An iterative inversion method using transient electromagnetic data to predict water-filled caves during the excavation of a tunnel

Abstract

The correct interpretation of full-space transient electromagnetic data has always constituted a critical safety problem during tunnel excavation projects. Targeting the interpretation of water-filled caves under narrow tunnel conditions, we have developed an iterative inversion method based on 3D finite-difference time-domain (FDTD) forward calculations and a direction algorithm. In total, 125 groups of 3D FDTD forward calculation results are analyzed to identify the correlations between the response data and the geometric conditions of the cave. A direction algorithm is established based on the correlations, thereby increasing the iterative inversion convergence speed. Using the proposed iterative inversion method, the location and volume of the water-filled cave in front of the tunnel face are successfully inverted. Through an iterative program, the inversion results of simulations involving the detection of water-filled caves under tunnel conditions are accurately analyzed, and the relative error is less than 10%. The application of the iterative inversion method to the Mingyue Mountain Tunnel project suggests that this method is capable of interpreting the size of water-filled caves and it is valid for a narrow tunnel face with only a single available measurement point. The proposed iterative inversion method can be used alone or in combination with other detection techniques, thereby providing engineers with a better early warning system for detecting water-filled caves in tunnels.

Vehicle-Borne Transient Electromagnetic Numerical Characteristic Parameter of Water-Bearing Body behind Tunnel Linings

“…Introduction to Tunnel Detection System. TEM technology has been widely used for the detection of different media [7,10,14]. In operational tunnels, the normal surrounding rock and WBB have very different electrical resistivity; thus, the transient electromagnetic technology has the potentiality and ability to detect the normal surrounding rock and WBB behind tunnel linings, and it can even investigate the development scale of the WBB.…”

“…e VBTEM is based on improved detection by the transient electromagnetic method (TEM) in many engineering applications, such as geological advance prediction of tunnels, underground gas storage, and metal mineral exploration [7][8][9][10][11][12][13][14]. e VBTEM has the common characteristics of transient electromagnetic in other fields, but also has its differences, which means that some issues need to be discussed before applying VBTEM technology into operational tunnels.…”

“…Finally, the detection distance of the low-resistivity target of VBTEM is different from the traditional TEM. For example, Fitterman and Stewart [7] studied the transient electromagnetic sounding for groundwater exploration at approximately 1000 m with a 100 × 100 m 2 transmitter loop, Hördt [8] adopted an innovative instrumentation with a 40 × 40 m 2 transmitter loop and found an unknown and unexpected buried valley complex at 300 m, Li et al [14] predicted and detected water-filled caves ahead of the tunnel excavation face at 30 m with a 2 × 2 m 2 transmitter loop. However, the WBB behind tunnel linings of VBTEM is too close.…”

“…e time range requirement of ground TEM and tunnel TEM range from microsecond level to millisecond level [9,13,14], while the time range requirement of VBTEM is required to be at least microsecond level or lower, so the transmitter type and coil parameters of VBTEM need to be reselected and redesigned, respectively. Because of the lower level of time range, the early transient electromagnetic signals need to be paid more attention, which means the technical parameters of the VBTEM equipment should be proposed.…”

“…Meanwhile, unlike ground TEM and tunnel TEM, the scale of the WBB cannot be very large, and it exists on a certain scale behind the tunnel lining; thus, whether its scale can be effectively detected remains to be further investigated. Due to the limitation of tunnel cross section, only a limited number of measurement points can be measured in operational tunnels, just like tunnel TEM [14,15], so the singlepoint electromagnetic signal technology also needs to be referenced in VBTEM. Similarly, in the case of a single measurement point and short-distance, detecting the WBB behind tunnel linings in operational tunnels needs to be profoundly investigated, and physical characteristics of the WBB behind tunnel linings also needs to be further described.…”