The Gauss-Newton method has good convergence properties when used for the solution of seismic and electromagnetic inversion problems. One main issue is the high numerical cost. The numerical cost can be reduced if the optimization domain can be decoupled from the simulation domain such that the number of optimization parameters is much smaller than the number of grid nodes required for accurate simulation results. Overparameterization can be avoided. The decoupling can be achieved in a rigorous manner with the use of node-based basis functions. We provide a generic derivation of the method that is easily specialized for seismic and electromagnetic problems. The transformations between the optimization domain and the simulation domain are most effective if both domains can be described by rectilinear grids. A variable seabed depth causes difficulty. We introduce a transform from the true bathymetry to a flat seabed that solves this problem. The method is validated by application to synthetic and real electromagnetic data sets. The real data are acquired at the slow spreading Mohns Ridge located east of Greenland and southwest of Svalbard. We provide a discussion on the interpretation of these data for an inverse scheme using the transverse isotropy with a vertical symmetry axis approximation. We offer some insights on how to interpret inversion results in the case of exploration for marine minerals. The interpretation differs from a hydrocarbon exploration setting owing to the presence of vertical conductors due to the formation water circulation and vertical resistors due to volcanic intrusions.

We present a new approach that allows for the inversion of quantities derived from the observed data using non-diagonal data covariance matrices. For example, we can invert approximations of apparent resistivity and phase instead of magnetotelluric impedance using this methodology. Compared to the direct inversion of these derived quantities, the proposed methodology has two advantages: i) If an inversion algorithm allows for the specification of a full data covariance matrix, users can invert for arbitrary derived quantities by specifying the appropriate covariance matrix instead of having to rely on the inversion code to have implemented this feature. ii) It is fully compatible with the assumptions of least-squares optimization and thus avoids potential issues with bias when inverting quantities that are non-linear functions of the original data, We discuss the theory of this approach and show an example using magnetotelluric data. However, the same method can be applied to other types of geophysical data, for example gravity gradient measurements.

Profiling electromagnetic distributed acquisition systems (DAS), such as MIMDAS and Titan-24, are commonly used for near-surface exploration. Such systems allow more rapid acquisition compared to standard magnetotelluric (MT) approaches. Instead of recording horizontal electric and magnetic fields at every site, DAS acquire a single (along-profile) component of the electric field at every site, but the perpendicular component is measured at every second or so site, and the magnetic field sensors are normally positioned only at a couple of locations within the study area. It is common practice to apply standard MT inversion algorithms to invert such DAS data, despite the lack of full four component measurements at each site. This is only valid in the strictly two-dimensional (2D) case with the geoelectric strike perpendicular to the acquisition profile and for the transverse magnetic (TM) mode, and also under certain assumptions on frequency range and resistivities. In case of the 2D transverse electric (TE) mode and in the general 3D case, employing standard MT inversion software will lead to erroneous results. Therefore, we have developed a new 3D inversion algorithm that considers the correct positioning of all sensors. We investigate the ability of the new inversion to recover the subsurface resistivity and compare the results to standard MT inversion of DAS data. The newly developed inversion code is also useful for conventional MT surveys when data from some channels is lost. With the new approach missing fields can be easily substituted by fields from another site. Presentation Date: Wednesday, October 17, 2018 Start Time: 1:50:00 PM Location: 213A (Anaheim Convention Center) Presentation Type: Oral

