Research On Induced Polarization Effect In Central-Loop Transient Electromagnetic Soundings

Central loop configuration is the most commonly used device within the depthsof1km in the transient electromagnetic method (TEM). In the field work, sometimesTEM response will appear negative phenomenon. People carried out deep research onthis negative phenomenon, It is concluded that the negative phenomenon is due to aburied polarizable body.the reversal phenomenon of induced polarization (IP) signin the data acquisition and processing is often considered as the interference factors,and as the great impact on inversion of the data, so it has also great differencewith the actual geological information. Therefore, in the measurement of transientelectromagnetic, appearance of sign inversion is not considered as a disturbance toprocess, we should make full use of the regularities of transient electromagneticresponse influenced by induced polarization, so as to better get the geological targetinformation.This paper firstly Shows the current research at home and abroad on the transientelectromagnetic soundings influenced by induced polarization effect;and a mechanismhypothesis model is put forward to explain the induced polarization effect in thetransient electromagnetic response;secondly, the forward modeling based on layeredmodel is carried out in the frequency domain for central loop device,the conversionfrom frequency domain to the time domain.In the transformation, this paper analyzesthe problem of transform accuracy of three methods of frequency-time domaintransform (polygonal approximation method of cosine transform, digital filteringalgorithm of sine transform,digital filtering algorithm of cosine transform),then thereason that cause the error accuracy in three methods are analyzed.Finally, this paper summarizes the regularities of the transient electromagneticresponse influenced by parameters of Cole-Cole model (DC resistivity, chargeability,time constant, the frequency correlation coefficient) in half space,and analyzes theregularities of the transient electromagnetic response influenced by inducedpolarization in layered model.Through the study on the transient electromagnetic response influenced by theIP effect, this paper draws the following conclusion. 1. Mechanism model of induced polarization effect was proposed by the authorscan explain the process to produce the IP effect of transient in transientelectromagnetic response.2. Through the study of the three methods of transformation, the digital filteringmethod based on sine transform has higher computing precision than theother two methods,and it can meet the accuracy requirements.3. In the half space, the parameters of the Cole-Cole model of DC resistivity andfrequency correlation coefficient can make the transient response occur twosign reversals in a certain range.4. In the layered model, the transient electromagnetic response with the IP effecthas much more obvious characteristics than the electromagnetic responsewithout IP effect in the curve figure.