The purpose of this research is to apply time-domain electromagnetic method (TEM) to detect the underground metals. We concentrated on the three topics: to recognize the characteristics of the transient voltage by metal, to create a powerful transmitter for enhancing the signal to noise ratio, and to design better detection modes for the efficiency of field work. With those tools in use, it is not only easily judging the existence of metal conductors but also detect their locations.
The fact that the electric conductivity of metal (about 107S/m) is much larger than that of the background stratum (100-10-3S/m), together with the slower decay of eddy electric current of metal, the transient voltage of the metal is more anomalous than that of the background stratum. This phenomenon is the key point used to judge if the metals exist or not. We created a powerful transmitter; it synchronized with the TEM consol and transmitting current as high as 40 A. Moreover, many transmitters can be used in parallel to produce as much current as needed. For example, three sets of transmitter parallel connected will produce current 120 A in the transmitter loop. Other than enhance the signal to noise ratio, it can increase the depth of detection. Thus, promotes the detection effects, especially under the noisy engineering environments. After the many case studies in various environmental conditions and engineering requirements, we also designed better detection modes for the efficiency in the fieldwork, that is the rough detection mode and the detail detection mode. One thing worth mentioning, experience tell us that by changing the transmitter current and checking the anomaly inverse is a worthy tip to reconfirm the signal.
Finally, two successful cases are shown. Case 1, in Pingtong survey area, the rough detection mode was used to explore the position and shape of the underground steel plate, the depth of the steel stake was analyzed and the validity of the exploration was proved. Case 2, in Nantou survey area, the detailed detection mode to determine the position and length of the underground sleeve stakes was undertaken; correlated with the original design was obtained. Furthermore, by surrounding the coils around the bridge pillars and doing TEM survey, the depth of the underground stakes connected with the bridge pillars could be obtained.||en_US|