SEAFLOOR ELECTROMAGNETIC METHODS CONSORTIUM A research proposal submitted by Steven Constable and Kerry Key Institute of Geophysics and Planetary Physics Scripps Institution of Oceanography La Jolla CA 92093-0225 U.S.A. [email protected][email protected]Version date: July 2013 SUMMARY Scripps Institution of Oceanography (SIO) has been a pioneer in the development and use of marine electromagnetic methods, and is actively working with industry to apply marine EM methods to offshore hydrocarbon exploration and other commercial applications. Since electrical conductivity is a strong indicator of porosity and pore fluid properties, EM methods have applications both for mapping geological structure and for assessing the resistivity of hydrocarbon reservoirs. Two techniques are in common use: Magnetotelluric sounding (MT) is a passive method well suited for reconnaissance basin characterization or to specifically assist in regions of poor seismic performance, such as areas of salt, basalt, or carbonate lithologies. Controlled source electromagnetic (CSEM) sounding is an active technique which cannot probe as deeply as MT but is preferentially sensitive to resistive structures, providing a good complement to the MT method and, under favorable circumstances, capable of directly assessing the resistivity of targets identified as potential hydrocarbon reservoirs. This proposal is to provide core funding for continued research on electromagnetic methods as an offshore explo- ration tool. It supports further development of the equipment, survey techniques, software (processing, analysis, and inversion), testing, and theory for the marine MT and CSEM methods. Importantly, it supports students and postdocs working in marine EM methods, who will graduate with skills critical to continued research and commercial devel- opment. The proposal does not support specific field surveys except insofar funding for instruments, infrastructure, and personnel enhances our capability to conduct field trials, and provides the opportunity to study data collected on surveys supported by other funding. More information and a current list of sponsors can be found at http://marineemlab.ucsd.edu/semc.html 1
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SEAFLOOR ELECTROMAGNETIC METHODS CONSORTIUM
A research proposal submitted by
Steven Constable and Kerry KeyInstitute of Geophysics and Planetary Physics
Scripps Institution of OceanographyLa Jolla CA 92093-0225
Scripps Institution of Oceanography (SIO) has been a pioneer in the development and use of marine electromagneticmethods, and is actively working with industry to apply marine EM methods to offshore hydrocarbon exploration andother commercial applications. Since electrical conductivity is a strong indicator of porosity and pore fluid properties,EM methods have applications both for mapping geological structure and for assessing the resistivity of hydrocarbonreservoirs. Two techniques are in common use: Magnetotelluric sounding (MT) is a passive method well suited forreconnaissance basin characterization or to specifically assist in regions of poor seismic performance, such as areasof salt, basalt, or carbonate lithologies. Controlled source electromagnetic (CSEM) sounding is an active techniquewhich cannot probe as deeply as MT but is preferentially sensitive to resistive structures, providing a good complementto the MT method and, under favorable circumstances, capable of directly assessing the resistivity of targets identifiedas potential hydrocarbon reservoirs.
This proposal is to provide core funding for continued research on electromagnetic methods as an offshore explo-ration tool. It supports further development of the equipment, survey techniques, software (processing, analysis, andinversion), testing, and theory for the marine MT and CSEM methods. Importantly, it supports students and postdocsworking in marine EM methods, who will graduate with skills critical to continued research and commercial devel-opment. The proposal does not support specific field surveys except insofar funding for instruments, infrastructure,and personnel enhances our capability to conduct field trials, and provides the opportunity to study data collected onsurveys supported by other funding.
More information and a current list of sponsors can be found at http://marineemlab.ucsd.edu/semc.html
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INTRODUCTION
Scripps Institution of Oceanography (SIO) carried out pioneering work in the field of marine electromagnetic methodsin the 1970’s and 1980’s, with the work of Charles (‘Chip’) Cox and Jean Filloux, with later contributions from AlanChave, Spahr Webb, and Steven Constable. These academic projects, funded by various government agencies, weredesigned to study the electrical conductivity structure of normal oceanic lithosphere in deep water. Application tohydrocarbon exploration was considered in the early 1980’s, but typical exploration water depths of around 300 mwere too shallow to make marine EM methods attractive for a number of reasons. These included noise from watermotion, the effect of the atmosphere on controlled source methods, and cost effectiveness.
