Magnetic fields of galvanic or inductive origin can be distinguished by their phase behaviour. Pure distortion fields are in-phase with the regional electric field. Hence the magnetic phase of the galvanic response approaches the regional impedance phase, as distortion effects outweigh the inductive response of an anomaly with increasing period. Hypothetical event analysis is suited extremely well to examining the common phase content of a magnetic response function array. This technique can recover the regional strike direction as well as the regional impedance phases. I have tested its application extensively by a series of 3D modelling studies, which incorporate regional 2D and local 3D structures. The approach has also been used to investigate two datasets. For the Iapetus array, which covers an area of 200 km x 300 km in N-England/S-Scotland, the technique revealed a common regional strike azimuth of ca. N125°E in the period range 500s - 2000s. This direction differs from the strike indicated by the induction arrows, which seem influenced mainly by local current concentrations along the east-west striking Northumberland Trough and the north-west striking Southern Upland Fault. Both impedance phases are positive and differ by ca. 10°, which supports the assumption of distortion fields in the dataset and that the regional structure is 2D. In the case of the BC87 profile data from British Columbia (Canada), a regional strike direction of N60°E could be recovered at long periods, which is in agreement with the results derived from earlier magnetotelluric decomposition procedures. The negative phase angle, however, indicates that the local fields are of inductive origin. Hence we do not obtain any information on the regional impedance phase. The modelling studies confirm, that with hypothetical event analysis it is always possible to recover the regional strike direction from distorted data, even if a strong regional vertical field is present in the dataset or if the local fields are generated by induction processes rather than distortion. The determination of the regional impedance phases, on the other hand, is far more sensitive to deviations from the physical distortion model.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:661177 |
| Date | January 1998 |
| Creators | Ritter, Patricia |
| Publisher | University of Edinburgh |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://hdl.handle.net/1842/11894 |
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