An airborne geophysical survey of the Whin Sill between Haltwhistle and Scots Gap, south Northumberland

A detailed airborne geophysical survey was made of part of south Northumberland, at a flying height of 75 m with magnetic, electromagnetic (VLF-EM) and radiometric equipment mounted in a helicopter. The area of 440 km2 covered by the survey includes the outcrop of the Whin Sill, its down-dip extension and the Haydon Bridge mining district. There was some indication from available data of a spatial relationship between magnetic anomalies, attributable to faulting in the Whin Sill, and some of the known mineral veins, as well as evidence from ‘Landsat’ imagery of a broader structural control to the distribution of the mineral occurrences of the area. Particular importance was therefore attached to the magnetic results and their structural interpretation. Details are given of the equipment, survey procedure and map compilation based on information supplied by the geophysical contractor for the survey (Sander Geophysics Limited). General aspects of the interpretation of the magnetic and electromagnetic data are discussed, and detailed consideration is given to the principal features revealed by the magnetic data. The aeromagnetic map shows a clear correlation between the distribution of anomalies and the mapped outcrops of the sill, and in drift-covered areas allows more accurate delineation of the subcrop of the sill. The magnetic data also indicate that the outcrop pattern consists of a series of linear segments and it is suggested that the form of the sill was controlled during intrusion by the pre-existing joint or fault system, as well as being extensively modified by later faulting. Linear magnetic anomalies occur over the down-dip extension of the sill though it is not clear if these are necessarily entireIy due to faulting. In the Settlingstones Mine area the magnetic anomalies are clearly related spatially to the known veins and have been used to guide the search for vein extensions, while comparable anomalies elsewhere suggest new sites to be considered for detailed exploration. The VLF and radiometric data provide little obvious additional information at this stage, but further more detailed interpretation is desirable.