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Copyright © 2000 by The American Association of Petroleum Geologists. All Rights reserved.Paper presented at the AAPG Hedberg Research Conference, "Integration of Geologic Modelsfor Understanding Risk in the Gulf of Mexico," September 20–24, 1998 (Galveston, Texas).

Subsalt Exploration Using the Marine Magnetotelluric Method

Steven Constable, Scripps Institution of Oceanography, http://mahi.ucsd.edu/Steve/

AbstractWe have developed instrumentation and techniques tocollect magnetotelluric(MT) data on the continentalshelves, allowing subsurface electrical resistivity to bemapped from the surface to depths of tens ofkilometers. Electrical methods are particularlyvaluable in areas of poor seismic performance, such assub-salt, sub-basalt, and sub-carbonate prospects. Ourequipment was tested in 1997 over a deepwater, 3Dsalt sheet in the Gulf of Mexico. When inverted withmodern 2-D interpretation packages, MT data yieldedestimates of base salt depths that were within 10% ofthose obtained by state of the art 3-D seismicinterpretation. We have worked with industry to makethe methodology commercially available, and haveconducted four proprietary surveys to date, both in theGulf of Mexico and the Mediterranean.

IntroductionThe magnetotelluric (MT) method is an establishedtechnique which uses measurements of naturallyoccurring electromagnetic fields to determine the

electrical resistivity of subsurface rocks. Resistivityinformation may be used to map major stratigraphicunits, determine relative porosity, or decide betweentwo or more competing geological interpretations.When an MT survey is accompanied by seismic,gravity, or magnetic data, joint interpretation of alldata leads to a more complete understanding of thesubsurface than is possible through use of any onetechnique alone. The MT method can be used as areconnaissance tool for basin characterization, orspecifically to assist in regions of poor seismicperformance and productivity. Typical of the latter aresediments buried under salt, basalt, or carbonate unitswhich generate strong reflections and reverberationsand make imaging the buried sediments difficult usingacoustic methods alone.

Although salt has a high acoustic contrast withsurrounding sediments, salt also has a high resistivity,making it a potential candidate for mapping byelectrical methods. Modeling shows that givensufficient quality and quantity of MT data, saltstructure, and in particular base of salt, can be

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resolved with accuracies approaching 5% of depth ofburial (Hoversten et al., 1998). Electrical methodsprovide a data stream that is independent of seismic,gravity, and geological results, and so can greatlyreduces risk when incorporated into the explorationprogram: Gravity models, intrinsically non-uniquewhen used alone, can be tested for compatibility withconstraints from an electrical survey. Seismic velocitymodels can be improved using both depth andporosity estimates obtained form resistivity models.And, ideally, preliminary structural information fromelectrical surveys could be used to design higherquality and more cost effective 3-D seismic surveys.Early attempts to use the MT method in the marineenvironment (Hoehn and Warner, 1983) failed, mainlybecause the then current technology and methodologywas not sufficient for the task. This situation haschanged. Modern progress in land MT surveying suchas remote reference data acquisition, robust dataprocessing, multi-site acquisition, and multi-dimensional modeling and inversion have togethermade the MT method much more reliable than it wasin the past. We have coupled these advances with thedevelopment of an effective marine instrument systembased on strong existing programs in marinegeophysics and instrumentation at Scripps Institutionof Oceanography (SIO) (Constable et al., 1998).

