Large-depth hydrogeological detection in the North China-type coalfield through short-offset grounded-wire TEM

摘要

The North China-type coalfield is the most important coalfield in China. With the development of Chinese economy, shallow coal resources (< 600 m) have been exhausted and large-depth detection is being conducted. Water-inrush is disastrous for coal mining. However, large-depth (600-1200 m) detection of water-filled zones in the North China-type coalfield is a difficult geophysical task due to its low-resistivity coverage. The traditional loop-source transient electromagnetic method (TEM) cannot realize large-depth detection. On the other hand, high-resolution detection of LOTEM is limited by the problem of recording points and large volume effect. To explore the large-depth targets with high-resolution, Short-offset grounded TEM (SOTEM) was applied in the North China-type coalfield. SOTEM is one branch of grounded-wire TEM. The response of SOTEM is calculated using superposing-dipole method rather than the direct-dipole approximation, which is used in the calculation of the response of LOTEM. This paper analyzes the comparison of the resolution and detection depth between SOTEM and loop-source TEM at first. Compared to the loop-source TEM with frequently used parameters, the detection depth of SOTEM with the usual setup parameters is about twice of magnitude, and the resolution capability of SOTEM at large depth is higher than that of loop-source TEM. Then, the resolution capability and detection depth between LOTEM and SOTEM are analyzed. For the grounded-wire TEM, offsets are strictly related to the detection depth. So, the detection depth of LOTEM with larger receiver-transmitter offset will be greater. However, at the depth of 600-1200 m, the resolution capability to conductive targets of SOTEM is higher than that of LOTEM. At last, SOTEM with a 1000-m long source and 1000-2000 m offset is applied to the 1200 m-depth exploration of water-filled zones in the North China-type coalfield. The induced voltage is observed and transformed into magnetic field. Then, the data is inverted using the least mean fourth algorithm. The distribution of water on the top and bottom surface of the large-depth No. 4 coal seam is given, and is verified by the drilling result.