Inversion of controlled source audio-frequency magnetotellurics data for a horizontally layered earth

We present a technique for inverting controlled source audio-frequency magnetotelluric (CSAMT) data to recover a 1-D conductivity structure. The earth is modeled as a set of horizontal layers with constant conductivity, and the data are apparent resistivities and phases computed from orthogonal electric and magnetic fields due to a finite dipole source. The earth model has many layers compared to the number of data points, and therefore the solution is nonunique. Among the possible solutions, we seek a model with desired character by minimizing a particular model objective function. Traditionally, CSAMT data are inverted either by using the far-field data where magnetotelluric (MT) equations are valid or by correcting the near-field data to an equivalent plane-wave approximation. Here, we invert both apparent resistivity and phase data from the near-field transition zone and the far-field regions in the full CSAMT inversion without any correction. Our inversion is compared with that obtained by inverting near-field corrected data using an MT algorithm. Both synthetic and field data examples indicate that a full CSAMT inversion provides improved information about subsurface conductivity.

[1]  Y. Sasaki,et al.  Resistivity imaging of controlled‐source audiofrequency magnetotelluric data , 1992 .

[2]  David E. Boerner,et al.  A spatial and spectral analysis of the electromagnetic sensitivity in a layered earth , 1989 .

[3]  Stan E. Dosso,et al.  Linear and non-linear appraisal using extremal models of bounded variation , 1989 .

[4]  Douglas W. Oldenburg,et al.  Inversion of time-domain electromagnetic data for a horizontally layered Earth , 1993 .

[5]  Gerald W. Hohmann,et al.  Controlled‐source audiomagnetotellurics in geothermal exploration , 1980 .

[6]  D. Oldenburg,et al.  METHODS FOR CALCULATING FRÉCHET DERIVATIVES AND SENSITIVITIES FOR THE NON‐LINEAR INVERSE PROBLEM: A COMPARATIVE STUDY1 , 1990 .

[7]  R. D. Jacobson,et al.  Results of a controlled-source audiofrequency magnetotelluric survey at the Puhimau thermal area, Kilauea Volcano, Hawaii , 1987 .

[8]  David E. Boerner,et al.  Orthogonality in CSAMT and MT measurements , 1993 .

[9]  D. Oldenburg,et al.  Generalized subspace methods for large-scale inverse problems , 1993 .

[10]  Philip E. Wannamaker,et al.  Tensor CSAMT survey over the Sulphur Springs thermal area, Valles Caldera, New Mexico, U.S.A., Part II: Implications for CSAMT methodology , 1997 .

[11]  K. Zonge,et al.  Use of Electrical Geophysics to Detect Sources of Groundwater Contamination , 1985 .

[12]  P. Wannamaker.  Tensor CSAMT survey over the Sulphur Springs thermal area, Valles Caldera, New Mexico, U.S.A., Part I: Implications for structure of the western caldera , 1997 .

[13]  D. W. Strangway,et al.  Audio-frequency magnetotellurics with a grounded electric dipole source , 1975 .

[14]  Gerald W. Hohmann,et al.  Controlled‐source audiofrequency magnetotelluric responses of three‐dimensional bodies , 1991 .

[15]  Comparison of induced polarization and controlled-source audio-magnetotellurics methods for massive chalcopyrite exploration in a volcanic area , 1997 .

[16]  G. W. Hohmann,et al.  4. Electromagnetic Theory for Geophysical Applications , 1987 .

[17]  Kenneth L. Zonge,et al.  9. Controlled Source Audio-Frequency Magnetotellurics , 1991 .

[18]  Further evidence of electrical anomalies over hydrocarbon accumulations using CSAMT , 1983 .

[19]  R. Parker,et al.  Occam's inversion; a practical algorithm for generating smooth models from electromagnetic sounding data , 1987 .