Local and regional induction in the British Isles

Electric currents induced in the shallow seas and deep ocean around the British Isles have a profound effect upon the electromagnetic fields observed at stations on the land. The configuration of anomalous currents changes with frequency, and causes corresponding changes in the geomagnetic transfer functions. Magnetic variations have been recorded at a dense network of sites in southern Scotland and northern England. Single­ station transfer functions have been used to generate hypothetical event maps of the anomalous vertical field, and hence to infer the configuration of the anomalous internal currents. At periods exceeding 2000 s, the vertical field is dominated by the effects of electric currents to the west, presumably in the Atlantic Ocean . In the period range 400-2000 s, anomalous currents are concentrated in a thin sheet comprising the shallow seas, the t hick sequences of post-Caledonian sedimentary rock which underlie them, and the extensions of the sedimentary basins into the land. The response of the individual basin is determined not only by its local conductivity structure, but also by the extent of its connection to the shallow seas,i.e. its regional importance within the conductive sheet. At periods less than 200 s on the other hand, the anomalous fields at inland sites are principally determined by the local geological structure . These results confirm conclusions reached from theoretical studies of electromagnetic induction in a heterogeneous surface layer (Park, Orange & Madden); that electromagnetic response data measured in a 3-D environment such as the Northumberland Basin must be interpreted using 3-D models . If one or 2-D models are used, the data must be corrected on the basis of regional measurements.