This changed in the 1990’s, when increasing costs associated with deepwater exploration drove an interest in usingmarine EM methods to mitigate drilling risk. Initial activity was associated with using the magnetotelluric (MT)method to map base of salt in the Gulf of Mexico. The SIO Seafloor Electromagnetic Methods Consortium (SEMC)was born in 1994 out of a desire to develop instrumentation, field practices, and processing and modeling codes forcontinental shelf marine MT. Later, both Statoil and ExxonMobil used the SEMC to help develop the controlled sourceelectromagnetic (CSEM) method for estimating the resistivity of prospective hydrocarbon reservoirs.
Air (resistive)
Seawater (very conductive)
CSEM Transmitter
Seafloor (variable conductivity)
Magnetotelluric source fields
CRIP
PS IN
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ITUTION OF OCEANOGRAPHY
UCSD Electric and magnetic field recorders
The magnetotelluric (MT) method is an established technique that has been used on land for exploration duringthe past 50 years. MT sounding uses measurements of naturally occurring electromagnetic fields to determine theelectrical resistivity of subsurface rocks. Resistivity information may be then used to map major stratigraphic units,determine relative porosity, or decide between two or more competing geological interpretations. The MT method canbe used as a reconnaissance tool for basin characterization, or to assist in regions of poor seismic performance andproductivity. Typical of the latter are sediments buried under salt, basalt, or carbonate units. With the developmentof marine CSEM for hydrocarbon detection and delineation, MT has found a new role in characterizing backgroundconductivity to aid in CSEM interpretation, and in joint inversion with CSEM to greatly improve resolution.
The modern application of the MT method to marine continental shelf exploration was developed by the SEMC incollaboration with scientists at UC Berkeley and AOA Geophysics starting in 1994, when a prototype instrument wastested off San Diego. Since then SIO has developed a new seafloor instrument system that has seen well over 1000deployments with an instrument loss rate of less than 1% and a data recovery rate of better than 95%.
The horizontal electric dipole (HED) controlled source EM (CSEM) method was developed at SIO in the late 1970’sby Charles Cox and colleagues, with a view to studying the shallower, resistive, part of the oceanic lithosphere in deep
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water, which was otherwise invisible to the MT method. Although it has long been known that the HED-CSEM methodis very sensitive to thin resistive layers, and attempts to develop industry projects started at SIO as early as 1984, itwas not until exploration moved into water depths of 1000 m or more that the CSEM technique became useful, sinceit helps if geological targets are electromagnetically shielded from the atmosphere at the sea surface. The motivationto use CSEM for hydrocarbon exploration came mainly from industry (specifically Statoil and ExxonMobil), but SIOpersonnel and equipment were critical elements in most of the early oilfield CSEM surveys carried out until around2003.
Although great progress has been made in the past ten years, the collection of seafloor electromagnetic data isstill a technologically sophisticated exercise, and we still make improvements in areas such as motional noise onmagnetic sensors, electric field noise characteristics, continuously towed receiver arrays, and instrument reliabilityand performance. Of equal importance is the development of processing, analysis, modeling, and inversion softwareappropriate to the marine environment. Finally, it is desirable to pursue new areas of research, such as the joint useof CSEM and MT in surveys, the ability to use other EM field normalizations than the traditional apparent resistivityand phase, measurement of vertical electric field, joint interpretation of EM and gravity/seismics data, and so on.Continued activity by the SEMC supports all of these enterprises, but most importantly supports young scientists, inthe form of student and postdocs, to work in the field of marine EM.
PROJECT ORGANIZATION
It is proposed that the work described below be funded by a cooperating group of sponsors from the petroleum and otherindustries. Participation can be reviewed yearly, but a three-year funding agreement is preferred to reduce paperwork.The normal funding level is US$15,000 per year per company. This funding level has been constant for over a decade,and we discussed increasing this amount at the 2010 annual business meeting of the consortium, but it is clear that thetiming is not good for such an increase. Work will be under the direction and supervision of Steven Constable andKerry Key.
