The frequency characteristics of anomalous vertical fields observed in the British Isles

The frequency characteristics of anomalous verti- cal fields are presented and summarised in a consistent man- ner across a substantial area of the UK. The period range considered is from 10 to 10000 s. The results have been necessarily divided into coastal and non-coastal observa- tions. An effective coast-effect has been observed at only one location, on the west coast of Ireland. At other coastal sites, individual frequency characteristics are observed that appear dependent on site location. The controlling regional influence of the deep ocean and shelf seas at periods in excess of 1000 s is noted and to some extent defined in the results obtained across a major portion of theUK land-mass. When mainland results are compared on a regional ba- sis, spatially consistent features emerge in both the real and imaginary induction arrows. The results demonstrate that the in-quadrature component of the anomalous vertical field, although often small (i.e. <0.1), can be determined with sufficient accuracy to provide information as charac- teristic and detailed as that of the in-phase component. When transfer function estimates are well-resolved, it is demonstrated that the frequency characteristics observed in the azimuth differences and phase of the anomalous verti- cal field can be used to characterise the response observed at a given location. In particular, the detailed behaviour of such parameters with frequency may indicate the degree of near-field 2-dimensionality. A characteristic period, pre- dicted by simple 2-dimensional models, is clearly observed at a number of locations. It is pointed out that such a period is diagnostic of the structural parameters defining the geoelectric anomaly.