Subsurface structure of SE Scotland from broadband magnetotelluric measurements

Abstract

Broadband (1000-0.01 Hz) magnetotelluric (MT) soundings have been undertaken at 22 sites in SE Scotland. Twelve of these were located in a 12 km × 6 km array near Duns where previously a deep resistivity sounding had been undertaken using the Habberjam square array technique. These sites lie to the SE of an MT traverse which crossed both the Southern Upland Fault and the previously determined Southern Uplands magnetovariation anomaly. The results from the Duns array have been used for a comparison of the resistivity and MT sounding techniques and for extension 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 station spacing and broadband soundings are clearly demonstrated by the Duns study, in which static shift at one site and downward biasing at another have been easily detected. Comparison of the electrical sounding methods demonstrates that apart from the usefulness of tensorial resistivity soundings for studies of the surficial layers, electrical resistivity structure can be obtained more easily and with much greater depth and lateral resolution by means of broadband MT soundings. An interesting feature in the electrical resistivity structure in the Duns area is the presence of a good conductor at about 1 km depth at the most easterly sites. This requires more extensive study. Further examination of the resolution of the published 2D model for the MT traverse across 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 Ireland is noted.