Meetings will be held yearly with the sponsors (usually in conjunction with the annual SEG meeting) to review progressand provide sponsors’ representatives an opportunity to confer with the PI on the research. Results of project researchwill be made available to all participating sponsors through yearly reports and meetings. In particular, since 2007 wehave been holding an annual 2-day workshop at Scripps in March where in-depth presentations describe our activities.All the talks from these workshops are posted on the consortium website, amounting to over 100 slide sets covering allaspects of marine EM. All software developed, and data collected, is made available in detail and in a timely fashion.
FACILITIES
The marine EM laboratory at Scripps has a fleet of 50–60seafloor instruments, all capable of being equipped withmagnetotelluric and vertical electric field sensors, or evenhydrophones. We have two fully tested 500 A transmitterswith GPS stabilized waveform control capable of operatingon any standard coaxial deeptow cable. Our laboratory staffpresently includes 2 engineers and 2 technicians all trainedto build and operate this equipment, and 4 students andpostocs all with seagoing experience. We have extensivecomputing resources, including access to various academicclusters operated by the San Diego Supercomputer Center.SIO operates one of the largest research fleets in the world,with 2 ocean-class, 1 regional-class, and 1 local-class ves-sels.
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PROPOSED WORK
It is anticipated that the direction of the research will change as experience is acquired. Also, the sponsors will have theopportunity to confer with the PIs at the annual meetings. However, some or all of the following research directionswill be pursued.
Instrument upgrades: We are continuously developing and testing improvements to our seafloor EM receivers anddeeptowed transmitters. This not only allows us to help push the state of the art, but provides an in-house capability tocollect high quality data sets for industry-sponsored academic projects.
Integrated MT decomposition and inversion: Currently, MT data rotation and decomposition are done before, andindependently of, inversion. It is feasible to modify inversion codes so that data rotation and decomposition are part ofthe inversion process, and either specified as fixed parameters or solved for along with the model parameters.
2D/3D modelling capability: Currently, only 2D inversion can be accomplished in a routine manner, and only witha limited number of codes. However, 3D forward and inversion codes are becoming available and need to be adaptedfor seafloor use, although most of the currently available codes are proprietary. We are pursuing a project to providethe industry and academia with a suite of open-source, publicly available codes to carry out 1D and 2D forward andinverse modeling, 3D forward modeling, and possibly 3D inversion at some future time. Initial progress has been andwe currently have a 2D forward and inverse joint MT/CSEM code capable of handling bathymetry. We are currentlyworking on a 3D CSEM forward code.
Adaptive finite element modeling: Unstructured triangular finite elements allow for modeling of arbitrarily complexstructures, making them highly suitable for modeling complicated seafloor models that represent realistic geology andbathymetry. Adaptive refinement is used to automatically produce a numerically accurate finite element mesh.
Joint inversion: Integration of EM data to the interpretation of other geophysical parameters is essential. Jointinversion with gravity data presents a realistic possibility. Joint inversion with seismic data will be more difficult butis being attempted.
Stochastic Inversion: Statistical methods exist for assessing model uniqueness and resolution when the problem canbe reduced to a small number of parameters and the forward solution is very fast (i.e. sparsely parameterized 1Dmodels). We are investigating how these stochastic inversion schemes might be applied to more complicated modelspaces.
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Integration of MT with controlled source methods: Since controlled source EM is preferentially sensitive to resistivestructure, while MT best resolves conductive features, combined use of the two methods, particularly in areas of shallowresistive rocks such as basalt, offers the expectation of increased resolution.
Resolution studies for CSEM in oilfield characterization: A better understanding of the capabilities and limitationsof the CSEM method can be obtained by model studies in 1D, 2D, and 3D.
Marine EM for gas hydrate characterization: Gas hydrates present a hazard to offshore drilling and infrastructre,and/or a potential hydrocarbon reserve. Seismic methods define the bottom of hydrates very well (the bottom simulatingreflector, or ‘BSR’), but EM methods provide a way to quantify the thickness and porosity of these features. We havecollected several data sets over potential hydrate targets, and also have started to investigate the electrical properties ofgas hydrate in the laboratory.
