浅海瞬变电磁全波形响应特征及探测能力分析

瞬变电磁法在浅海工程勘探等领域受到了越来越多的关注.目前浅海瞬变电磁仍处于应用初期,相关研究少且未有成熟装备问世,有必要研究其探测能力并为观测系统选取最佳观测参数.本文以几种典型发射波形为例,采用褶积算法细致分析了不同发射波形条件下浅海瞬变电磁on-/off-time响应受海底介质电导率、磁导率及发射波形脉宽等参数的影响特征与规律;通过三维正演并设定极限探测深度阈值,进一步分析不同发射波形on-/offtime期间浅海瞬变电磁探测能力及对典型三维目标体的极限探测深度.基于本文研究成果,可为浅海瞬变电磁探测装置设计、观测系统的参数选取及试验参数的选取等提供了一些有价值的理论借鉴.

Exploration capability of transmitter current waveform on shallow water TEM response

The transient electromagnetic (TEM) method in shallow sea has received munch attention as an effective geophysical tool in engineering and environment investigation. Since it is still at its infancy, then it is necessary to study its exploration capability and select the best observation parameters for the investigation system. The advantages of using central loop systems at sea are twofold:simplified navigation and simplified operation. This paper investigates the effect and exploration capability of different TEM transmitter waveforms on measuring dBz/dt and Bz response in shallow sea. First, the characteristic of the transmitter pulse shape is calculated by spectrum analysis. The pulse shapes considered include the square, trapezoid, triangular and half-sine shape. Then, using the convolution algorithm, we analyze the influence of seafloor conductivity, permeability, waveform plus width and the on-and off-time dBz/dt and Bz signals on the exploration capability. Finally, in order to simulate the three-dimensional marine TEM response, the vector finite element method (VFEM) is used to simulate the on-and off-time dBz/dt and Bz response over a 3D conductor which buried in the seafloor, and calculate the exploration depth of different transmitting waveforms, on-and off-time TEM response. The accuracy of our algorithm is checked with previous work. The result shows a good agreement. The following observations are obtained based on numerical tests:1) TEM response in shallow sea has good resolving power to the conductive seafloor, but poor one to the resistive seafloor. 2) TEM response in shallow sea increases with the increasing seafloor permeability, Bz response is more sensitive to the permeability changes than dBz/dt response, and triangular and half-sine waves are more strongly influenced by the change in seafloor permeability than trapezoid wave. 3) Depending on the detected target, the exploration capability and efficiency of shallow sea TEM can be enhanced by choosing appropriate pulse width. 4) The on-and off-time Bz response has deeper exploration depth than the dBz/dt response. 5) The effect in turn-off time of trapezoid waves on the exploration capability of on-time response is greater than off-time response, both for Bz and dBz/dt response. The presented work provides theoretical basis for both the development of shallow sea TEM and selection of observation and interpretation parameters in the future.