SEAFLOOR ELECTROMAGNETIC METHODS CONSORTIUM A research proposal submitted by

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1 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 U.S.A. sconstable@ucsd.edu kkey@ucsd.edu 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 exploration 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 development. 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 1

2 CRIPPS INSTITUTION OF OCEANOGRAPHY UCSD INTRODUCTION Scripps Institution of Oceanography (SIO) carried out pioneering work in the field of marine electromagnetic methods in the 1970 s and 1980 s, with the work of Charles ( Chip ) Cox and Jean Filloux, with later contributions from Alan Chave, Spahr Webb, and Steven Constable. These academic projects, funded by various government agencies, were designed to study the electrical conductivity structure of normal oceanic lithosphere in deep water. Application to hydrocarbon exploration was considered in the early 1980 s, but typical exploration water depths of around 300 m were too shallow to make marine EM methods attractive for a number of reasons. These included noise from water motion, 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 using marine 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 for continental shelf marine MT. Later, both Statoil and ExxonMobil used the SEMC to help develop the controlled source electromagnetic (CSEM) method for estimating the resistivity of prospective hydrocarbon reservoirs. Air (resistive) Magnetotelluric source fields CSEM Transmitter Seawater (very conductive) Electric and magnetic field recorders Seafloor (variable conductivity) The magnetotelluric (MT) method is an established technique that has been used on land for exploration during the past 50 years. MT sounding uses measurements of naturally occurring electromagnetic fields to determine the electrical 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 can be used as a reconnaissance tool for basin characterization, or to assist in regions of poor seismic performance and productivity. Typical of the latter are sediments buried under salt, basalt, or carbonate units. With the development of marine CSEM for hydrocarbon detection and delineation, MT has found a new role in characterizing background conductivity 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 in collaboration with scientists at UC Berkeley and AOA Geophysics starting in 1994, when a prototype instrument was tested off San Diego. Since then SIO has developed a new seafloor instrument system that has seen well over 1000 deployments 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 s by Charles Cox and colleagues, with a view to studying the shallower, resistive, part of the oceanic lithosphere in deep 2

3 water, which was otherwise invisible to the MT method. Although it has long been known that the HED-CSEM method is very sensitive to thin resistive layers, and attempts to develop industry projects started at SIO as early as 1984, it was not until exploration moved into water depths of 1000 m or more that the CSEM technique became useful, since it helps if geological targets are electromagnetically shielded from the atmosphere at the sea surface. The motivation to use CSEM for hydrocarbon exploration came mainly from industry (specifically Statoil and ExxonMobil), but SIO personnel and equipment were critical elements in most of the early oilfield CSEM surveys carried out until around Although great progress has been made in the past ten years, the collection of seafloor electromagnetic data is still a technologically sophisticated exercise, and we still make improvements in areas such as motional noise on magnetic sensors, electric field noise characteristics, continuously towed receiver arrays, and instrument reliability and performance. Of equal importance is the development of processing, analysis, modeling, and inversion software appropriate to the marine environment. Finally, it is desirable to pursue new areas of research, such as the joint use of CSEM and MT in surveys, the ability to use other EM field normalizations than the traditional apparent resistivity and 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, in the 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 other industries. 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 the timing is not good for such an increase. Work will be under the direction and supervision of Steven Constable and Kerry Key. Meetings will be held yearly with the sponsors (usually in conjunction with the annual SEG meeting) to review progress and provide sponsors representatives an opportunity to confer with the PI on the research. Results of project research will be made available to all participating sponsors through yearly reports and meetings. In particular, since 2007 we have 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 all aspects 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 seafloor instruments, all capable of being equipped with magnetotelluric and vertical electric field sensors, or even hydrophones. We have two fully tested 500 A transmitters with GPS stabilized waveform control capable of operating on any standard coaxial deeptow cable. Our laboratory staff presently includes 2 engineers and 2 technicians all trained to build and operate this equipment, and 4 students and postocs all with seagoing experience. We have extensive computing resources, including access to various academic clusters 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 vessels. 3

