SOME UNUSUAL LOW BOUGUER ANOMALIES OF SMALL EXTENT IN CENTRAL IRELAND AND THEIR CONNECTION WITH GEOLOGICAL STRUCTURE*

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Abstract
During a regional gravity survey in Central Ireland a series of intense Bouguer anomalies of very limited extent was encountered. They are all negative, the largest being ‐6.4 mgal and each covers an area of the order of one square km. Gradients up to 9 mgal/km have been measured. The country rock is Carboniferous Limestone, density 2.7 g/cm3, but outcrops are scarce particularly in the region of the anomalies and the surface is in general covered by glacial gravels, drift and bog. Most of the anomalies lie on an approximate straight line running northeast‐southwest for a distance of 115 km.An analysis indicates that the cause of each anomaly is a mass with a negative density contrast of at least 0.3 g/cm3, probably 0.5 g/cm3, and so positioned that its top is not deeper than 0.5 km from the surface.As no rock with a density less than 2.5 g/cm3 is known to exist in the neighbourhood, it is postulated that the cause of the anomalies lies in a sequence of solution cavities in the limestone now filled with water or lightly packed rock debris and gravel aligned along a large fault. No such system has been known or suspected in Ireland.The presence of such large faults would have serious consequences on the possibility of oil accumulation while the infillings of the cavities might have economic importance.