The source of magmatic features along the Namibian continental margin, and therefore the processes which lead to the opening of the South Atlantic ocean, are still debated controversially. One big question is weather hotspot volcanism was fed by a deep reaching plume or by heterogeneities of the middle and upper mantle. In an attempt to gain a better understanding of the involved magmatic processes, a 3D inversion of magnetotelluric data with an integrated seismically constrained density model was conducted. Integration was accomplished by adding a cross-gradient constraint of the density model to the inversion, which enforces model resemblance at structural boundaries. The impact of this cross-gradient constraint with a preexisting density model is limited to this model's resolution, because the cross-gradient only works at structural boundaries within the constraint model. Its benefits include enhancing of resistivity structures in the inversion model. Additionally, the density constraint does not overprint resistivity structures which are not imaged by gravimetric methods (i.e. resistivity variations due to mineral composition). An observed high resistivity anomaly below the continental margin and Walvis Ridge coincides well with seismically observed high velocity underplating. This feature is interpreted to mark magmatic intrusions from a plume source, initiating continental breakup. The eastern termination of the high resistivity structure correlates with the onset of seaward dipping reflectors in seismic data. Therefore, it marks the transition from continental to oceanic regime (continent-ocean boundary). The theory of a plume source of the magmatic features is supported by the local planar extent of roughly estimated 70 000km² and the deep reaching form of the underplating. This form with three arms at a 120° spread is suggestive for the rift arms of a hot spot impingement into the crust. However, since the aforementioned amount of intrusive material is rather small for the impact of a road plume head, the preferred model includes a plume that stopped ascending in the mid-mantel. The underplated magmatic features would then be fed by smaller plumes or hotspot-like dikes rising from this deep mantle plume. A continental breakup solely driven by plate-forces is unlikely for the South Atlantic, as lithospheric thinning and subsequent magmatism would have resulted in a larger volcanic area due to mantle heterogeneities, than the observed local magmatic underplating.

We present a three-dimensional resistivity model derived from a large-scale magnetotelluric experiment offshore Namibia. To obtain the model we use the inversion code mtinv based on an integral equation approach (see Avdeev&Avdeeva, 2009, Avdeeva et al., 2012 and Moorkamp et al., 2010). The code allows positioning of MT sites at the sea-floor and inclusion of bathymetry. However, the bathymetry is approximated with a rectilinear mesh, which is not optimal as a large portion of the computationally feasible grid is used up by bathymetry, at the same time reducing the resolution of the upper region. Nevertheless, we observe a good correlation of the boundaries of large-scale resistive anomalies with seismically identified lower crustal high-velocity structures. This gives us confidence in the large-scale features of the resistivity model. Together with information from 2-D seismic profiles and on-shore geology the resistivity model provides valuable insight into the spatial distribution of increased magmatic activity.

The static distortion of magnetotelluric (MT) impedances is a common problem that can prevent detailed imaging of the subsurface. The effect of distortion on the undistorted impedance Z can be described as a multiplication with an unknown, real-valued matrix C. Inverting the observed impedance Zobs = C · Z without any consideration of distortion can result in strong artefacts, particularly in the near-surface. As a consequence, a variety of approaches have been developed to remove as much of the distortion effects as possible or compensate for them in the inversion. However, these either reduce the number of data and thus potentially reduce resolution, or make assumptions about the properties of the matrices C and Z which might not be generally valid.

The Namibian continental margin marks the starting point of the Tristan da Cunha hotspot trail, the Walvis Ridge. This section of the volcanic southwestern African margin is therefore ideal to study the interaction of hotspot volcanism and rifting, which occurred in the late Jurassic/early Cretaceous. Offshore magnetotelluric data image electromagnetically the landfall of Walvis Ridge. Two large-scale high resistivity anomalies in the 3-D resistivity model indicate old magmatic intrusions related to hot-spot volcanism and rifting. The large-scale resistivity anomalies correlate with seismically identified lower crustal high velocity anomalies attributed to magmatic underplating along 2-D offshore seismic profiles. One of the high resistivity anomalies (above 500Ωm) has three arms of approximately 100km width and 300km to 400km length at 120° angles in the lower crust. One of the arms stretches underneath Walvis Ridge. The shape is suggestive of crustal extension due to local uplift. It might indicate the location where the hot-spot impinged on the crust prior to rifting. A second, smaller anomaly of 50km width underneath the continent ocean boundary may be attributed to magma ascent during rifting. We attribute a low resistivity anomaly east of the continent ocean boundary and south of Walvis Ridge to the presence of a rift basin that formed prior to the rifting.