4D-EM: An obvious extension of the use of CSEM methods to explore for hydrocarbons is to use them to monitorreservoirs during production. This ‘4D-EM’ application will require a whole new set of issues to be resolved withregard to instrumentation, data processing, and interpretation.
New instrument systems: We are developing towed E-field receivers, surface-towed time and frequency domaintransmitters, long baseline E-field gradiometers, joint CSEM/seismic sensors, and other novel instrument systems.
Yuguo Li postdoc 2005–2008, project scientist 2008–2009
David Myer student 2005–2012
Brent Wheelock student 2005–2012
Samer Naif student 2009–present
Dyan Connell student 2009–2011 (Masters)
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Anand Ray student 2010–present
Vanessa Brown student co-advised with Satish Singh, IPG Paris, 2009–2012
Peter Kannberg student, 2011–present
Derrick Hasterok postdoc, 2010–2012
Dallas Sherman student, 2012–present
SEMC WORKSHOPS
In March 2007, 2008, 2009, 2010, 2011, 2012, and 2013 we held 2-day workshops at Scripps to present work beingcarried out under consortium sponsorship. These workshops have proved popular with sponsors, attracting over 30attendees. The workshops are free of charge and only consortium members are invited to attend (some academic guestsare occasionally invited to join the meetings). All the talks from these workshops are posted on the consortium website. We expect to continue these workshops indefinitely.
PUBLICATIONS ACKNOWLEDGING SEMC SUPPORT:
Constable, S., A. Orange, G.M. Hoversten, and H.F. Morrison, 1998. Marine magnetotellurics for petroleum explorationPart 1. A seafloor instrument system. Geophysics, 63, 816–825.
Hoversten, G.M., H.F. Morrison and S. Constable, 1998. Marine magnetotellurics for petroleum exploration Part 2.Numerical analysis of subsalt resolution. Geophysics, 63, 826–840.
Heinson, G., A. White, S. Constable, and K. Key, 1999. Marine self potential exploration. Bull. Aust. Soc. Explor.Geophys., 30, 1–4.
Hoversten, G.H., S. Constable, and H.F. Morrison, 2000. Marine magnetotellurics for base salt mapping: Gulf ofMexico field-test at the Gemini structure. Geophysics, 65, 1476–1488.
Eidesmo, T., S. Ellingsrud, L.M. MacGregor, S. Constable, M.C. Sinha, S. Johanson, F.N. Kong, and H. Westerdahl,2002. Sea Bed Logging (SBL), a new method for remote and direct identification of hydrocarbon filled layers indeepwater areas. First Break, 20, 144–152.
Ellingsrud, S., T. Eidesmo, S. Johansen, M.C. Sinha, L.M. MacGregor, and S. Constable, 2002. Remote sensing ofhydrocarbon layers by seabed logging (SBL): Results from a cruise offshore Angola. The Leading Edge, 21,972-982.
Kerry Key, 2003. Application of broadband marine magnetotelluric exploration to a 3D salt structure and a fast-spreading ridge. Ph. D. Thesis, University of California, San Diego.
Key, K., S. Constable, and C. Weiss, 2004. Mapping 3D salt using 2D marine MT: Case study from Gemini Prospect,Gulf of Mexico. SEG Expanded Abstracts, 23, 596.
deGroot-Hedlin, C. and S.C. Constable, 2004. Inversion of magnetotelluric data for 2D structure with sharp resistivitycontrasts. Geophysics, 69, 78–86.
Zhdanov, M.S., L. Wan, S. Constable, and K. Key, 2004. New development in 3-D marine MT modeling and inversionfor off-shore petroleum exploration. SEG Expanded Abstracts, 23, 588.
Constable, S., and C.J. Weiss, 2006. Mapping thin resistors (and hydrocarbons) with marine EM methods: Insightsfrom 1D modeling. Geophysics, 71, G43–G51.
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Weitemeyer, K., S. Constable, K. Key, and J. Behrens, 2005. The Use of Marine EM Methods for Mapping GasHydrates. Contributed paper at 2005 Offshore Tech. Conf., Houston, USA.
