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2 Acta Geophysica

2 2020

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Three dimensional magnetotelluric inversion using L BFGS

Lu Libin, Wang Kunpeng, Tan Handong, Li Qingkun

Acta Geophysica

2020

Vol. 68, no. 4

1049--1066

The gradient-based optimization methods are preferable for the large-scale three-dimensional (3D) magnetotelluric (MT) inverse problem. Compared with the popular nonlinear conjugate gradient (NLCG) method, however, the limited-memory Broyden–Fletcher–Goldfarb–Shanno (L-BFGS) method is less adopted. This paper aims to implement a L-BFGS-based inversion algorithm for the 3D MT problem. And we develop our code on top of the ModEM package, which is highly extensible and popular among the MT community. To accelerate the convergence speed, the preconditioning technique by the afne linear transformation of the original model parameters is used. Two modifcations of the conventional L-BFGS algorithm are also made to get a comparable convergence rate with the NLCG method. The impacts of the preconditioner parameters, the regularization parameters, the starting model, etc., on the inversion are evaluated by synthetic examples for both L-BFGS and NLCG methods. And the real MT Kayabe dataset is also inverted by the inversion algorithms. The synthetic tests show that through our L-BFGS inversion algorithm the similar resistivity models can be obtained with that from the NLCG method. For the real data inversion, the L-BFGS method performs more efciently and reasonable results could be obtained by less iterations of the inversion process than the NLCG method. Thus, we suggest the common usage of the L-BFGS method for the 3D MT inverse problem.

Case study: joint seismic refection and CSAMT data interpretation for mineral explorations in Fujian, China

Liu Guofeng, Meng Xiaohong, Tan Handong, Chen Zhaoxi, Ni Jianhui, Liu Lanbo

Acta Geophysica

2020

Vol. 68, no. 5

1373--1385

The Southwestern area of Fujian Province in China contains a major metallogenic belt. There have been three major tectonic movements since the Late Paleozoic, and the thrusts here are the main mineral deposits. These occurred between the Early Carboniferous and Late Permian eras. Due to dramatic undulations in the surface and associated complex underground structures, a single geophysical method cannot provide reliable imaging results. This is largely due to difculties in data acquisition and processing. Seismic exploration provides one example. Using this method, raw shot gathers have a low signal-to-noise ratio because of the undulations in the topography. Furthermore, strong lateral velocity variation makes the migration process exceedingly difcult. This means that interpretations of the resulting refection seismic profle give rise to uncertainty. For this reason, two-dimensional refection seismology and controlled source audio-frequency magnetotel lurics (CSAMT) electromagnetic sounding techniques were performed. After initially completing conventional processing on the seismic refection data, subsequent pre-stack depth migration (PSDM) over the rugged topography yielded a much better image. It was able to identify thrust faults and magma intrusions. Then, using CSAMT, inversion was conducted with the same topography as the seismic refection. By combining the migrated seismic profle, inverted resistivity profle, and borehole data, a vertical geological model was constructed. This proved the existence of an overlapped thrust, and it also indicated the possible presence of deeper mineralization.