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Volume 49, Issue 6
Displaying 1-13 of 13 articles from this issue
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Alan G. Jones, Adam Schultz1997 Volume 49 Issue 6 Pages 727-737
Published: June 20, 1997
Released on J-STAGE: May 25, 2007
JOURNAL FREE ACCESSDownload PDF (34814K) -
Jaume Pous, Juanjo Ledo, Alex Marcuello, Pilar Queralt1997 Volume 49 Issue 6 Pages 739-744
Published: June 20, 1997
Released on J-STAGE: May 25, 2007
JOURNAL FREE ACCESSA two-dimensional resistivity model was obtained for the COPROD2R data set by trial-and-error forward model fitting of the apparent resistivities and phases. Two major conductivity zones were found to explain the data in the zone where the NACP anomaly is located. The first one coincides with previous interpretations of the NACP anomaly, which results in four separate conductive bodies. The second is another deeper conductor, whose top is located at 26 km depth with its base deeper than 40 km. A test removing this deeper conductor was made in order to ensure its detectability.View full abstractDownload PDF (15650K) -
A. K. Agarwal, K. J. Macdonald, J. T. Weaver1997 Volume 49 Issue 6 Pages 745-756
Published: June 20, 1997
Released on J-STAGE: May 25, 2007
JOURNAL FREE ACCESSThe small local minimum in the long-period B-polarization phase responses above the NACP anomaly in the COPROD2 data is re-examined with a view to understanding its role in determining whether the anomaly is a multi-body or continuous conductive structure. It is found by trial-and-error numerical modelling and by further optimization of a model with continuous structure that the B-polarization phase response is not profoundly affected by the NACP anomaly itself, but is extremely sensitive to the resistivity distribution in the sedimentary layer to the extent that quite minor resistivity variations can turn a local minimum into a local maximum and vice versa, without necessarily affecting the short period responses in an unreasonable manner. A further modelling exercise with a multi-body anomaly shows that its response does not necessarily give rise to a local minimum in the phase response, as sometimes claimed. The investigation presented here is not intended to establish whether the NACP anomaly is a continuous or broken conductive structure but it does suggest that the evidence that has been cited in favour of a multi-body anomaly may not be sufficient by itself to settle the question with certainty.View full abstractDownload PDF (29204K) -
Patricia Ritter, Oliver Ritter1997 Volume 49 Issue 6 Pages 757-766
Published: June 20, 1997
Released on J-STAGE: May 25, 2007
JOURNAL FREE ACCESSGalvanic distortion of the regional MT responses was inferred from the analyses of the BC87 dataset presented at the MT-DIW1 in 1992. Since perturbation currents also produce an anomalous magnetic field, the GDS response functions were suspected to be distorted in the long period range. In order to reveal the correct regional strike direction, hypothetical event analysis was applied to the data from all sites. With this technique, common regional information, such as strike and impedance phase, can be recovered from distorted response functions. Although the BC87 GDS data are noisy, and the distortion is more of an inductive than of a purely galvanic nature, the resulting regional strike direction of 60° agrees well with the azimuth suggested from previous MT decomposition methods. Thin sheet modelling studies of the geometry of the deep crustal conductor (L-shaped or oblique) at 1000 s imply that along the BC87 profile induction arrows are generally produced by current concentrations around a resistive region associated with the Nelson Batholith. Comparison of the model induction arrows with those determined from the BC87 data indicates that the model featuring the oblique, NE striking conductor (SABC) may be favoured.View full abstractDownload PDF (29306K) -
Alan D. Chave, Alan G. Jones1997 Volume 49 Issue 6 Pages 767-789
Published: June 20, 1997
Released on J-STAGE: May 25, 2007
JOURNAL FREE ACCESSThe BC87 magnetotelluric data exhibit distortions due to three-dimensional structures at all scale sizes, from very local to regional. Previous work has shown that these distortions can sometimes, but not always, be described by an electric field galvanic distortion model, and hence can be removed using a tensor decomposition approach. This work extends the analysis to include galvanic distortion of the magnetic field, and shows its importance for many of the BC87 sites. However, even a combined electric and magnetic field galvanic distortion approach fails for quite a few of the sites studied. This is pervasive for periods shorter than 10 s, suggesting local 3D inductive effects, but model inadequacy at longer periods may indicate a breakdown of the model assumptions, particularly the requirement that the regional electric field be both uniform across the distorting body and comparable to that at the observation point.View full abstractDownload PDF (61049K) -