Constable, S., 2005. Hydrocarbon Exploration Using Marine EM Techniques. Contributed paper at 2005 OffshoreTech. Conf., Houston, USA.
Key, K., 2005. Joint interpretation through combined visualization of marine electromagnetic and seismic data.Contributed paper at 2005 Offshore Tech. Conf., Houston, USA.
Weitemeyer, K.A., S.C. Constable, K.W. Key, and J.P. Behrens, 2006. First results from a marine controlled-source electromagnetic survey to detect gas hydrates offshore Oregon. Geophys. Res. Lett., 33, L03304,doi:10.1029/2005GL024896.
Key, K.W., S.C. Constable, and C.J. Weiss, 2006. Mapping 3D salt using 2D marine MT: Case study from GeminiProspect, Gulf of Mexico. Geophysics, 71, B17–B27.
Constable, S., 2006. Marine electromagnetic methods–A new tool for offshore exploration. The Leading Edge, 25,438–444.
Weitemeyer, K., S. Constable, and K. Key, 2006. Marine EM techniques for gas-hydrate and hazard mitigation. TheLeading Edge, 25, 629–632.
Weiss, C.J., and S. Constable, 2006. Mapping Thin Resistors in the Marine Environment, Part II: Modeling andAnalysis in 3D. Geophysics, 71, G321–G332.
Medin, A.E., R.L. Parker, and S. Constable, 2006. Making sound inferences from geomagnetic sounding. Phys. EarthPlanet. Int., 160, 51–59.
Key, K.W., and C.J. Weiss, 2006. Adaptive finite element modeling using unstructured grids: the 2D magnetotelluricexample. Geophysics, 71, G291–G299.
Constable, S., and L.J. Srnka, 2007. An introduction to marine controlled source electromagnetic methods forhydrocarbon exploration. Geophysics, 72, WA3–WA12.
Li, Y., and K. Key, 2007. 2D marine controlled-source electromagnetic modeling: Part 1 – An adaptive finite-elementalgorithm. Geophysics, 72, WA51–WA62.
Li, Y., and S. Constable, 2007. 2D marine controlled-source electromagnetic modeling: Part 2 – The effect ofbathymetry. Geophysics, 72, WA63–WA71.
Orange, A., K. Key, and S. Constable, 2009. The feasibility of reservoir monitoring using time-lapse marine CSEM.Geophysics, 74, F21–F29.
Constable, S., K. Key, and Lewis, L., 2009. Mapping offshore sedimentary structure using electromagnetic methodsand terrain effects in marine magnetotelluric data. Geophysical Journal International, 176, 431–442.
Key, K., 2009. 1D inversion of multicomponent, multifrequency marine CSEM data: Methodology and syntheticstudies for resolving thin resistive layers. Geophysics, 74, F9–F20.
Constable, S., 2010. Ten years of marine CSEM for hydrocarbon exploration. Geophysics, 75, 75A67–75A81.
Weitemeyer, K., and S. Constable, 2010. Mapping shallow geology and gas hydrate with marine CSEM surveys. FirstBreak, 28, 97–102.
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Myer, D., S. Constable, and K. Key, 2010. A marine EM survey of the Scarborough gas field, Northwest Shelf ofAustralia. First Break, 28, 77–82.
Key, K., and A. Lockwood, 2010. Determining the orientation of marine CSEM receivers using orthogonal Procrustesrotation analysis. Geophysics, 75, F63–F70.
Van Beusekop, A.E., R.L. Parker, R.E. Bank, P.E. Gill, and S. Constable, 2011. The 2-D magnetotelluric inverseproblem solved with optimization. Geophysical Journal International, 184, 639–650.
Key, K., and S. Constable, 2011. Coast effect distortion of marine magnetotelluric data: Insights from a pilot studyoffshore northeastern Japan. Physics of the Earth and Planetary Interiors, 184, 194–207.
Myer, D., S. Constable, and K. Key, 2011. Broad-band waveforms and robust processing for marine CSEM surveys.Geophysical Journal International, 184, 689–698.