Geomar, Helmholtz Centre for Ocean Research, Wischhofstr. 1-3, 24148 Kiel, Germany 6 now at University of Leicester, University Road, Leicester LE1 7RH, United Kingdom 7 now at GEUS, Øster Voldgade 1, 1350 Copenhagen, Denmark 8 Institute of Geophysics, Polish Academy of Science, ul. Księcia Janusza 64, 01452 Warsaw, Poland 9 University of Barcelona, C. Marti I Franques, 08028 Barcelona, Spain 10 now at Forschungsanstalt für Wasserschall und Geophysik, WTD71, Klausdorfer Weg 2, D-24148 11 Kiel, Germany 12 Earthquake Research Institute, The University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-0032, 13 Japan 14 GFZ German Research Centre for Geosciences, Telegrafenberg 14473, Potsdam, Germany 15

Detecting a salt dome overhang is known to be problematic by seismic methods alone. We used magnetotellurics (MT) as a complementary method to seismics to investigate the detectability of a salt dome overhang. A comparison of MT responses for 3D synthetic salt models with and without overhang shows that MT is very sensitive to shallow salt structures and suggests that it should be possible to detect an overhang. To further investigate the resolution capability of MT for a salt dome overhang, we performed a 3D MT inversion study and investigated the impact of model parametrization and regularization. We showed that using the logarithms of the conductivities as model parameters is crucial for inverting data from resistive salt structures because, in this case, commonly used Tikhonov-type stabilizers work more equally for smoothing the resistive and conductive structures. The use of a logarithmic parametrization also accelerated the convergence and produced better inversion results. When the Laplace operator was used as a regularization functional, we still observed that the inversion algorithm allows spatial resistivity gradients. These spatial gradients are reduced if a regularization based on first derivatives in contrast to the Laplace operator is introduced. To demonstrate the favorable performance when logarithmic parametrization and gradient-based regularization are employed, we first inverted a data set simulated for a simple model of two adjacent blocks. Subsequently, we applied the code to a more realistic salt dome overhang detectability study. The results from the detectability study are encouraging and suggest that 3D MT inversion can be applied to decide whether the overhang is present in the shallow salt structure even in the case when only profile data are available. However, to resolve the overhang, a dense MT site coverage above the flanks of the salt dome is required.

The limited-memory quasi-Newton method with simple bounds is used to develop a novel, fully 3D magnetotelluric MT inversion technique. This nonlinear inversion is based on iterative minimization of a classical Tikhonov regularized penalty function. However, instead of the usual model space of log resistivities, the approach iterates in a model space with simple bounds imposed on the conductivities of the 3D target. The method requires storage proportional to 2 ncp N, where N is the number of conductivities to be recovered and ncp is the number of correction pairs practically, only a few . These requirements are much less than those imposed by other Newton methods, which usually require storage proportional to N M or N N, where M is the number of data to be inverted. The derivatives of the penalty function are calculated using an adjoint method based on electromagnetic field reciprocity. The inversion involves all four entries of the MT impedance matrix; the x3D integral equation forwardmodeling code is used as an engine for this inversion. Convergence, performance, and accuracy of the inversion are demonstrated on synthetic numerical examples. After investigating erratic resistivities in the upper part of the model obtained for one of the examples, we conclude that the standard Tikhonov regularization is not enough to provide consistently smooth underground structures. An additional regularization helps to overcome the problem.

The limited-memory quasi-Newton method with simple bounds is used to develop a novel, fully 3D magnetotelluric (MT) inversion technique. This nonlinear inversion is based on iterative minimization of a classical Tikhonov regularized penalty function. However, instead of the usual model space of log resistivities, the approach iterates in a model space with simple bounds imposed on the conductivities of the 3D target. The method requires storage proportional to [Formula: see text], where [Formula: see text] is the number of conductivities to be recovered and [Formula: see text] is the number of correction pairs (practically, only a few). These requirements are much less than those imposed by other Newton methods, which usually require storage proportional to [Formula: see text] or [Formula: see text], where [Formula: see text] is the number of data to be inverted. The derivatives of the penalty function are calculated using an adjoint method based on electromagnetic field reciprocity. The inversion involves all four entries of the MT impedance matrix; the [Formula: see text] integral equation forward-modeling code is used as an engine for this inversion. Convergence, performance, and accuracy of the inversion are demonstrated on synthetic numerical examples. After investigating erratic resistivities in the upper part of the model obtained for one of the examples, we conclude that the standard Tikhonov regularization is not enough to provide consistently smooth underground structures. An additional regularization helps to overcome the problem.

