Measurement of static shift in MT and CSAMT surveys

Abstract

Controlled source audio-frequency magnetotelluric (CSAMT) surveys have found frequent application in mineral exploration, and there have also been a number of attempts to use magnetotelluric (MT) soundings in the same application. The advantages of CSAMT over conventional electromagnetic methods lies in its ability (through measurements of the electric field) to discriminate resistive as well as conductive zones, and in its depth-of-penetration capability. "Static shift" however can be a common problem in many MT and CSAMT soundings. It severely distorts any depth calculations and degrades the resolution of conductivity structure inferred from scalar CSAMT measurements. Static shift also significantly restricts the MT-method in its use for geological studies of the upper and lower crust; and its application to mineral, hydrocarbon, and geothermal resource exploration. Theoretical investigation using modelling has shown that measurements of an Inductive Source Telluric Field (ISTF) may closely predict and thus potentially remove the static shift at MT and CSAMT stations. Applications of the method in the field require the additional setup of a low-power transmitter near the survey line. The main survey electrodes, cables and receiver are then used to collect additional data for corrections. This process can be run in parallel with the MT or CSAMT data acquisition, and requires little additional survey time. Initial tests of the concept in field studies near Sydney and over a skarn prospect in the Parkes vicinity proved very encouraging. The 'along-line variation' indicative of static shift was reduced by the correction by a factor of about 5 in the Parkes case, and the 'corrected' CSAMT section appeared smooth and regular as opposed to having the vertical striping typical of static problems.