A new wide-band radiomagnetotelluric measurement system with HTC-Squid

Abstract

The radiomagetotelluric (RMT) method estimates the resistivity distribution of soil by measuring and evaluating the em-fields of radiotransmitters in the very low-, long- and medium wave frequency range. The method potentialy does not require many personnel for field measurements. It has all the features for fast recording of complex geological structures. Presently available RMT instruments have serious limitations. Therefore, we have designed and tested the following improvements: Estimating full impedance tensor To explore complex shaped subsoils knowledge of full electrical impedance tensor is indispensible. We introduce a method to estimate all four tensor elements in the radio frequency range. This makes it possible to evaluate RMT-data by using conventional and powerful MT inversions programs. Fast speed in measurement Progress in measurement is determined by the time to move the instrument from one point to another, registration time and time necessary to process the time series of typically some ten million samples per channel. To minimize the registration and the processing time we build a very fast digital time series preprocessing electronics. It calculates impedance tensor for 16 frequencies in RMT range in parallel from one registration within a few seconds. Using superconducting quantum interference device (SQUID) magnetometer A liquid nitrogen cooled SQUID Magnetometer was tested. SQUIDs are invaluable for the near future especially if wideband registration with mobile instruments, as presented here, is required. Extended frequency range The RMT technique is often unable to penetrate highly conductive overburden like dump sites, clay layers or contaminated ground water. The reason for this is that the lowest available frequency is too high. Therefore, we have combined the RMT frequency range (10 kHz to 1600 kHz) with the CSAMT (frequency: 1 kHz to 10 kHz). This permits a threefold increase in the depth of penetration. At low frequencies we use a light battery-driven horizontaly orienteted magnetic transmitter to generate the em-field. This is necessary because of absence of radio transmitters below 10 kHz.