航空瞬变电磁激电效应特性研究

Characteristics of induced-polarization effects in airborne transient EM

航空瞬变电磁法具有探测速度快、成本低、无需在地面作业等优点,广泛应用于矿产勘查、环境工程和地下水等领域.由于存在激电效应,导致航空瞬变电磁响应曲线出现异常衰减和符号反转现象.常规的实电阻率反演方法通常对负值响应予以剔除,反演结果与真实模型偏差很大,降低了反演结果准确性和可信度.为了研究航空瞬变电磁激电效应的特性,本文基于Cole-Cole模型表示激电效应,采用汉克尔数值滤波和正弦数值滤波实现了航空瞬变电磁一维正演模拟.将总响应分解为感应响应和极化响应,对激电效应的物理机制提出了一种新的解释,分析了地电参数、装置系数对响应曲线特性的影响以及响应对不同激电参数的灵敏度.研究表明,利用零频电阻率计算感应响应能有效地解释航空瞬变电磁激电效应的物理机制.当零频电阻率越大,极化效应越强,极化体埋深越浅,发射线圈高度越低,更容易出现符号反转现象.电磁响应对电阻率灵敏度最大,对充电率、时间常数和频率相关系数灵敏度很小.

Airborne Transient Electromagnetic(ATEM) has the advantages of fast detection speed, low cost, and no need to operate on the ground. It is widely used in mineral exploration, environmental engineering and groundwater. Response curve abnormal attenuation and sign reversal(negative response) in the airborne transient electromagnetic frequently observed in the field measurement, due to induced-polarization effects. The conventional real resistivity inversion algorithms usually delete the negative response, and the inversion model is different from the true model, which reduces the accuracy and reliability of the inversion model.Based on the Cole-Cole model, we have developed a 1 D modeling algorithm for ATEM by using Hankel numerical filtering and Sine numerical filtering. We have established different geoelectic models and different launcher coefficients to study the effects of different parameters on the electromagnetic response and the sensitivity of the Cole-Cole model parameters are analyzed. The total response is decomposed into the induced response and the polarized response and we propose a new physical interpretation for the induced-polarization effects in ATEM.The research shows, using zero-frequency resistivity to calculate the induced response can effectively reveal the physical mechanism of the induced-polarization effects in ATEM. The larger the zero-frequency resistivity, the stronger the polarization, the shallower the buried depth of the polarized body, and the lower the transmitter height, the more likely to observe the sign reversal. The response is big sensitive to zero-frequency resistivity and has small sensitivity to chargeability, time constant and frequency dependence.