We examine the resolution and sensitivity of water-borne boat-towed multi-frequencyradio magnetotelluric (RMT) data for delineating weak zones in crystalline environmentunder a lake or a river. 2D modeling of RMT data in joint transverse electric (TE) andtransverse magnetic (TM) and determinant mode of 40 profiles over two water passages fromLake Malaren near the city of Stockholm was used for this purpose. The 2D modeling revealsthat the estimated models have responses that fit the observed data reasonably well. Themodels show an overall agreement when compared with existing marine seismic results,bathymetry data and other available geological knowledge from the site. A low-resistivityzone observed in the middle of all profiles is suggested to be from a fracture/fault zonestriking in the direction of the water passages. Because RMT is an inductive-based method,the presence of conductive lake sediments shields scanning the underlying fractured bedrock.To reliably interpret the RMT data, synthetic modeling using a three-layered generic modelwith a conductive layer of varying thickness sandwiched between resistive water andfractured crystalline bedrock was performed. The modeling results suggest that lowerfrequency signal is required to have greater depth of penetration that would better resolve thebedrock and fracture zone although some evidence of fractured bedrock can still be inferredfrom the data. The study also shows the advantage of having a water layer that acts as a nearhomogeneous medium that eliminates near surface static shift effects. We propose that theboat-towed RMT acquisition system should be upgraded to enable controlled-source acquisition to improve the depth penetration and to overcome the shortcomings of usingradio-frequencies as signal.
I'm happy that Carotori Tontini agrees that his previous expressions for the wavenumber representation of the magnetic field from a Gaussian magnetization distribution (Tontini, 2005) are incorrect. Nonetheless, I find it difficult to accept his comments on most of the other issues raised in my discussion.
Thurston (2001) proposes a method and presents synthetic and practical results that we find difficult to understand both theoretically as well as practically.
We have studied a gabbro intrusion in northern Sweden, using 3D inversion of airborne magnetic data, ground-based gravity data, and petrophysical measurements on outcrop samples. Gabbro intrusions are of interest because they are potential hosts of Cu-Ni and platinum group element mineralization. We developed a joint inversion algorithm and applied it to both potential-field data sets to obtain spatial distributions of density and magnetic susceptibility. The distributions were coupled through a nonrigidly enforced parameter relationship determined from the petrophysical samples. We managed the problem of balancing the influence of the two data sets by a novel adaptive reweighting scheme that enforced the discrepancy principle for each data set independently. We demonstrated in tests with synthetic data that neither individual nor joint inversions gave reliable estimates for the depth extension of the intrusive body, the near-surface details, or any complex geometrical features. However, the joint inversion improved the image of the interface between the intrusion and the surrounding rocks and revealed that the density and susceptibility models satisfied the observed petrophysical relationship, which, in turn, caused the structures in the models to align. The geometry of the intrusion was an intrinsic result of the inversion, based on the two distinct petrophysical trends for the gabbro and the surrounding rocks. The inferred shape was simple and concise, and was therefore a useful and testable hypothesis about the subsurface geology that was in agreement with both potential-field data sets and the petrophysical information.
SUMMARY We consider a model for fracture-generated anisotropy in the crust represented by a layered earth with azimuthal anisotropy. A theory has been developed to compute electromagnetic fields induced by grounded electric dipoles on such a model. A vector potential is used to derive fields. Reflection coefficients are obtained by matrix recurrence formulae. Comparisons with one-dimensional isotropic models and models of homogeneous half-spaces with azimuthal anisotropy show that our algorithm gives correct results. Results are presented in terms of controlled-source tensor magnetotelluric (CSTMT) impedance tensor and tipper vector. The existence and characteristics of azimuthal anisotropy are revealed by them. We suggest the use of Parkinson’s vectors to delineate anisotropy in practice because magnetic fields are less susceptible to static distortion than electric fields.
The inherent nonuniqueness in modeling magnetic data can be partly reduced by adding prior information, either as mathematical constructs or simply as bounds on magnetization obtained from laboratory measurements. If a good prior model can be used as a reference model, then the quality of estimated models through an inverse approach can be greatly improved. But even though data on magnetic properties of rocks might exist, their distribution may often be quite irregular on local and regional scales, so that it is difficult to define representative classes of rock types suitable for constraining geophysical models of magnetization. We have developed a new way of constructing a reference model that varies only laterally and is confined to the part of the terrain that lies above the lowest topography in the area. To obtain this model, several estimated 2D magnetization distributions were constructed by data inversion as a function of the iteration number. Then, a suitable 2D model of the magnetization in the topography was chosen as a starting point for constructing a 3D reference model by modifying it with a vertical decay such that its average source depth was the same for all horizontal positions. The average source depth of the reference model was chosen to satisfy the average source depth obtained from analyzing the radial power spectrum of the area studied. Finally, the measured magnetic data were inverted in three dimensions using the given reference model. For a selected reference model, shallow structures indicated a better overall correlation with large remanent magnetizations measured on rock samples from the area. Throughout the entire model, the direction of magnetization was allowed to vary freely. We found that the Euclidean norm of the estimated model was reduced compared with the case where the magnetization direction was fixed.
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