4 PROPOSED WORK It is anticipated that the direction of the research will change as experience is acquired. Also, the sponsors will have the opportunity to confer with the PIs at the annual meetings. However, some or all of the following research directions will be pursued. Instrument upgrades: We are continuously developing and testing improvements to our seafloor EM receivers and deeptowed transmitters. This not only allows us to help push the state of the art, but provides an in-house capability to collect high quality data sets for industry-sponsored academic projects. Integrated MT decomposition and inversion: Currently, MT data rotation and decomposition are done before, and independently of, inversion. It is feasible to modify inversion codes so that data rotation and decomposition are part of the 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 with a limited number of codes. However, 3D forward and inversion codes are becoming available and need to be adapted for seafloor use, although most of the currently available codes are proprietary. We are pursuing a project to provide the industry and academia with a suite of open-source, publicly available codes to carry out 1D and 2D forward and inverse modeling, 3D forward modeling, and possibly 3D inversion at some future time. Initial progress has been and we currently have a 2D forward and inverse joint MT/CSEM code capable of handling bathymetry. We are currently working on a 3D CSEM forward code. Adaptive finite element modeling: Unstructured triangular finite elements allow for modeling of arbitrarily complex structures, making them highly suitable for modeling complicated seafloor models that represent realistic geology and bathymetry. 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. Joint inversion with gravity data presents a realistic possibility. Joint inversion with seismic data will be more difficult but is being attempted. Stochastic Inversion: Statistical methods exist for assessing model uniqueness and resolution when the problem can be reduced to a small number of parameters and the forward solution is very fast (i.e. sparsely parameterized 1D models). We are investigating how these stochastic inversion schemes might be applied to more complicated model spaces. 4

5 Integration of MT with controlled source methods: Since controlled source EM is preferentially sensitive to resistive structure, while MT best resolves conductive features, combined use of the two methods, particularly in areas of shallow resistive rocks such as basalt, offers the expectation of increased resolution. Resolution studies for CSEM in oilfield characterization: A better understanding of the capabilities and limitations of 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 simulating reflector, or BSR ), but EM methods provide a way to quantify the thickness and porosity of these features. We have collected several data sets over potential hydrate targets, and also have started to investigate the electrical properties of gas hydrate in the laboratory. 4D-EM: An obvious extension of the use of CSEM methods to explore for hydrocarbons is to use them to monitor reservoirs during production. This 4D-EM application will require a whole new set of issues to be resolved with regard to instrumentation, data processing, and interpretation. New instrument systems: We are developing towed E-field receivers, surface-towed time and frequency domain transmitters, long baseline E-field gradiometers, joint CSEM/seismic sensors, and other novel instrument systems. RECENTLY DELIVERED PRODUCTS OF THE SEMC: Sharp boundary OCCAM-2DMT inversion code OCCAM2DMT inversion code version 3.0 (fully f90 compatible, dynamic memory allocation) SEMC Workshop talks from 2007, 2008, 2009, 2010, 2011, 2012, 2013 Marine EM shortcourse slide sets Gemini MT data set MARE2DCSEM forward modeling code for 2D CSEM fields MARE2DMT forward modeling code for 2D MT fields MT processing codes Joint CSEM/MT 1D inversion codes MARE2DEM, a joint CSEM/MT 2D inversion code SEMC Web site: STUDENTS AND POSTDOCS FUNDED BY THE SEMC: Kerry Key student , postdoc , research scientist 2008 present (soon to be a professor!) James Behrens student , postdoc Karen Weitemeyer student , postdoc , project scientist, Ashley Medin student Yuguo Li postdoc , project scientist David Myer student Brent Wheelock student Samer Naif student 2009 present Dyan Connell student (Masters) 5

