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Fine detection on water abundance of sandstone by joint inversion of seismic and CSAMT: a case study in the western Ordos Basin, China

Dong Yi, Cheng Jiulong, Wen Laifu, Cheng Peng, Li Fei, Jiao Junjun, Huang Qisong

Acta Geophysica

2022

Vol. 70, no 5

2045--2056

The Ningdong mining area in the western Ordos Basin, China, mainly mines coal seam in Yan'an group, where its overlying rock is a thick sandstone layer of the Middle Jurassic Zhiluo Formation. This rock layer poses a direct water hazard threat to the coal mining if it is water-rich. The water abundant rock layer in the upper strata of Zhiluo Formation forms a low-resistivity overburden layer, decreasing the resolution of controlled source audio magnetotelluric (CSAMT) method in detecting water-bearing in the lower part of Zhiluo Formation sandstone layer of coal seam direct roof. Therefore, under the influence of the low resistivity overburden of the upper sandstone, how to accurately detect the aquosity of the lower sandstone layer is of great importance to the safe mining of coal mines in the region. On the basis of CSAMT detection, combined with high-resolution seismic exploration method, the joint inversion of seismic and CSAMT is realized by using cross-gradient operation between the seismic wave impedance attributes clustered by particle swarm algorithm and CSAMT inversion model. The seismic data fitting term in the joint inversion objective function is discarded, and the pseudo-2D inversion method is used for CSAMT to reduce the calculation cost of the inversion. A 3D geological model conforming to the hydrogeological characteristics of the Ningdong mining area is established, and the joint inversion test between seismic and CSAMT is conducted, proving the feasibility and applicability of joint inversion to detect the water enrichment of sandstone in this area. The accuracy of the seismic and CSAMT joint inversion results is verified by combining the engineering example of water abundance detection in the sandstone layer in Maiduoshan coal mine that accorded with the typical hydrogeological characteristics of Ningdong mining area and the results of later downhole drilling exposures, which is remarkably better than the single method. The research shows that the joint inversion of seismic and CSAMT can accurately identify the water abundance of the lower sandstone layer and its range under the influence of the upper low resistance sandstone overburden and achieve the purpose of fine detection of the water abundances of the lower sandstone layer of the Zhiluo Formation. The joint inversion can provide important safety geological guarantee for the mining of coal seams in the Ningdong mining area in the western part of Ordos Basin.

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.

Perspektywy badań magnetotellurycznych w Polsce

Miecznik J., Pepel A.

Geologia / Akademia Górniczo-Hutnicza im. Stanisława Staszica w Krakowie

2003

T. 29, z. 3--4

219-232

Pięćdziesiąt lat stosowania metody magnetotellurycznej do rozwiązywania różnorodnych zagadnień geologicznych potwierdziło jej prospekcyjną efektywność. Najczęściej magnetotelluryka jest wykorzystywana w zagadnieniach geologii strukturalnej, zwłaszcza w aspekcie poszukiwań złóż węglowodorów, oraz w badaniach podstawowych do oceny własności fizycznych skorupy ziemskiej i jej struktury. Rejestracja niskich częstotliwości wariacji pola elektromagnetycznego umożliwia uzyskanie informacji o własnościach górotworu na znacznych głębokościach. Dlatego też celowe jest włączenie badań magnetotellurycznych do programów sejsmicznych POLONAISE'97 i CELEBRATION'2000. Rejestrując wysokie częstotliwości, możemy metodą magnetotelluryczną kartować przypowierzchniowe struktury i anomalie opornościowe. Do tych celów stosuje się odmianę magnetotelluryki zwaną Controlled Source Audio Frequency Magnetotellurics (CSAMT). Wymagania w zakresie dokładności odtworzenia fizycznych i geometrycznych parametrów górotworu implikują rozwój nowych metod inwersji danych pomiarowych. Można tu wskazać na metodę sieci neuronowych ANN (Artificial Neural Networks).

The effectiveness of the magnetotelluric metod in solving geological problems has been proven for fifty years of its applications. Most often, the magnetotelluric method is applied in structural geology, particularly in hydrocarbon prospecting, and in the basic researches to determine physical properties and the structure of the Earth's crust. Measurement of MT field components over a low-frequency range gives information on the geology at great depth. Therefore, it seems justified to include the magnetotelluric investigations to projects POLONAISE'97 and CELEBRATION'2000. The magnetotelluric measurements over a high-frequency range can be successfully applied to map resistivity anomalies from near-surface geological structures. The Controlled Source Audio Frequency Magnetotellurics (CSAMT) is advantageous in such studies. The needs for high-accuracy reproduction of physical and geometrical parameters of the earth gave risk to the development of new methods of data inversion such as the Artificial Neural Networks (ANN).