InstrumentationThe SIO seafloor electromagnetic recorder (Figure 1)incorporates an acoustic navigation and releasesystem, a modern digital data logger, custom electricfield preamplifiers, low-noise electrodes designed forseafloor use, and commercial broad-band magneticsensors in custom underwater housings. Loggingelectronics reside in a 15 cm inside diameter 7076-T6aluminum tube which is anodized and painted toresist corrosion by seawater and terminated by twoend-caps sealed with O-rings. One endcap has ports tostart the computer, purge damp air from theinstrument, and connect an external computer to theinternal SCSI disk drive. The other endcap has high-pressure, underwater connectors for linking thesensors to the logger inputs. The entire system has amaximum operation depth of 6000 m.The logger pressure case is supported in apolyethylene framework which protects theinstrument form damage during handling andsupports the five glass flotation spheres, acousticrelease package, tow magnetometer coils, four 5 melectrode arms, and a concrete anchor. A magneticcompass, equipped with a timed release to lock theneedle mechanically, records the orientation of thesystem after deployment on the seafloor. The acousticrelease unit serves both to locate the instrumentunderwater and to release the package from theseafloor at the end of the recording period.

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During marine operation, a concrete anchor is attachedto the plastic frame by means of a release systemactuated by acoustic command, releasing the anchorfrom the instrument and allowing the positivelybuoyant package to float to the surface for recovery.Data acquisition at remote reference sites can beeffected with logger units identical to those used onthe sea floor. By working with autonomous vehicles,rather than moorings, the instrument system is morecompact (8 units, plus ancilliary equipment, fit intoone wide-body air freight container), less susceptibleto motional noise induced by mooring cables, andcapable of being deployed and recovered in deepwater in a timely fashion from a modest size ship.

Field TrialsDuring the summers of 1996, 1997, and 1998 weconducted marine MT field trials over the “Gemini”prospect in the Gulf of Mexico (Figure 2), a 3D saltsheet well mapped by 3D seismic data and containinga sub-salt discovery well (Figure 3). In 1996 instrumentproblems corrupted the seafloor magnetic data, andresults relied on combining seafloor electric fields withmagnetic reference data collected on land ( so-called‘hybrid’ data). Results were encouraging, butcompromised by the absence of seafloor magneticmeasurements. We returned in 1997 with improvedequipment and a much more focussed objective tocollect a single profile of high-quality, interpretabledata as a demonstration of the method. Our surveyprofile was chosen on the basis of an experimental

design study which assessed the success of 2-Dmodeling the 3-D structure and also the size of theexpected signal. The water depth of the profile isalmost constant at 950 m. A line of 9 high quality MTsites was obtained in over the deep, relatively thin, 3Dsaltstructure. Raw timeseries of seafloor electric andmagnetic fields were remote reference processed toobtain MT responses in a 1 s to 1000 second periodband.Figure 4 shows the interpretation of these data usingthe Occam’s inversion 2-D MT code of deGroot-Hedlinand Constable (1990). This algorithm generates thesmoothest model that fits the field data, based on thephilosophy that only structure required by the MTresponses is included in the final model. When theinversion is started from a featureless half-space, itgenerates a resistive region where the saltbody isknown to be, showing that without any otherinformation the MT method is sensitive to saltstructure. The salt resistor is smooth, of course,reflecting the intrinsic resolution of the MT methodwhen no other information is included.The first additional information we can include in theinversion is the location of top-salt, obtainable fromrelatively inexpensive 2-D seismic profiling. We dothis by relaxing the requirement that the model besmooth at the known location of the salt surface.Resolution of base salt improves only slightly, but themodel begins to look more geologically realistic.Beneath stations 5 and 6 the salt is too thin to be

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detected by the MT method, and so the smoothinversion removes it.Although smooth models are useful indications of MTresolution, we know that the contacts between salt andsediment are not smooth, but sharp. Recentdevelopments in MT inversion technology (Smith etal.,1997) allow us to include this geologicalinformation in the interpretation, and invert for a saltbody having sharp boundaries at both top-salt andbase-salt surfaces. The result is shown in Figure 5.Again, the match between MT-determined base-saltand seismically-determined base salt is excellentexcept under stations 5 and 6, where the MT modelsalt is now too thick. The sharp boundary inversion isa regularized inversion similar to the smoothinversion, and in this case variations in salt thicknessare penalized. Thus, the inversion makes the salt asthick as possible where there is no data constraint, ineffect placing an upper bound on possible thickness.

