Summary

Geothermal exploration in urban and suburban Beijing areas is a challenging task because of the high-level ambient noises. The paper summarizes several successful applications of the CSAMT method to geothermal exploration in the Beijing area. By carefully selecting the output power, operating frequency, transmitter length, and receiver spacing, we were able to accurately locate geothermal reservoirs deeper than 2000 meters. In the Beijing area, the CSAMT is particularly useful in locating hot-water-bearing faults.

Introduction

The geothermal resources in Beijing and suburban areas are usually in the Wumishan formation of Jixian system. The burial depths are normally greater than 2000 meters. Because of the large burial depths, geological information about the geothermal resources is scarce. Geophysical methods were demanded to help locate the geothermal reservoirs. One of the biggest challenges for geophysical surveys was the high levels of civil and environmental noises because the surveys must be conducted in or around the city.

The controlled source audio-frequency magnetotellurics (CSAMT) method (Piao, 1990; He, 1990; Fu, 1991; Misac, 1992; Fang, 1993; Bi, 2002) was used because of its high signal-to-noise ratio. The CSAMT method also allowed us to explore deep into the subsurface, which is particularly useful to investigate the deeply buried geothermal resources. Since 1994, we have conducted more than 20 CSAMT surveys in Beijing area. Interpretation of the results has led to successful drilling of over 10 geothermal wells. This paper summarizes our CSAMT surveys and demonstrates the ability of the CSAMT method to reject high ambient noises.

Geological and geophysical characteristic of Beijing
geothermal areas

Beijing is located in the secondary tectonic element of North China (Bi, 2002). From Mesoproterozoic, the Beijing area has formed massive littoral facies deposit formation. Chang-cheng system, Ji-xian system and Qing-bai-kou system are distributed widely in the area. The deposition thickness is huge. There are many water-rich strata, such as carbonates and sandstone in Ji-xian system. There also are some water-rich strata in Cambrian and Ordovician. After the Cenozoic era, the northern China fault was formed in the Himalayas movement. As a section of the northest China fault, a series of faulted-basins were formed in the Beijing area in Eogene, making some water-rich strata buried deeply.

A geothermal reservoir is formed when there is a cap rock like shale, detritus rock layer, Cambrian, Eogene.

If there are some crevasses, hot water will migrate along the crevasse. Some geothermal strata are formed in this way, which are the primary target of our CSAMT survey. The basis for the CSAMT method for geothermal exploration is the resistivity contrast between the geothermal reservoirs and their surrounding rocks. The resistivity of the quaternary deposits in the Beijing area is usually 10-100Om. The resistivity of the shale is usually 100-200Om. The resistivity of the sandstones is generally 200-400Om, and the resistivity of calcareous rock is 300- 1000Om. The resistivity of the volcanic rocks in the Jurassic system is perhaps in the range of 1000-2000Om.

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