Abstract

Dr M. Bacon reported briefly upon the results of two reversed refraction lines shot by I.G.S. in the area north-west of Liverpool Bay in January 1971. The results show a velocity of 3.9 km/s. overlying 4.6 km/s.; the interface dips up from west to east so that the thickness of the upper layer varies from 2 km in the west to only 0.3 km in the east. Tentatively we might assign a Triassic age to the 3.9 km/s. material; the underlying higher velocity may be a deeper horizon in the Permo-Trias (perhaps salt), or possibly the Carboniferous.

Assuming densities related to seismic velocities by the Nafe-Drake curve, the gravitational effect of the lower velocity material would be to produce a maximum anomaly of only 8 mgal.

The Authors welcomed the refraction result presented by Dr Bacon and hoped that a more extended series of refraction experiments would be carried out by I.G.S. However, the –8 mgal gravity effect computed by Dr Bacon for the low velocity layer is rather less than one would expect from the gravity observations. The discrepancy may have arisen because Dr Bacon used the Nafe-Drake relationship to calculate the density of the low velocity layer relative to the underlying intermediate 4.6 km/s. layer, rather than to the deep basement beneath the basin which would probably have a velocity of about 5.8 km/s. If we relate the density of the upper low velocity layer to that of the deep basement, and include the gravity contribution from the