Two‐dimensional inversions of lithospheric‐probing magnetotelluric (MT) data at a total of 20 sites acquired along an approximately east–west 300‐km‐long profile across the Wopmay orogen in the Northwest Territories, Canada, provide electrical resistivity models of the boundary between the Archean Slave craton and the adjacent Proterozoic Bear Province. An analysis of distortion effects and structural dimensionality indicates that the MT responses are primarily one‐dimensional or only weakly two‐dimensional with a depth‐independent geoelectric strike angle of N32°E, consistent with regional structural geology. The regional‐scale model, generated from the longer period responses from all of the sites along the profile, reveals significant lateral variations in the lithospheric mantle. Resistive cratonic roots are imaged to depths of ∼200 km beneath both the Slave craton and the Hottah terrane of the Bear Province. These are separated by a less resistive region beneath the Great Bear magmatic zone, which is speculatively interpreted as a consequence of a decrease in the grain size of olivine in the Wopmay mantle, caused by localized shearing, compared to its neighboring cratonic roots. Focused two‐dimensional models, from higher frequency responses at sites on specific sections of the profile, reveal the resistivity structure at crustal depths beneath the region. These suggest that the root of the Slave craton crosses beneath the Wopmay orogen, and that the Wopmay fault zone does not penetrate into the lower crust. A comparison of these results with those obtained during the Lithoprobe project farther south shows striking along strike variations in the conductivity structure associated with the Wopmay orogen.

PreviousNext No AccessSEG Technical Program Expanded Abstracts 2007Hydrocarbon reservoir detectability study for marine CSEM methods: Time domain versus frequency domainAuthors: Anna AvdeevaMichael CommerGregory A. NewmanAnna AvdeevaDublin Institute for Advanced Studies, IrelandSearch for more papers by this author, Michael CommerLawrence Berkeley Nat'l Lab, USASearch for more papers by this author, and Gregory A. NewmanLawrence Berkeley Nat'l Lab, USASearch for more papers by this authorhttps://doi.org/10.1190/1.2792497 SectionsAboutPDF/ePub ToolsAdd to favoritesDownload CitationsTrack CitationsPermissions ShareFacebookTwitterLinked InRedditEmail Abstract We present a comparative study about the detectability of a hydrocarbon reservoir in a marine environment, using controlled‐source electromagnetic (CSEM) methods both in the time and frequency domain. The target is a thin resistive body buried at a certain depth under the sea floor. Depth of the sea, and depth, thickness and resistivity of the reservoir are variable model parameters. For different sets of these parameters we calculated synthetic electromagnetic (EM) responses using a parallel version of the three‐dimensional (3D) time‐domain finite‐difference forward modeling code by Commer and Newman (2004) and the 3D frequency‐domain finite‐difference code by Newman and Alumbaugh (1995). To compare the responses quantitatively, signal‐to‐noise ratios (SNR) were calculated as a function of time/frequency and source‐receiver separation. SNR was calculated using the scattered field response of the reservoir as signal and the response of the background, in our case a two‐layered model, as noise.Permalink: https://doi.org/10.1190/1.2792497FiguresReferencesRelatedDetailsCited ByElectromagnetic Applications in Methane Hydrate Reservoirs1 January 2022Optimization and miniaturization of magnetic sensors for marine EMStefan L. Helwig, Kurt M. Strack, and Valery Korepanov14 October 20092.5-D modeling of cross-hole electromagnetic measurement by finite element method29 May 2008 | Petroleum Science, Vol. 5, No. 2Marine time domain CSEM: an emerging technologyKurt Strack and Norman Allegar15 December 2008 SEG Technical Program Expanded Abstracts 2007ISSN (print):1052-3812 ISSN (online):1949-4645Copyright: 2007 Pages: 3124 publication data© 2007 Copyright © 2007 Society of Exploration GeophysicistsPublisher:Society of Exploration Geophysicists HistoryPublished: 14 Sep 2007 CITATION INFORMATION Anna Avdeeva, Michael Commer, and Gregory A. Newman, (2007), "Hydrocarbon reservoir detectability study for marine CSEM methods: Time domain versus frequency domain," SEG Technical Program Expanded Abstracts : 628-632. https://doi.org/10.1190/1.2792497 Plain-Language Summary PDF DownloadLoading ...