GP41A-0765
Long Term Behavior of the Impedance Tensor at Nagycenk Geophysical Observatory and its Statistical Relation to Magnetospheric Processes and Solar Wind Parameters
In the Szechenyi Istvan Geophysical Observatory at Nagycenk, a parallel monitoring and registration of geomagnetic and telluric variations has been carried on for more then fifty years. At the first approach the spectral energy distribution of the minute mean value horizontal telluric and magnetic components has been compared. The impedance tensor's variation spectra has been calculated by using four days time intervals covering four years. Based on the plane wave assumption some stable behavior of the transfer function is expected. On the contrary certain periods has been found in the time variation of certain spectral components. This modulation was recognized both in the amplitude and phase of all tensor element and also in case of the considered tensor invariants. Dominant spectral peaks has been shown at periods of 27 days, 6 month and 1 year related to the Carrington-rotation and the Earth orbiting respectively. The relation of impedance function variation in time domain to solar wind and magnetospheric parameters has also been analyzed.So as to extend our examination on longer time interval (on solar cycle scale), the digitization of the whole analogue telluric and magnetic records has been started. The investigation of the deviation of estimated apparent resistivity curves resulting from the above variation of the impedance tensor is also proposed.
GP41A-0766
Geomagnetic, Magnetotelluric, Geothermal and Seismic Signatures of Magnetic Phase Transition in the Crust
In a GRL paper in 2005 the authors assumed that the second-order magnetic phase transition in the Earth's crust may be a possible source of geomagnetic and magnetotelluric anomalies. In that paper two consequences were mentioned: (1) some geomagnetic anomalies of unknown origin could be perhaps explained by a significant enhancement of the magnetic susceptibility at the Curie (Neel) depth, (2) this phenomenon might be also responsible for some magnetotelluric crustal conductivity anomalies. Recently we have found that the magnetotelluric effect of a hypothetical one-dimensional magnetic phase transition is a very thick and extremely high-resistivity pseudolayer, and the multidimensional responses are very mixed. In some places in the Pannonian basin the second-order magnetic phase transition can be probably really excluded, but in some other places it seems to be the only reasonable explanation for the observed geophysical anomalies. Possible geothermal and seismic signatures of this phenomenon will also be discussed. Acknowledgement: Hungarian Scientific Research Fund, T68475
GP41A-0767
Progress in modeling whole-Earth electromagnetic induction for ground-based observations: Peering through the crust
Although results from nearly all branches of global geophysics over the last few decades have painted a consistent view of Earth's mantle as an inherently three-dimensional body of remarkable physiochemical and thermal complexity, a critical step in the defensible inference of mantle properties is the proper accounting for near-surface or crustal structure. Results will be presented on the algorithmic maturation of a previously- developed, fully three-dimensional electromagnetic induction modeling code. Using a higher-order finite differences method, the code solves directly for the induction field over a mesh of nested spherical shells, each of which is isoparametrically discretized into a set spherical triangles from successive refinement of an initial octohedron. This discretization has the advantage of providing roughly uniform coverage of the refined spherical triangles, thus avoiding undesirable node clustering at the poles, and ensures a globally- compatible circulation of triangle edges. Results on the predicted global distribution surface-based response function estimates at long periods are shown to be influenced by the lateral variations in the crust and its coupling with the resistive mantle below. Preliminary results will also be presented on the electromagnetic response of the thermal structure due to a hypothetical mantle convection model.
GP41A-0768
Development of geospace proxies for Sq and Dst solid Earth induction studies
The present generation of geomagnetic field models based on ground-based and satellite magnetometer datasets has achieved unprecedented spatial resolution. Perhaps the greatest uncertainty in the models lies in a description of time-varying external sources and the associated spatiotemporal distribution of electric currents induced inside the heterogeneous solid Earth and conductive oceans. The limiting factor to progress in geomagnetic induction has long been the outstanding source/structure trade-off. Spatiotemporal geomagnetic variations recorded by satellite magnetometers are caused by fluctuations in the external source and the consequent induction of currents that are characteristic of deep Earth structure. Furthermore, the distribution of induced currents depends on the external source fluctuations. There is no unique decomposition of induction signals into source-originating and structure-originating parts. The spatiotemporal variations are explained by trading off the external source signature against the deep Earth conductivity structure and making certain simplifying assumptions about the external field spatiotemporal fluctuations. In this contribution we examine the possibility of using proxy geospace variables such as solar irradiance or the interplanetary magnetic field as indicators of the external magnetospheric and ionospheric source currents in an effort to reduce the source/structure ambiguity that exists in solid Earth induction studies.
