Research On TEM Detection Technology For Water-accumlating Gob In Coal Mines

Since recent years, water penetration from gob of abondaned mines became a significant feature of water inrush in coal mines, threating serousely safe production in coal mines. There is not any regularity for occurrence of gob. The distribution of gob is often isolated and discontinous, so the detection of gob became difficult problem. Transient electromagnetic method(TEM) has special advantage and evident result as an approach for detection of water-accumulating gob. Aiming at some common key technologies in detection of water-accumulating gob by TEM with configuration of central loop—fixed loop source, the paper carried out systematic study, and obtained following results:Through forward calculation of different ground electric models of water-accumulating gob in single layer, air-filled highly resistant gob and gob in multiple layer developed in coal measures, the characteristics of transient electromagnetic response and section of gobs under different conditons were analysed. For water-accumulating gob in highly resiastant surrounding rocks, transient electromagnetic signals are evident, but air-filled highly resitant gob has weak transient electromagntic response on curves and sections, and is difficult to show its location.The transient electromagnetic installation conmmonly sued in detection of water-accumulating gob in coal mines is configuration of central loop—fixed loop sources,systematic theoric analysis and numerical modeling were made in the paper in term of rim effect. Calculation of primary field shows that in emession rim, the primary field is quite different at defferent places and has the maximum value at the center. Secondary field analysis of uniform half space and stratified ground electric model indicated response of vertical magnetic field for different emission side length, and the attenuation of vertical magnetic field is different at the same offset. When the offset is zero, the energy of vertical is the strongest. At the extent of about 1/3 of the center of emission rim, the stimulated field density varies a little, can be basically considered as uniform field. The type of disturbance during TEM detection and the attributes of noise on curves were analyzed. From the threory of wavelet analysis, identification and suppression of disturbance were discussed. The variation characteristics of Lipschitz exponentαwas used to disguinsh anomaly and non-anomaly. The examples of noise suppression showed the feasibility of the suppression. The paper also analyzed and studied the disturbance caused by topography and the result of topographic correction was analyzed from the view of practice.The mechanism of additional effect is very complicated. Experiment indicated that the additional effect of shallow water-accumulating gob had significant impact on transient electromagnetic signals. And the additional effect could not be simply incorporated into disturbance. The cause of additional effect was analyzed theorically. The examples confirmed that a the application of additional effect in detection of water-accumulating gob can produce ideal results.During detection of water-accumulating gob in coal mines by TEM, different operation parameters should be selected. The cases of detection of gob in different gerological conditions showed that as a detection approach TEM had good result for detection of water-accumulating gob in coal mines.