Toshihiro Uchida1997 Volume 49 Issue 6 Pages 791-800
Published: June 20, 1997
Released on J-STAGE: May 25, 2007
JOURNAL FREE ACCESSTwo-dimensional inversion has been applied to Electromagnetic Array Profiling (EMAP) data obtained in Oklahoma, USA. Since the data comprise scalar impedances measured along a survey line, a TM-mode inversion was performed on the original data, under the assumption that the survey line is perpendicular to the geologic strike. The inversion method applied is a linearized least-squares scheme with smoothness regularization. The “optimum smoothness” is selected based on a statistical criterion, ABIC (Akaike's Bayesian Information Criterion), which is derived from Bayesian statistics and the entropy-maximization theorem. Starting from a homogeneous earth as an initial guess, the inversion iteratively modifies the model structure until the observations are matched in a statistical sense and parameter modification becomes almost zero. The final two-dimensional models generally show a very conductive host formation (less than 10 Ω·m) with two large resistive bodies of approximately 100 Ω·m embedded near the middle of the survey line.View full abstractDownload PDF (27294K) -
Yasuo Ogawa1997 Volume 49 Issue 6 Pages 801-806
Published: June 20, 1997
Released on J-STAGE: May 25, 2007
JOURNAL FREE ACCESSThe Oklahoma EMAP dataset was analyzed using a two-dimensional inversion algorithm which includes static shifts as free parameters. Model misfit was minimized while simultaneously minimizing the resistivity roughness norm and the static shift L2 norm. The tradeoff parameters between the model misfit and these two norms were determined to minimize the Akaike's Bayesian Information Criterion (ABIC).View full abstractDownload PDF (12309K) -
F. E. M. (Ted) Lilley1997 Volume 49 Issue 6 Pages 807-815
Published: June 20, 1997
Released on J-STAGE: May 25, 2007
JOURNAL FREE ACCESSThe MTDIW2-PNG magnetotelluric data are examined as Mohr circles. Within their error bounds, of the ten sites five may be classified as 1-D immediately. Another four sites are 2-D; the only strongly 3-D data-set is from site PNG121. Visual examination of this simple presentation of the data gives much of the information that comes from more sophisticated analyses and decompositions, and can be used to check the latter. For example, from these basic figures it is possible to give principal directions for the 2-D cases, and observe (where it is present) the characteristics of frequency-independent “static shift”. Other characteristics also become evident from examining the data in real and quadrature pairs, notably that the real data are noisier than the quadrature data (a phenomenon attributed to lightning strikes, which would put noise predominantly into the real data). There is a suggestion of a systematic effect in the two-dimensionality of the data as recorded, as the sites were operated as pairs, and generally odd-numbered sites are two-dimensional in character, while even-numbered sites are one-dimensional.View full abstractDownload PDF (21499K) -
Jaume Pous, Juanjo Ledo, Alex Marcuello, Pilar Queralt1997 Volume 49 Issue 6 Pages 817-825
Published: June 20, 1997
Released on J-STAGE: May 25, 2007
JOURNAL FREE ACCESSThe PNG data have been analysed to determine the presence of the lower Darai limestones. As a first step, Groom and Bailey decomposition method was applied, which indicated that the electrical structure was approximately two-dimensional and striking N120°. Accordingly, two-dimensional forward modeling of the rotated data was performed. The final model showed the resistivity distribution with alternating resistive Darai limestones and conductive Ieru stratigraphic units. In particular, the model exhibited the resistive lower Darai limestones along the whole profile, which was the question posed at the MT-DIW2. A sensitivity test was performed to ensure the presence of the lower Darai limestones was resolvable.View full abstractDownload PDF (30256K) -
A. K. Agarwal, J. T. Weaver1997 Volume 49 Issue 6 Pages 827-842
Published: June 20, 1997
Released on J-STAGE: May 25, 2007