GP41A-0769
Inverse 3-D time-domain modelling of global EM induction in the Earth: A sensitivity study
We present a new method of inverse 3-D global electromagnetic induction modelling which is based on the time-domain approach. The core of the inverse algorithm is a computationally effective evaluation of gradient of the misfit function in the space of model parameters using the solution of the adjoint problem. We derive rigorous formulations of adjoint problem in the time domain and apply the quasi-Newton method to solve the non-linear inverse problem with respect to 3-D conductivity structure. Using the synthetic data from the preparatory "End to End" simulations for the upcoming SWARM mission, we study the sensitivity of the presented method to conductivity variations at different depths with respect to the spatio-temporal resolution of satellite data. Effects of model regularization, noise in data and inaccuracies in the 3-D magnetospheric field model on the inversion results are also addressed.
GP41A-0770
3-D Frequency-Domain Inversion of Global Induction Data
We present an efficient 3-D inversion scheme to recover 3-D mantle conductivity from global induction data. It exploits limited-memory quasi-Newton method, along with optimized forward problem solver, based on volume integral equation approach. The substantial dramatic saving in computational load gives the usage of "adjoint" approach to calculate efficiently gradients of misfit. In support of the efficient gradient calculation, explicit forms for elements of Green's tensors for two new types of the source have been derived. We validate our inversion scheme by inverting the synthetic C-responses calculated in realistic 3-D conductivity model of the Earth. We discuss alternative response functions to deal with.
GP41A-0771
Inversion of Large-Scale 3D MT data sets such as EMSCOPE with ModEM, a Modular EM Inversion System
We discuss the opportunities for efficient inversion of large magnetotelluric (MT) data sets in the scope of our flexible modular system for gradient based inversion of electromagnetic data (ModEM). This general electromagnetic (EM) inversion system has been developed in the past couple of years using an "object oriented" approach. It has been specifically designed to allow easy implementation and testing of various inversion algorithms, and to facilitate tailoring of parametrization and regularization schemes to specific applications. Ultimately, we expect the modular approach to more readily allow inversion of novel data types and joint data sets. Here, we discuss and provide examples of synthetic inversions of traditional MT data types. Memory-efficient NLCG and hybrid schemes provide immediate alternatives to the Gauss-Newton approach. This makes the MT component of ModEM efficient enough that it can be realistically applied to inversions of large-scale data sets such as those collected in the framework of EM component of EarthScope (EMSCOPE). We also describe the model covariance, efficiently implemented using a recursize autoregression scheme. The scheme allows smoothing in a depth-dependent manner, which is necessary to adequately compensate for the effects of the small-scale near-surface structures that a sparse data set is not able to resolve, without compromising the resolution at depths. Prior knowledge of other kinds can also be applied, such as turning off smoothing across an interface and freezing features for hypothesis testing. We conclude that further development of flexible inverse tools such as ModEM is crucial for making best use of the most current and extensive MT data sets available today.
GP41A-0772
Three-dimensional magnetotelluric modeling using finite element method: A study on tetrahedron element and hexahedron element
To solve the Magnetotelluric (MT) forward problem we apply the finite element (FE) method directly to the second- order Maxwell's equations formulated in terms of the electric fields, i.e. without introducing scalar potentials. To avoid spurious solutions, edge elements are used instead of the more usual nodal elements. Both tetrahedral and hexahedral elements are considered. In the first case, there are 6 basis functions (and unknowns) associated with each element; for hexahedral elements the number of unknowns per element is 12. In either case the resulting coefficient matrix is large, sparse, symmetric, and complex but not Hermitian. However, there are differences in the pattern of complex entries and the pattern of nonzero entries. The maximum number of nonzero entries per equation resulting from the hexahedra is 33, while the maximum number of nonzero entries per equation for tetrahedra varies, depending on mesh topology. Both systems of equations can then be solved by various kinds of preconditioned iterative solver. Efforts to develop an effective and efficient preconditioner will be discussed, and convergence rates, accuracy and storage for the two types of elements will be compared, using the standard COMMEMI 3D model.
