Magneto-Telluric Investigations in the Irish Sea and Southern Scotland

The magneto-telluric field, in the spectrum 8 s to 100 min, has been recorded at four stations, Eskdalemuir (lat 55°17′N, long 3°11′W), Portpatrick (54°51′N, 5°5′W), Port Erin (54°5′N, 4°45′W) and Nefyn (52°56′N, 4°33′ W). Records of E were obtained simultaneously on two short cables orientated N–S and E–W at Eskdalemuir, the H variations being obtained from the observatory records there. At the other stations E was recorded on six cables (9–120 km long), five of which cross part or all of the Irish Sea. The measurements were made on each cable in turn. At each station the component of H orthogonal to the cable was also measured. Assuming a top layer of resistivity 3000 ohm-metres, the profiles at Eskdalemuir indicate a thickness for this layer of 12 km. Underlying this is a layer of resistivity 45 ohm-metres which extends to a depth of 30 km; beneath this the resistivity is about 2500 ohm-metres. These results do not depend critically upon the resistivity assumed for the top layer. After corrections have been made for the estimated thickness and resistivity of the sediments in the Irish Sea, the results from five of the six long cables confirm the presence of a low resistivity (10–100 ohm-metres) layer in the middle or lower part of the crust. On comparison of the magnetic field records with the magnetograms of Eskdalemuir observatory, it was found that, although the shape of the pulsations was similar at all stations, the amplitude ratios between stations showed considerable dependence upon the period of the oscillations. This can be explained largely by anomalies in conductivity near Nefyn and Port Erin. It appears unlikely that sedimentary basins in the Irish Sea can account for more than a small part of the anomalies. The effect of finite dimensions of the source is discussed ; corrections for a possible variation of the field dimension have been made and it is found that these do not materially affect the results. Possible induction effects of the Atlantic Ocean have been considered and it is concluded that these have been negligible in this survey.