6 Anand Ray student 2010 present Vanessa Brown student co-advised with Satish Singh, IPG Paris, Peter Kannberg student, 2011 present Derrick Hasterok postdoc, 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 being carried out under consortium sponsorship. These workshops have proved popular with sponsors, attracting over 30 attendees. The workshops are free of charge and only consortium members are invited to attend (some academic guests are occasionally invited to join the meetings). All the talks from these workshops are posted on the consortium web site. We expect to continue these workshops indefinitely. PUBLICATIONS ACKNOWLEDGING SEMC SUPPORT: Constable, S., A. Orange, G.M. Hoversten, and H.F. Morrison, Marine magnetotellurics for petroleum exploration Part 1. A seafloor instrument system. Geophysics, 63, Hoversten, G.M., H.F. Morrison and S. Constable, Marine magnetotellurics for petroleum exploration Part 2. Numerical analysis of subsalt resolution. Geophysics, 63, Heinson, G., A. White, S. Constable, and K. Key, Marine self potential exploration. Bull. Aust. Soc. Explor. Geophys., 30, 1 4. Hoversten, G.H., S. Constable, and H.F. Morrison, Marine magnetotellurics for base salt mapping: Gulf of Mexico field-test at the Gemini structure. Geophysics, 65, Eidesmo, T., S. Ellingsrud, L.M. MacGregor, S. Constable, M.C. Sinha, S. Johanson, F.N. Kong, and H. Westerdahl, Sea Bed Logging (SBL), a new method for remote and direct identification of hydrocarbon filled layers in deepwater areas. First Break, 20, Ellingsrud, S., T. Eidesmo, S. Johansen, M.C. Sinha, L.M. MacGregor, and S. Constable, Remote sensing of hydrocarbon layers by seabed logging (SBL): Results from a cruise offshore Angola. The Leading Edge, 21, Kerry Key, Application of broadband marine magnetotelluric exploration to a 3D salt structure and a fastspreading ridge. Ph. D. Thesis, University of California, San Diego. Key, K., S. Constable, and C. Weiss, 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, Inversion of magnetotelluric data for 2D structure with sharp resistivity contrasts. Geophysics, 69, Zhdanov, M.S., L. Wan, S. Constable, and K. Key, New development in 3-D marine MT modeling and inversion for off-shore petroleum exploration. SEG Expanded Abstracts, 23, 588. Constable, S., and C.J. Weiss, Mapping thin resistors (and hydrocarbons) with marine EM methods: Insights from 1D modeling. Geophysics, 71, G43 G51. 6

7 Weitemeyer, K., S. Constable, K. Key, and J. Behrens, The Use of Marine EM Methods for Mapping Gas Hydrates. Contributed paper at 2005 Offshore Tech. Conf., Houston, USA. Constable, S., Hydrocarbon Exploration Using Marine EM Techniques. Contributed paper at 2005 Offshore Tech. Conf., Houston, USA. Key, K., 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, First results from a marine controlledsource electromagnetic survey to detect gas hydrates offshore Oregon. Geophys. Res. Lett., 33, L03304, doi: /2005gl Key, K.W., S.C. Constable, and C.J. Weiss, Mapping 3D salt using 2D marine MT: Case study from Gemini Prospect, Gulf of Mexico. Geophysics, 71, B17 B27. Constable, S., Marine electromagnetic methods A new tool for offshore exploration. The Leading Edge, 25, Weitemeyer, K., S. Constable, and K. Key, Marine EM techniques for gas-hydrate and hazard mitigation. The Leading Edge, 25, Weiss, C.J., and S. Constable, Mapping Thin Resistors in the Marine Environment, Part II: Modeling and Analysis in 3D. Geophysics, 71, G321 G332. Medin, A.E., R.L. Parker, and S. Constable, Making sound inferences from geomagnetic sounding. Phys. Earth Planet. Int., 160, Key, K.W., and C.J. Weiss, Adaptive finite element modeling using unstructured grids: the 2D magnetotelluric example. Geophysics, 71, G291 G299. Constable, S., and L.J. Srnka, An introduction to marine controlled source electromagnetic methods for hydrocarbon exploration. Geophysics, 72, WA3 WA12. Li, Y., and K. Key, D marine controlled-source electromagnetic modeling: Part 1 An adaptive finite-element algorithm. Geophysics, 72, WA51 WA62. Li, Y., and S. Constable, D marine controlled-source electromagnetic modeling: Part 2 The effect of bathymetry. Geophysics, 72, WA63 WA71. Orange, A., K. Key, and S. Constable, The feasibility of reservoir monitoring using time-lapse marine CSEM. Geophysics, 74, F21 F29. Constable, S., K. Key, and Lewis, L., Mapping offshore sedimentary structure using electromagnetic methods and terrain effects in marine magnetotelluric data. Geophysical Journal International, 176, Key, K., D inversion of multicomponent, multifrequency marine CSEM data: Methodology and synthetic studies for resolving thin resistive layers. Geophysics, 74, F9 F20. Constable, S., Ten years of marine CSEM for hydrocarbon exploration. Geophysics, 75, 75A67 75A81. Weitemeyer, K., and S. Constable, Mapping shallow geology and gas hydrate with marine CSEM surveys. First Break, 28,