GP41A-0773
Can the resolution of marine MT be enhanced with inter-station transfer functions?
Recent broadband marine magnetotelluric (MT) surveys performed by industry and academia have utilized large arrays of receivers that simultaneously record the seafloor electric and magnetic fields, resulting in data sets of 10's to 100's of MT stations. The primary MT data product used for forward and inverse modeling is the MT impedance tensor, which relates the horizontal electric and magnetic fields recorded at each station to the seabed conductivity. However, the advent of large instrumentation arrays allows for inter-station transfer tensors to be computed using multiple-station MT processing codes. Transfer tensors can be computed between electric or magnetic field recordings from any pair of stations in an array. Similarly, hybrid impedance tensors can also be computed between electric and magnetic field recordings from any pair of stations. While the inclusion of inter-station transfer tensors can greatly increase the size of an MT data set, the impact these data may have on the resolution of the seabed conductivity structure is not known. This study examines the behavior and sensitivity of inter-station transfer tensors for some representative offshore conductivity structures. Synthetic inversions of inter-station transfer functions and MT impedances are used to demonstrate how these additional data can offer improvements in the resolution of the inverted model conductivity structure.
GP41A-0774
Magnetotelluric images of the southern edge of the Gorda Plate, Northern California
Two parallel east-west magnetotelluric (MT) transects spanning a distance of 250km and spaced approximately 125km apart provide a comparison between the conductivity structure of the subducting Gorda plate at latitude 40°40'N and the slab window to the south at latitude 39°40'N. The northernmost profile of 14 long period and broadband instruments spans the northern Sacramento Valley, Lassen Volcanic National Park, the Sierra Nevada and the northwesternmost portion of the Basin and Range. . The southern profile spans the central Sacramento Valley just north of Sutter Buttes, crosses the Sierra Nevada, and terminates in the western Basin and Range northeast of Reno, NV. This profile, consisting of only 10 MT stations, is incomplete due to the fires in Plumas National Forest this past summer and will be completed in 2009. Instruments from the EMSOC consortium recorded high quality data at periods ranging from 0.01s to 20,000s at all sites. Despite the incomplete nature of the southern profile, comparisons between the two sections can be made.
GP41A-0775
Magnetotelluric Transect Through Yosemite Reveals Evidence of Sierran Delamination
Recent studies have shown evidence of lithospheric delamination beneath the Sierra Nevada. In an east- west magnetotelluric profile, 12 long-period and 14 short-period stations were installed along a 240 km transect spanning from the San Joaquin Valley to the Nevada border (through Yosemite) at latitude 37° 50" N. Broadband and long period instruments from the EMSOC consortium were used to record data with periods ranging from 0.01 to 20000s, allowing the conductivity structure beneath the Sierra to be imaged to a depth of 200km. Resulting 2-D models reveal the batholith's resistive root extends to a depth of just 40km with a highly conductive feature from a depth of 65 to 100km directly below and dipping eastward. Further, there is a resistive body at depths greater than 120 km and 140 km thick resistive section to the west under the San Joaquin Valley. The location of the conductor is roughly coincident with a region of low wavespeed from tomographic studies. Together, we interpret the conductor as rising asthenosphere from the east that is displacing the underlying Sierran lithosphere.
