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.