Concluding RemarksMapping the base of thin, resistive salt is a challengingand difficult MT problem; electrical methods are notused to survey salt on land and perhaps could not beused for this purpose onshore. However, our offshoresuccess results form the uniform electrical nature ofseawater and the complete lack of cultural electricalnoise, allowing us to produce quality Mt data free ofstatic shifts and bias. After obtaining the results shownin this paper, we returned to the Gemini prospect in

1998 and collected the beginning of a 3-D set in orderto test our capabilities over the more complex parts ofthe salt structure, and filled in the line collected in 1997to examine issues of data density versus resolution(Figure 3). Since the inception of this project in 1994,SIO has assisted in creating a commercial marine MTcapability, and in summer 1998 two proprietarysurveys were conducted in the Gulf of Mexico,following earlier commercial operations in theMediterranean.Acknowledgements: This work was conducted incollaboration with Mike Hoversten and FrankMorrison of U.C. Berkeley, and Arnold Orange andRansom Reddig of AOA Geophysics. Part of this workwas funded by a consortium of companies thatincluded Agip, Andarko, AOA Geophysics,BG,BP,BHP, Chevron, Shell, and Texaco.

ReferencesConstable, S., A. Orange, G.M. Hoversten, and H.F>

Morrison, 1998. Marine magnetotellurics forpetroleum exploration 1. A seafloor instrumentsystem. Geophysics, 63 816-825.

DeGroot-Hedlin, C. and Constable, S.C., 1990. Occam’sinversion to generate smooth, two-dimensionalmodels from magnetotelluric data. Geophysics 55,1613-1624.

Hoehn, G.L., and B>N> Warner, 1983. Magnetotelluricmeasurements in the Gulf of Mexico at 20 m depth.

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In “CRC Handbook of Geophysical Exploration at Sea”,ed. R. Geyer, CRC Press, Inc., pp.397-416.

Hoversten, G.M., H.F. Morrison and S. Constable,1998. Marine magnetotellurics for petroleumexploration 2. Numerical analysis of subsaltresolution. Geophysics 63, 826-840

Hoversten,J.T., G.M. Hoversten, H.F. Morrison, and E.Gasperikova, 1997. Sharp boundary inversion of 2-D magnetotelluric data. Contributed paper atE.A.G.E. Ann. Mtg., Geneva, Switzerland.

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Figure 1: Line drawing of seafloor MT instrument. A concrete anchor sinks the device to the seafloor.The anchor is released by the acoustic unit on receipt of a command code, and the device rises to thesurface with the help of the glass flotation spheres. The electric dipole arms are 5 m long pipesterminated with silver-silver chloride electrodes.

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Figure 2: Bathymetry map of the Gulf of Mexico showing the location of the Gemini prospect in 1 kmwater off New Orleans.

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Figure 3: Footprint of the Gemini salt sheet, based on the 3-D seismic volume provided by sponsorcompanies, and the locations of MT sites collected during 3 summer field season. MC292 is Geminidiscovery well.

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Figure 4: Inversion of the 1997 MT profile shown in Figure 3, using the regularized algorithm ofdeGroot-Hedlin and Constable (1990). In the upper panel, no constraints other than the data wereapplied to the inversion, which seeks the smoothest model compatible with the data. In the lowerpanel, the inversion was again started from a half-space, but in this case the smoothness penalty onthe top of the salt was relaxed, allowing the inversion to place a sharp conductivity jump at thisprescribed surface.

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Figure 5: Inversion of the 1997 MT profile shown in Figure 3, using the sharp-boundary code of Smithet al. (1997) (Hoversten et al., 1999).As in the smooth inversion, top-salt is constrained to be at theseismically-determined depth, and base salt is defined by a sharp resistivity contrast. Again, thesection through the seismic salt volume is shown by the white line. The heavy black line shows wherethe base-salt boundary was started during the inversion.