GP41A-0776
Electrical Conductivity Beneath the Payún Matrú Volcanic Field in the Andean Back-arc of Argentina near 36.5S: Insights into the Magma Source and Results of 3D Modeling
Southern Mendoza and northern Neuquen Provinces, south of the Nazca flat slab in western Argentina, have widespread, geologically young basaltic volcanism, but no historic activity. The youngest basalts, erupted in the vicinity of the large Payún Matrú volcanic center have essentially no arc signature. Kay, et al. (2006) and Folguera, et al. (2006) argue that this back-arc igneous province is the result of extension due to trench roll-back following steepening of a flat slab that existed in the middle to late Miocene. Magnetotelluric data collected in 2005 at 18 sites along an east-west profile from 70W to 67W have been used to probe the source of the Payún Matrú basalts. These data imply significantly 3D structure. However, preliminary analysis of an arguably 2D region at the center of the profile allows tentative identification of a conductive mantle plume surfacing at Payún Matrú that rises from below 200 km depth. This suggests that Payún Matrú Volcanic Field is sampling mantle deeper than 200 km, perhaps just above the subducted Nazca slab or perhaps from the mantle transition near where the Nazca slab penetrates into it at 400 km. The 2008 fieldwork extends the earlier profile to a spatial array extending from Laguna Llancanelo north of Payún Matrú to beyond the Cortaderas Lineament that bounds the basaltic province to the south. These 19 new sites consist of 5 to 10 days of 4 Hz horizontal electric and 3-component magnetic field time- series. Data processing in the field suggests that the deep crust or upper mantle has northwest-southeast striking structure increases in conductivity to the southwest of Payún Matrú. This underlies shallow structure with north-south strike between Payún Matrú and the Colorado River. This complexity explains our initial difficulty with 2D interpretation. We will display a preliminary 3D interpretation of these new data.
GP41A-0777
Estimation of regional resistivity structure beneath the Kyushu, southwestern Japan, as inferred from the Network-MT survey
Network-MT observations, which use telephone line networks as long baseline telluric measurements (Uyeshima, 1990), were carried out in the Kyushu district, southwestern Japan, from 1993 to 1998. The Kyushu district is the typical high angle subduction zone in Japan, which the Philippine sea plate subducts beneath the Eurasian plate, and some active volcanoes (for example, the Aso volcano, the Kirishima volcano group and Sakurajima volcano) are located along the volcanic front. We reanalyzed these data sets to determine regional scale deep electrical conductivity structure. In this reanalysis, we tried to choose triangular elements of the Network-MT again in order to obtain independency of each triangular element, and calculated magnetotelluric responses for each triangular element showing more suitable spatial distribution in survey area. Furthermore, comparing the geology and tectonics, we estimated electrical tendency to by the phase tensors analysis (Caldwell et al., 2004). As a preparatory step for imaging three-dimensional modeling, we carried out several tow dimensional inversion analyses to the Network-MT impedance responses across the characteristic geology, tectonics and volcanoes. In these two-dimensional inversions, we used the REBOCC code (Siripunvaraporn and Egbert, 1999), and reconsidered the horizontal and vertical smoothing factors while considering the intervals of the observation sites along each model profile. Then we obtained the final resistivity model of each profile which was judged expressing well the information of the MT responses. As a preliminary result, one of the resistivity models, whose profile goes along around the Kirishima volcano group, we obtained a remarkable conductor beneath the Kirishima volcano which shows a good agreement with the previous result of ULF MT survey (Ichiki et al., 2000). Further, we found that the bottom of this conductor extends to the subducting Philippine Sea Plate. However, at the present stage, we could not treat measured voltage differences for several kilometers electrode spacings in the inversions. For deducing detail electrical structure, we should make an improvement the inversion code for Network-MT. In this presentation, we would like to explain details of our reanalysis and obtained two-dimensional models, and introduce the future direction of this study.
GP41A-0778
Magnetotelluric Data from the Tien Shan and Pamir Continental Collision Zones, Central Asia
We present magnetotelluic (MT) data obtained within the framework of the multi-disciplinary Tien Shan - Pamir Geodynamic program (TIPAGE). The dynamics of the Tien Shan and Pamir orogenic belts are dominated by the collision of the Indian and Eurasian continental plates. With the geophysical components, we intend to image the deepest active intra-continental subduction zones on Earth (the N-dipping Hindu Kush and the S-dipping Pamir zones) and to establish how the highest strain over the shortest distance that is manifested in the India-Asia collision zone is accommodated structurally. The MT data were recorded in summer 2008 at 80 stations (12 combined broad-band (BB) / long period (LMT) sites and 68 BB sites) in the Pamir mountain ranges in Tajikistan with a spacing of approximately 2 km between BB sites and 14 km for the BB/LMT sites. These stations form the 156 km long, southern part of an approximately 340 km long profile from Osh in Kyrgyzstan via Sarytash, the Kyrgyz-Tajik border, Karakul and Murgab to Zorkul in southern Tajikistan. We present examples of the MT data, which is of exceptionally high quality in this very remote area, and show preliminary 2D inversion results.
