Magnetotelluric and Raiomagnetotelluric investigations, an example on Midsommar Island in Sweden



Recent studies show that electromagnetic methods are robust and widespread geophysical methods that can be employed to delineate the electrical conductivity of earth materials. In this paper Magnetotelluric and Radio-magnetotelluric methods have been used to map the Earth's structures.
The Magnetotelluric (MT) method is a frequency-domain electromagnet (EM) sounding technique used to investigate the electrical structure of the earth's surface. The method exploits naturally existing EM fields as a signal source. The MT method has a wide range of applications, from shallow investigations (geotechnics, ground water, and environment) to moderate and deep target in exploration of natural resources (mineral, geothermal and petroleum) depending on frequency band used.
In general the Radio-magnetotelluric (RMT) uses the MT-principles by employing artificial transmitters far off the measuring site in the frequency range from about 10 kHz up to 300 kHz. This is called Controlled source RMT method. At the same time we can take the advantage of using signals from powerful communication transmitters for submarines in the very low frequency range (VLF) from 10 to 30 kHz. In this study we used only the latter case.
Data were collected in year 2000 at one site located on Midsommar Island, west of Stockholm, Sweden. This work has been done under the coverage of Bj?rk? Energy Project which has been designed to map the structure at depth with geophysical methods and by drilling. Combined data including RMT data which reflects the characteristics of the uppermost part of the earth and MT data reflecting the characteristics of the deeper parts of the earth’s upper crust at the island are considered. Hence, a good coverage from surface to deeper part of the earth can be obtained.
For RMT measurements radio signals with frequency ranges from 15 to 30 kHz have been used while in MT method three bands have been applied with frequency ranges from 256 Hz to 8192 Hz, 8 Hz to 256 Hz, and 0.25 s to 8 Hz for band 1 to band3, respectively. In some frequencies a weak correlation were observed due to the noisy data which were collected mostly from band 3. So these data were put aside and we placed more emphasis on the rest.
MT and RMT data, resulted from 1D inversion, had been processed in order to obtain resistivity variation with depth. As a result, electrical structures from upper parts of the crust to 3000 meters depth have been plotted for this site. RMT data are being used to evaluate the characteristics of structures from the surface to 200 meters depth, which demonstrate decrease in the electrical resistivity. This parameter increases down to 600 meters depth then there is a decline in the electrical resistivity in deeper parts. From 900 meters to 1300 meters, a conducting zone has been detected by using MT data.
Inversion results correlate perfectly with borehole data which had been drilled to 964 meters depth on the island. From the surface to 200 meter depth low electrical resistivity about 300 can be detected which interpreted as brackish water. From 200 meters to 670 meters the electrical resistivity increases to about 5000and the lithology of this part indicates Jotnian sandstone with sharp boundary. From 670 meter to 900 meter the electrical resistivity exceeds. From 900 meters to 960 meters depth the electrical resistivity decreases to about 600 most probably due to the electrical nature of the breciated quartz lithology at that depth. From this depth down to 1300 meter a conducting zone is detected probably due to the abundant fractures in rocks which contain fluids and the MT data confirms this by showing resistivity lows.
In regard to the frequency gap between MT and RMT data, it is suggested to combine another electromagnetic method such as, CSMT which data can be controlled better with less noisy data while filling this gap. Professor Laust Pedersen from Uppsala University is apprecitaed for providing the data to be reprocessed and reported.