JOURNAL FREE ACCESSMagnetotelluric data obtained from a region of Papua New Guinea have been inverted with the aid of a two-dimensional scheme in which the simplest 'least-blocked' model compatible with the data is sought. The average strike direction for the entire region and for all periods is estimated to be approximately N60W. In the rotated frame skew is small everywhere, but ellipticity values remain quite large at some sites, which raises some uncertainty about the assumption of two-dimensionality. Since the data also indicate the presence of local galvanic distortion, a static shift correction has been applied; both the original (rotated) data and the static-shift corrected data have been used to generate 'least-blocked' models. The final models for the two data sets are found to resemble each other quite closely. Both reveal a descending conductive layer beneath a resistive limestone cover of thickness 0.6-0.8 km on southwestern side of the profile increasing to around 2.1-2.4 km on the northeastern side. The one notable difference is that the descending conductive layer divides into two parts separated by a resistive limestone layer in the model obtained from static-shift corrected data, but not in the other.View full abstractDownload PDF (46437K) -
Hiroaki Toh, Makoto Uyeshima1997 Volume 49 Issue 6 Pages 843-856
Published: June 20, 1997
Released on J-STAGE: May 25, 2007
JOURNAL FREE ACCESSConventional magnetotelluric (MT) analyses of the Papua New Guinea (PNG) dataset reveal that it is a very good two-dimensional (2D) dataset with relatively high SIN ratios up to 102 s. However, traditional Swift's angles are unstable at some sites and differ slightly site to site. This implies that weak but significant distortions are present at all sites. Hence, the regional 2D strike angle was sought by minimizing the global χ2 misfits derived from the site- and frequency-independent Groom-Bailey (GB) decomposition at seven selected sites. The other three sites were eliminated by χ2-test. The strike angle was determined to be 60° west of geographic north. One-dimensional (1D) Monte-Carlo inversion was performed using the Berdichevsky averages of the decomposed 2D principal tensor elements. The 2D model simply based on the 1D inversions was able to reproduce the observed impedances, except for the systematically high values of TM phase in the period range of 10-1 to 10 s. This indicates that a 1D approximation of the electrical conductivity structure beneath this region is reasonably valid. The 1D models indicated the existence of a thin (up to 2.5 km) resistive layer just below the surface, suggesting the 'single' Darai limestone.View full abstractDownload PDF (29838K) -
Yasuo Ogawa1997 Volume 49 Issue 6 Pages 857-867
Published: June 20, 1997
Released on J-STAGE: May 25, 2007
JOURNAL FREE ACCESSThe Papua New Guinea Magnetotelluric dataset was analyzed by applying Groom-Bailey tensor decomposition, and a consistent strike direction of N66° W was determined. The dataset was approximated by two-dimensional impedances, and frequency independent twist and shear. The static shift parameters (local anisotropy and site gain) were determined using a two-dimensional inversion where static shifts were also part of the model parameters. The model misfit was simultaneously minimized together with the following two norms: (1) roughness norm of the model, and (2) static shift L2 norm. The trade-off parameters between the model misfit and these norms were determined so as to minimize the Akaike's Bayesian Information Criterion (ABIC).View full abstractDownload PDF (24570K) -
Toshihiro Uchida1997 Volume 49 Issue 6 Pages 869-878
Published: June 20, 1997
Released on J-STAGE: May 25, 2007
JOURNAL FREE ACCESSTwo-dimensional inversion has been applied to magnetotelluric data obtained for petroleum exploration in Papua New Guinea. For the inversion, an impedance tensor is rotated to the general strike direction of the area, and TM, TE and determinant impedances are calculated. Distortion-corrected data are also derived to compare the results with the inversion of the original data. The inversion method applied is the linearized least-squares scheme with smoothness regularization. The optimum smoothness is selected based on a statistical criterion, ABIC, which is derived from Bayesian statistics and the maximum entropy theorem. The forward calculation is based on the finite-element method. Topography along the survey line is included in the mesh. The start model is a homogeneous earth, and the final models, obtained by the inversions of TM data, TM and TE data, and determinant data, are generally consistent with each other. A surface high-resistivity layer of 100-1, 000 Ω· m corresponds to Miocene limestone with a thickness of approximately 1 km. It is underlain by a thick low-resistivity layer of Mesozoic sedimentary rocks which form oil reservoirs.View full abstractDownload PDF (26639K)
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