GP41A-0779
Electrical Conductivity Images of South African Continental Collision Zones
Within the framework of the German-South African geo-scientific research initiative Inkaba yeAfrica several magnetotelluric (MT) field experiments were conducted along the Agulhas-Karoo transect in South Africa. This 600km long transect is designed to cross the Cape Fold Belt, the Namaqua-Natal Mobile Belt (NNMB), the Karoo Basin and the transition into the Kaapvaal Craton. At the same time, the transect crosses the Beattie Magnetic Anomaly and the Southern Cape Conductive Belt, two of Earth's largest continental geophysical anomalies. In this presentation we will focus on the ~ 1.2-1.0 Ga old convergent margin represented by the Kaapvaal craton-bounding Proterozoic Namaqua-Natal orogenic belt. Along the northernmost segment of the transect, we deployed over 120 MT sites along the 250km profile with a site spacing of 2-4km. With our experiment we focussed on a high-resolution image of the Mobile Belt–Craton transition on lithospheric scale. The MT data show a predominantly 2D behaviour; however, some sites show strong 3D effects, which are consistently observed over a 15km long segment. Anisotropic and 3D modelling studies show that these effects are caused by the unique setting of the extremely resistive rocks of the Kaapvaal Craton in combination with very conductive shallow structures. 2D- and 3D inversion results exhibit a shallow conductivity anomaly: a sub-horizontal, high conductivity band in the upper 5km of the NNMB. In view of other MT data collected in the area, we can map this anomaly over an area of 400km2. Based on deep borehole information, we are able to correlate the conductive band with the black shale and pyrite rich Whitehill formation within the Karoo basin, which seems to wedge out in vicinity of the Craton transition. In mid to lower crustal levels, the MT models show the transition from the untypically conductive rocks of the Proterozoic mobile belt and the extremely resistive rocks of the Kaapvaal Craton.
GP41A-0780
Electrical Conductivity Structure Of Central Victoria, Australia From Magnetotelluric Measurements
We perform MT soundings in north of Bendigo, Victoria Australia to investigate the deep subsurface geologic structure. The primary purpose of this survey is to figure out whether the discontinuity such as major faults extends northward and obtain conductivity image of Central Victoria, Australia, which help understanding gelogic structure. The time series of MT signal were measured over 11 days at 71 measurement sites together with at remote reference, which help enhance the quality of impedance estimation and its interpretation. The impedances are estimated by robust processing using remote reference technique and then inverted with 2D MT inversion. We can see that known faults are images clearly in the 2D MT inversion . Comparing resistivity images from 2D MT inversion with interpreted boundary from reflection seismic exploration along the same line, two interpretations match well each other.