Zhdanov, M.S., L. Wan, A. Gribenko., M. Cuma, K. Key, and S. Constable, 2011. Large-scale 3D inversion of marinemagnetotelluric data: Case study from the Gemini prospect, Gulf of Mexico. Geophysics, 76, F77-F87.
Weitemeyer, K., G. Gao, S. Constable, and D. Alumbaugh, 2010. The practical application of 2D inversion to marinecontrolled-source electromagnetic sounding. Geophysics, 75, F199–F211.
Constable, S., 2010. Ten years of marine CSEM for hydrocarbon exploration. Geophysics, 75, 75A67–75A81.
Li, Y., and S. Dai, 2011. Finite element modelling of marine controlled-source electromagnetic responses in two-dimensional dipping anisotropic conductivity structures. Geophysical Journal International, 185, 622–636,doi:10.1111/j.1365-246X.2011.04974.x.
Key, K., and J. Ovall, 2011. A parallel goal-oriented adaptive finite element method for 2.5-D electromagneticmodelling. Geophysical Journal International, 186, 137–154, doi:10.1111/j.1365-246X.2011.05025.x.
Key, K., 2011. Marine electromagnetic studies of seafloor resources and tectonics. Surveys In Geophysics, 33,135–167, doi:10.1007/s10712-011-9139-x.
Brown, V., K. Key and S. Singh, 2012. Seismically regularized controlled-source electromagnetic inversion. Geo-physics, 77, E57–E65.
Shahin, A., K. Key, P.L. Stoffa and R.H. Tatham, 2012. Petro-electric modeling for CSEM reservoir characterizationand monitoring. Geophysics, 77, E9–E20, doi:10.1190/geo2010-0329.1.
Connell, D., and K. Key, 2012. A numerical comparison of time and frequency-domain marine EM methods forhydrocarbon exploration in shallow water. Geophysical Prospecting, accepted.
Brown, V., M. Hoversten, K. Key, and J. Chen, 2012. Resolution of reservoir scale electrical anisotropy from marineCSEM data. Geophysics, 77, E147–E158, doi:10.1190/geo2011-0159.1.
Key, K., 2012. Is the fast Hankel transform faster than quadrature?. Geophysics, 77, F21–F30, doi:10.1190/geo2011-0237.1.
Myer, D., S. Constable, K. Key, M.E. Glinsky, and G. Liu, 2012. Marine CSEM of the Scarborough gas field, Part 1:Experimental design and data uncertainty. Geophysics, 77, E281–E299, doi:10.1190/GEO2011-0380.1.
Chen, J., G. Hoversten, K. Key, G. Nordquist, and W. Cumming, 2012. Stochastic inversion of magnetotelluricdata using sharp boundary parameterization and application to a geothermal site. Geophysics, 77, E265–E279,
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doi:10.1190/GEO2011-0430.1.
Ray, A. and K. Key, 2012. Bayesian inversion of marine CSEM data with a trans-dimensional self parametrizingalgorithm. Geophysical Journal International, 191, 1135–1151.
Constable, S., 2013. Review paper: Instrumentation for marine magnetotelluric and controlled source electromagneticsounding. Geophysical Prospecting, available online, doi: 10.1111/j.1365-2478.2012.01117.x.
Myer, D., S. Constable, and K. Key, 2013. Magnetotelluric evidence for layered mafic intrusions beneath theVøring and Exmouth rifted margins. Physics of the Earth and Planetary Interiors, 220, available online, doi:10.1016/j.pepi.2013.04.007.
Key, K., S. Constable, L. Liu, and A. Pommier, 2013. Electrical image of passive mantle upwelling beneath thenorthern East Pacific Rise. Nature, 495, 499–502.
Naif, S., K. Key, S. Constable, and R.L. Evans, 2013. Melt-rich channel observed at the lithosphere-asthenosphereboundary. Nature, 495, 356–359.
Myer, D., S. Constable, and K. Key, 2013. Magnetotelluric evidence for layered mafic intrusions beneath theVøring and Exmouth rifted margins. Physics of the Earth and Planetary Interiors, 220, available online, doi:10.1016/j.pepi.2013.04.007.
