全波形回线源地—井瞬变电磁全时响应特征分析

Characteristics analysis of full-time response of loop sourcesurface-borehole transient electromagnetic in full waveform

地—井瞬变电磁法在金属和煤等矿产勘查中发挥重要作用,其激发波形可以是任意形态,波形形态的选择对探测效果和数据质量具有重要影响,探讨不同发射波形对响应的影响可为电磁勘探系统的波形设计提供理论指导。基于回线源地—井瞬变电磁一维正演理论,通过模拟计算任意发射电流波形下的一维层状模型的水平和垂直分量的磁感应强度和感应电动势,来定性分析不同电流波形对瞬变响应的影响情况。研究表明,4种波形的响应曲线形态在有发射电流期间(on-time)具有较大的差异,而在无发射电流期间(off-time)一致且变化规律简单,方波激励的响应最强,三角波激励的响应最弱。三角波和半正弦波的B、dB/dt的各分量响应幅度在off-time期间随电阻率变化明显,而在on-time期间基本不随电阻率变化;方波和梯形波的B、dB/dt各分量在全时域(on-time、off-time)对地层的电阻率变化均有很好的分辨能力。

The surface-borehole transient electromagnetic method plays a crucial role in mineral exploration,especially for metals and coal.The excitation waveform in this method can take any form,and the selection of waveform shape significantly influences the detection efficiency and data quality.This paper conducts a theoretical analysis on the impact of different excitation waveforms on the response,providing theoretical guidance for waveform design in electromagnetic exploration systems.Based on the grounded-source transient electromagnetic one-dimensional forward modeling theory,the paper utilizes simulated calculations to analyze qualitatively the influence of various current waveforms on the transient response.The analysis includes horizontal and vertical components of magnetic induction and induced electromotive force for a one-dimensional layered model under arbitrary current waveforms.The research reveals that the response curve shapes of the four waveforms exhibit significant differences during the on-time period with current emission,while they are consistent and exhibit simple variations during the off-time period without current emission.The square wave excitation produces the strongest response,while the triangular wave excitation results in the weakest response.The amplitudes of B and d B/d t components for triangular and semi-sinusoidal waveforms show noticeable changes with variations in resistivity during the off-time period,whereas during the on-time period,they remain essentially unaffected by resistivity changes.The square wave and trapezoidal wave excitations demonstrate excellent discriminative capabilities for changes in the resistivity of the geological layer across the entire time domain(on-time and off-time).