GP41A-0781
MAGNETOTELLURIC INVESTIGATION IN THE JIYANG DEPRESSION AND ITS ADJACENT AREAS
The Jiyang Depression is a secondary tectonic unit of the Bohai Bay Basin, and is a Mesozoic-Cenozoic continental-facie depositional depression. The Jiyang Depression consists of the Dongying, Zanhua, Chezhen, and Huimin sags, and the Yihezhuang, Chenjiazhuang, Qingcheng, Binxian, Guangrao, Shouguang, and heaves. In order to delineate shallow and deep structures of the depression, we conduct a magnetotelluric (MT) investigation in the Jiyang Depression and its adjacent areas. We use a remote reference setup to reduce noise disturbance, when several magnetotelluric sounding stations are measuring synchronously at different sites. The acquired MT data are to be processed and interpreted. We execute time series processing with the remote reference technique. The surveyed area is seriously interfered by noise. To improve quality of the data, we perform noise identification and manual selection of the time series segments. Identification of polarization modes is an important task for MT data qualitative interpretation. We apply for a PCA (principal component analysis) based method to automatically identify apparent polarized resistivity curves of TE and TM modes. The method recognizes the distortion and major variation of TE and TM curves and distinguishes the two kinds of curves with principal component analysis. From the apparent resistivity-frequency profile result, we can see that the Chenning Uplift and the Chenjiazhuang Heave reveal a 3-layered structure of medium-low-high, from the shallow to the deep. The structure is especially distinct for the Chenning Uplift, where there is a laterally steady-spreading high- resistivity zone corresponding to the frequency band lower than 1 Hz. The Dongying Sag is characteristic of high-low-medium from the shallow to the deep, and has a low-resistivity anomaly zone with obvious and wide lateral extending in the medium frequency band, which reflects that the deposition thickness of the basin is relatively big. The Guangrao Heave and the Shouguang Heave are of a medium-low-high structure from the shallow to the deep. From the impedance phase-frequency profile image, it can be seen that electrical anomalies appear as a large variation in the cross direction, which indicates the complicated features of the research area. We use the 2-D Mackie inversion method to obtain underground electrical distribution of the surveyed area. We select the TE, TM, and joint TE-TM curves to realize inversion processing. The inversion result of the TM mode is used for further interpretation because the fitting conditions of the TM mode is better and the corresponding inversion result coincides well with geologic data. From the inversion result, we can see that the Chenning Uplift and the Changwei Depression to the south of the Guangrao Heave are of high-resistivity. The low-resisitivity range of the Dongying Sag shows that the bottom surface of the basin is about 8-10 km deep, which almost coincides with the published data. Generally speaking, the basement structures of four segments, the Chengning Uplift, the Chenzhen-Chenjiazhuang Heave, the Dongying Sag, and the Changwei Depression, differs greatly, and shows an overall feature that the uplift areas have root while the burial hills in the depression areas are with no root. It may indicate a prospective for pre-Cenozoic oil and gas resources.
GP41A-0782
Three-dimensional inversion of magnetotelluric data including sea effects obtained in Pohang, Korea
Three-dimensional (3D) inversion of magnetotelluric (MT) data is conducted to investigate geothermal energy resources in Pohang, Korea, which is located close to the East Sea. Since MT data obtained near sea are distorted by sea effects especially at low frequencies, the interpretation of MT data obtained in Pohang needs to consider sea effects. In order to confirm sea effects on Pohang MT data, we first construct a five-layered model with a realistic coastline and seafloor topography, and the model clearly shows that Pohang MT data are affected by sea effects at frequencies blow 1 Hz. Next, a 3D inversion algorithm based on the Gauss-Newton approach is utilized to produce a reliable resistivity model. Resistivity of sea water is fixed during inversion iterations by excluding sea blocks from an inversion domain. For an accurate interpretation of Pohang MT data, static shifts are also considered in inversion. Reasonable reconstructed images are obtained through the 3D inversion including sea effects and static shifts. The model is comparable with logging data, which are obtained from four boreholes in the study area, and can explain major geological features in Pohang.
GP41A-0783
Sea-effect correction of 3D magnetotelluric responses in Pohang, Korea
In order to investigate geothermal energy resources, Korea Institute of Geoscience and Mineral Resources (KIGAM) have conducted magnetotellurics (MT) surveys for years in Pohang, Korea. Since the survey site is close to sea, low-frequency MT data measured in Pohang are not free from sea effects. Consequently, inversion of MT data may result in misinterpretation of the subsurface if sea effects are not considered properly. The sea effect can be treated in the interpretation of MT data in two different ways: incorporating sea into an earth model in inversion and compensating sea effects before conducting inversion. The first approach has numerical difficulties due to the presence of electrically conductive sea, while the second method allows conventional inversion algorithms for a model without sea since sea effects are removed from MT data. This paper presents the method to compensate sea effects on MT data by correcting both electric and magnetic fields distorted by sea effects. Using a synthetic model with one or two-layered hosts, we validate the correction method. Nonlinear least-squares inversion of corrected MT data successfully recovers major features of the synthetic model. Corrected MT data observed in Pohang is also inverted and compared with former results from 3D inversion including sea effects. Approach of sea-effect removal from MT data helps to improve the geological comprehension of deep-seated structure in Pohang.
