symposium, Tulsa, Oklahoma II: Paper GG. 5 pp Kermabon AJ, Gehin C, and Blavier P (1969) A deep-sea electrical resistivity probe for measuring porosity and density of unconsolidated sediments. Geophysics 34:554-571 Lovell MA, Harvey PK, Jackson PD, Ball JK, Ashu P, Flint RC, and Gunn DA (1994) Electrical resistivity core imaging: Towards a 3-dimensional solution. In: Society of Professional Well Log An-
alysts 35th Annual Logging Symposium, Tulsa, Oklahoma II: Paper JJ. 5 pp Milkert D, Hentschke U, and Werner F (1995) Influence of storms on sediments in Eckernförde Bay. Proceedings of the Workshop Modelling Methane-Rich Sediments of Eckernförde Bay, Eckernf6rde, 26-30 June 1995 Rhoads DC, Aller RC, and Goldhaber MB (1977) The influence of colonizing benthos on physical properties and chemical diagenesis of the estuarine sea floor. In: Coull BC (Edv), Ecology of Marine Benthos. Colombia, South Carolina: University of South Carolina Press. pp 113-138 Schubel JR and Schiemer EW (1973) The cause of the acoustically impenetrable, or turbid, character of Chesapeake Bay sediments. Marine Geophysical Research 2:61-71 Schopper JR (1966) A theoretical investigation of the formation factor/permeability/porosityrelationship using a network model. Geophysical Prospecting 14:301-341 Taylor Smith D (197l) Acoustic and electrical techniques for seafloor sediment identification, In: Proceedings of the International Symposium on Engineering Properties of Sea--Floor Soils and Their Geophysicat Identification, Seattte, Washington. pp 235267 Winsauer WO, Sheann AM, Massun PH, and Williams M (1952) Resistivities of brine saturated sands in relation to pore geometry. Bulletin of the American Association of Petroleum Geologists 36:253-277
