| Abstrakt: |
The audiomagnetotelluric (AMT) sounding technique is used to obtain information on the electrical conductivity of the bedrock and it can be used therefore in the search for conducting mineral deposits such as massive sulphides. In 2011, new tensor instruments with two broadband electromagnetic (EM) receivers and necessary sensors for both high-frequency AMT and low-frequency magnetotelluric (MT) surveys were acquired by the Geological Survey of Finland (GTK). The instruments have been used in several target-scale AMT surveys where the depth of interest is from about 300 m to 2 km and the site spacing from 150 m to 1 km. This paper presents practical aspects of AMT surveys such as investigating the effects of diurnal variations in AMT source field intensity, the removal of manmade electromagnetic disturbances, the required duration of measurement, and generally performing surveys effectively. Another challenge that particularly relates to the frequencies utilized in AMT is the low signal strength at 1000-5000 Hz, which was also examined in the subtask. 3D forward modelling of AMT array data was also carried out. Field tests revealed that 15 min is an adequate duration for time series recording for good AMT data in the frequency range of 1-10 000 Hz. The use of a remote reference proved essential in the entire AMT frequency range. AMT instruments should record during the night in order to achieve good quality data in the frequency band of 1000-5000 Hz. To enable effective surveying, two field crews were trained, several practical field protocols were justified, and equipment was tailored in the project. To speed up measurements, metal sticks were tested as electrodes. A field example is presented that illustrates AMT array data from the Rovaniemi area, northern Finland. The data are visualized as sounding curves and maps. A 3D forward modelling study was carried out to model Tipper responses and off-diagonal elements of the impedance tensor. [ABSTRACT FROM AUTHOR] |