GP41A-0784
Removal of the sea effect in the magnetotelluric (MT) data using an iterative tensor stripping during inversion
When magnetotelluric (MT) data is obtained in vicinity of the coast, the sea can distort observed MT responses of subsurface, especially those of deep part. We introduce an iterative method to correct the sea effect, based on the previous topographic correction method which removes the distortions due to topographic changes in seafloor MT data. The method first corrects the sea effect in observed MT impedance, and then inverts corrected responses in a model space without the sea. Due to mutual coupling between sea and subsurface structure, the correction and inversion steps are iterated until changes in each result become negligible. The method is tested for 1-D and 2-D structure using synthetic MT data produced by 3-D forward modeling including surrounding seas. In all case, the method closely recovers the true structure assumed in generating synthetic responses after a few iterations. To test the applicability of the proposed method to field data, we apply the method to real MT data acquired from the Jeju Island, Korea. The distortions due to the surrounding sea start to appear below about 1Hz, and this result strongly suggests that the sea effect should be corrected in order to obtain more accurate and reliable interpretation on the subsurface.
GP41A-0785
2D Parameterized Inversions of Marine CSEM Data
As an alternative to the computational and algorithmic complexity of regularized inversion, we investigate the use of lightly parameterized 2D inversions of marine controlled source electromagnetic (CSEM) data. Since a priori information from seismics is often available, this overdetermined scheme offers a way to make use of additional information in an inversion that will run faster than a regularized, underdetermined one. The challenge for these simple inversions comes in finding parameterizations of conductivity models that are soluble, stable, and representative of realistic geologic structures. However, an advantage of such algorithms is that they can solve for sharp boundaries. We explore the use of a Levenberg-Marquardt type optimization to find a sharp boundary model with the smallest L2 norm misfit to the data. The advent of 2D forward modeling algorithms that use adaptively refined finite element meshes [Li and Key, 2007] enables automatic re-meshing of the inverted model structure between inversion iterations, ensuring numerical accuracy and providing great flexibility in the evolution of model structure. We test strategies for parameterization of several different scenarios using synthetic and real data, and study the stability of these inversion methods.
GP41A-0786
Field Tests of the Magnetotelluric Method to Detect Gas Hydrates, Mallik, Mackenzie Delta, Canada
The magnetotelluric method is not generally utilized at extreme latitudes due primarily to difficulties in making the good electrical contact with the ground required to measure the electric field. As such, the magnetotelluric technique has not been previously investigated to direct detect gas hydrates in on-shore permafrost environments. We present the results of preliminary field tests at Mallik, Northwest Territories, Canada, that demonstrate good quality magnetotelluric data can be obtained in this environment using specialized electrodes and buffer amplifiers similar to those utilized by Wannamaker et al (2004). This result suggests that subsurface images from larger magnetotelluric surveys will be useful to complement other techniques to detect, quantify and characterize gas hydrates.
GP41A-0787
Early Results of a Magnetotelluric Survey in the Nechako Sedimentary Basin, B.C., Designed to Assess the Method as a Tool in Oil and Gas Exploration
Magnetotelluric (MT) data was collected in the fall of 2007 within the Nechako Basin, B.C., Canada, to evaluate the usefulness of the technique for hydrocarbon exploration and to characterize the structure of the basin. The Mesozoic Nechako Basin is located within the Intermontane Belt of the Canadian Cordillera and includes overlapping sedimentary sequences deposited in response to terrane amalgamation to the western edge of ancestral North America. Beginning in the Late Cretaceous, regional transcurrent faulting and associated east-west extension were accompanied by the extrusion of basaltic lava that forms a laterally variable sheet covering much of the basin. The potential for hydrocarbons has been noted within several interior basins of British Columbia however, an important impediment to hydrocarbon exploration is the inability of traditional geophysical methods to see through the volcanic sequences. As the MT method is not hampered by these volcanics, a survey consisting of 734 combined high frequency and broadband sites was undertaken. Strike analysis shows lateral changes in the preferred geoelectric strike direction at periods between 0.1 and 10s, suggesting that localized structure is influencing the strike angle. Two-dimensional models along in the vicinity of Nazco, B.C. reveal a shallow conductive layer that likely represents the sedimentary packages of the Nechako Basin overlying a resistive layer that is interpreted as the crystalline basement rocks. Focused inversions illustrate a higher resolution of the shallow features within the Nechako sediments and reveal complex geologic structures. These models indicate that the method is capable of both penetrating and imaging the surface volcanics where they are thick. Distinct variations in the shallow conductive layer as well as the presence of resistive structures that break up the lateral continuity of the conductor are observed. These structures are an indication of fault systems that carry the deeper resistive material from depths towards the surface. Variations in the along profile conductivity within the sedimentary packages may result from changes in mineralogy of the sediments, changes in percent porosity or possibly chemical changes in the fluid within the pore spaces. Understanding these conductivity variations may contribute to a better understanding of the potential for hydrocarbon resources in the region.
