A Practical Magnetotelluric Method and Its Application in Taiwan

Abstract

大地電磁（Magnetotellunc）記錄系統為一套精密的電子儀器組合，可用來測量自然的地磁場強度變化及因之而感應出來的微弱地電流。系統中的磁力儀必須要能夠量出地磁場之X，Y，和Z三分量，而測量地電流變化的電位計必須要有極高之輸入阻抗以避免電流分路而造成過大之誤差。在地電流量度上最重要的部份是要設計一組非極化之電極，並且要設法與地面接觸良好，以消除假性電位差。另外在大地電磁測勘上要特別注意的是接收能量上的選擇。對不同之測勘深度要用不同的取樣頻率。在將信號數位化之前，往往了適當的使用低頻切除波波器，以減低低頻信號佔用過多之類比/數位轉換位元。在台灣地區做大地電磁測勘最大的麻煩出於環境因素的困擾，如人口過於密集，電磁干擾過高，及環境的污染皆影響到測量的結果。但是從另一面說來，由於臺灣地區之特殊環境，或許可由所測量出之數據中取得能夠對環境污染作定性歸類之有用資料。

The magnetotelluric recording instrument is a deliberately designed electronic system torecord the natural magnetic fluctuations and the induced extremely weak terrestrial current. Themagnetometer chosen must be capable of measuring the X, Y, and Z components of the magnetic field. A high input impedance d.c. potentiometer is strongly recommended for sensing theelectric field. The most important parts of the terrestrial current measurement are the nonpolarizing electrodes which can eliminate the spurious potential created by the electrochemicalpolarization of the electrodes. Furthermore, the electrodes must be carefully coupled to theground to avoid the instability of the measurement. Another critical factor in a magnetotelluric survey is the selection of the energy passband.The data should be recorded at different sampling rates for varying sounding depths. A low-cutfiltering of the data before A/D conversion may be employed to avoid the saturation of theconverter before the high frequency signal can be registered. The most troublesome problem in conducting magnetotelluric field work in Taiwan probably comes from environmental factors like the dense population, high man-made noise, and pollution. However, the magnetotelluric data collected in this area may provide some interestinginformation which could qualitatively determine the types of environmental pollution.