Three-dimensional Modeling of Controlled Source Audio Magnetotelluric Data with Topography

doi:10.19657/j.geoscience.1000-8527.2018.02.19

Abstract

Abstract:

The topography has a strong effect on the controlled source audio magnetotelluric (CSAMT) data, therefore, the topography should be considered when interpreting the CSAMT data. The real subsurface geolo-gical structure and the topography of the surface are usually more complex and are three-dimensional spaces in most cases. Based on the three-dimensional finite-difference CSAMT modeling algorithm for flat surface, this paper presents a new method to calculate the total electric field at the air-earth interface with topography from the total magnetic fields at subsurface staggered sampling points, modeling the three-dimensional CSAMT responses on topography. In the new algorithm, the pseudo-delta function is used to replace the source to directly calculate the total field, avoiding the limitation of selecting the appropriate background resistivity structure under complex geological structure conditions when the total field is separated into the background field and the secondary field. In order to simulate the total field directly, this study used the new boundary condition of the three-dimensional forward equation in the three-dimensional CSAMT modeling algorithm with topography. The correctness and validity of the algorithm are verified by comparing the response results obtained from the two models (the flat surface model with 3D anomaly and the three-dimensional trapezoidal-hill model) with the results of the previous algorithms.

Key words: CSAMT, three-dimensional topography, numerical simulation, pseudo-delta function

CLC Number:

P631

ZHONG Sumei, LIN Changhong, XIE Yuchun. Three-dimensional Modeling of Controlled Source Audio Magnetotelluric Data with Topography[J]. Geoscience, 2018, 32(02): 398-405.

Figures/Tables 5

Fig.1 View of the difference relationship used to calculate the total electric fields on the surface

Fig.2 Sketches of the horizontal surface three-dimensional anomalous body model

Fig.3 Results of 3D responses at frequency of 1,000 Hz calculated by the forward scheme (black circle) described in this paper(the horizontal surface three-dimensional anomalous body with a X-direction source located at -2.4 km, 0 km, 0 km) compared with responses calculated by the method given by Lin C H et al[11](black solid line,along the line x=0 m)

Fig.4 Sketches of the three-dimensional trapezoidal-hill models

Fig.5 Comparison of 3D responses (at frequency of 2 Hz) of the method given by this paper (black circle) and 3D FEM MT responses of the method given by Nam et al[7] (black solid line) for models of XY and YX

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