GP41A-0788
Why Are CSEM and Borehole Derived Resistivities in Basin Sediments Often Incompatible?
In a number of reported cases in offshore hydrocarbon provinces, electrical resistivities of sea floor sediments determined by Controlled Source Electro Magnetic (CSEM) surveys are found to be significantly greater than those measured by electrical well logging of boreholes at the same locations. In some instances the ratio between CSEM and bore hole log resistivities are as much as 5:1. A possible cause of this discrepancy is strong electrical anisotropy of the sea bed. Borehole logging techniques generally measure electrical resistivity using currents circulating in horizontal planes. CSEM surveys collected using 'end-on' source-receiver geometries, in contrast, are far more sensitive to the resistivity encountered by currents circulating in vertical planes. Grain alignment, aligned fractures and thin layering all can cause electrical resistivities to be different in the vertical compared to the horizontal direction. To investigate this we have examined electrical well log data and sediment samples from the North Sea. We have compared vertical (in-series) resistivities to horizontal (in-parallel) resistivities for a number of cases. Also, many reservoirs in the North Sea have been drilled using both vertical and deviated wells. By correlating the same sedimentary unit between different well logs from deviated and un-deviated bore holes we are able to compare resistivities measured at different inclination angles. Both sets of results are compared against laboratory measurements. Results for shale units indicate that electrical anisotropy can account for much of the discrepancy observed between CSEM and borehole resistivities. However sand layers do not show the same level of electrical anisotropy.
GP41A-0789
Joint Inversion of Magnetotelluric and Surface Wave Data in Anisotropic Media
Joint inversion of different kind of geophysical datasets can improve the model resolution and has been
commonly undertaken with datasets sensitive to the same physical parameter. Our work involves inverting
simultaneously for different physical parameters and is built on a joint inversion method originally applied to
teleseismic receiver functions and long-period magnetotelluric data. This is a challenging problem since the
two datasets are sensitive to different physical parameters (respectively, shear wave velocity and electrical
resistivity). A joint inversion using this approach has been applied successfully to recover one-dimensionnal
isotropic structure with both synthetic and real datasets from the Slave Craton (Moorkamp et al., 2007).
An
approximate agreement between geoelectric strike and seismic fast axis direction inferred from shear wave
splitting studies has been found in various regions, such as the Great Slave Lake shear zone (Eaton et al.,
2004), across the Grenville Orogen (Ji et al., 1996) and the Sao Francisco craton (Padilha et al., 2006). This
suggests a common origin is plausible in some situations for both seismic and electrical anisotropy.
These
observations motivate our attempt to jointly invert seismic and electrical anisotropic parameters.
We use
magnetotelluric and surface wave datasets, which both provide a good depth resolution.
Assuming that
seismic and electrical anisotropy have a common origin, we can thus expect superior resolution of azimuthal
anisotropy for lithospheric and sub-lithospheric depths combining these two techniques.
We examine the
capabilities and limitations of this new approach with synthetics datasets and we will apply this new joint
inversion of anisotropic parameters to real datasets. The Slave Craton in Northern Canada is of particular
interest but other regions will be investigated.