SUBSURFACE STRUCTURE OF SE SCOTLAND FROM BROAD-BAND MAGNETOTELLURIC MEASUREMENTS

Broadband (1000-0.01 Hz) magnetotelluric (MT) soundings have been unde rtaken at 22 sites in SE Scotland. Twelve of these were located in a 1 2 km x 6 km array near Duns where previously a deep resistivity soundi ng had been undertaken using the Habberjam square array technique. The se sites lie to the SE of an MT traverse which crossed both the Southe rn Upland Fault and the previously determined Southern Uplands magneto variation anomaly. The results from the Duns array have been used for a comparison of the resistivity and MT sounding techniques and for ext ension southward of the 2D modelling of the traverse data. In addition , several 1D MT inversion procedures have been implemented, including one recently developed from the Zohdy resistivity inversion method. No significant differences were found between the models resulting from application of these different algorithms. The advantages of close sta tion spacing and broadband soundings are clearly demonstrated by the D uns study, in which static shift at one site and downward biasing at a nother have been easily detected. Comparison of the electrical soundin g methods demonstrates that apart from the usefulness of tensorial res istivity soundings for studies of the surficial layers, electrical res istivity structure can be obtained more easily and with much greater d epth and lateral resolution by means of broadband MT soundings. An int eresting feature in the electrical resistivity structure in the Duns a rea is the presence of a good conductor at about 1 km depth at the mos t easterly sites. This requires more extensive study. Further examinat ion of the resolution of the published 2D model for the MT traverse ac ross the Southern Uplands Fault confirms the presence of a conducting zone at shallow depth (3-5 km) in the neighbourhood of the fault. The similarity in electrical resistivity structure along this traverse and along subsequent parallel traverses in central Scotland and in Irelan d is noted.