8 Myer, D., S. Constable, and K. Key, A marine EM survey of the Scarborough gas field, Northwest Shelf of Australia. First Break, 28, Key, K., and A. Lockwood, Determining the orientation of marine CSEM receivers using orthogonal Procrustes rotation analysis. Geophysics, 75, F63 F70. Van Beusekop, A.E., R.L. Parker, R.E. Bank, P.E. Gill, and S. Constable, The 2-D magnetotelluric inverse problem solved with optimization. Geophysical Journal International, 184, Key, K., and S. Constable, Coast effect distortion of marine magnetotelluric data: Insights from a pilot study offshore northeastern Japan. Physics of the Earth and Planetary Interiors, 184, Myer, D., S. Constable, and K. Key, Broad-band waveforms and robust processing for marine CSEM surveys. Geophysical Journal International, 184, Zhdanov, M.S., L. Wan, A. Gribenko., M. Cuma, K. Key, and S. Constable, Large-scale 3D inversion of marine magnetotelluric data: Case study from the Gemini prospect, Gulf of Mexico. Geophysics, 76, F77-F87. Weitemeyer, K., G. Gao, S. Constable, and D. Alumbaugh, The practical application of 2D inversion to marine controlled-source electromagnetic sounding. Geophysics, 75, F199 F211. Constable, S., Ten years of marine CSEM for hydrocarbon exploration. Geophysics, 75, 75A67 75A81. Li, Y., and S. Dai, Finite element modelling of marine controlled-source electromagnetic responses in twodimensional dipping anisotropic conductivity structures. Geophysical Journal International, 185, , doi: /j x x. Key, K., and J. Ovall, A parallel goal-oriented adaptive finite element method for 2.5-D electromagnetic modelling. Geophysical Journal International, 186, , doi: /j x x. Key, K., Marine electromagnetic studies of seafloor resources and tectonics. Surveys In Geophysics, 33, , doi: /s x. Brown, V., K. Key and S. Singh, Seismically regularized controlled-source electromagnetic inversion. Geophysics, 77, E57 E65. Shahin, A., K. Key, P.L. Stoffa and R.H. Tatham, Petro-electric modeling for CSEM reservoir characterization and monitoring. Geophysics, 77, E9 E20, doi: /geo Connell, D., and K. Key, A numerical comparison of time and frequency-domain marine EM methods for hydrocarbon exploration in shallow water. Geophysical Prospecting, accepted. Brown, V., M. Hoversten, K. Key, and J. Chen, Resolution of reservoir scale electrical anisotropy from marine CSEM data. Geophysics, 77, E147 E158, doi: /geo Key, K., Is the fast Hankel transform faster than quadrature?. Geophysics, 77, F21 F30, doi: /geo Myer, D., S. Constable, K. Key, M.E. Glinsky, and G. Liu, Marine CSEM of the Scarborough gas field, Part 1: Experimental design and data uncertainty. Geophysics, 77, E281 E299, doi: /geo Chen, J., G. Hoversten, K. Key, G. Nordquist, and W. Cumming, Stochastic inversion of magnetotelluric data using sharp boundary parameterization and application to a geothermal site. Geophysics, 77, E265 E279, 8

9 doi: /geo Ray, A. and K. Key, Bayesian inversion of marine CSEM data with a trans-dimensional self parametrizing algorithm. Geophysical Journal International, 191, Constable, S., Review paper: Instrumentation for marine magnetotelluric and controlled source electromagnetic sounding. Geophysical Prospecting, available online, doi: /j x. Myer, D., S. Constable, and K. Key, Magnetotelluric evidence for layered mafic intrusions beneath the Vøring and Exmouth rifted margins. Physics of the Earth and Planetary Interiors, 220, available online, doi: /j.pepi Key, K., S. Constable, L. Liu, and A. Pommier, Electrical image of passive mantle upwelling beneath the northern East Pacific Rise. Nature, 495, Naif, S., K. Key, S. Constable, and R.L. Evans, Melt-rich channel observed at the lithosphere-asthenosphere boundary. Nature, 495, Myer, D., S. Constable, and K. Key, Magnetotelluric evidence for layered mafic intrusions beneath the Vøring and Exmouth rifted margins. Physics of the Earth and Planetary Interiors, 220, available online, doi: /j.pepi