The use of shallow seismic techniques to characterize sub-surface quaternary deposits: the example of Porth Neigwl (Hells Mouth Bay), Gwynedd, N Wales

Forth Neigwl is a 6 km wide, drift-filled embayment, located on the Llyn Pe ninsula of northwest Wales, UK. The embayment is surrounded by hills formed of Lower Palaeozoic Pocks. Infilling the embayment is a 17 km(2) area of t hick Quaternary deposits which are well exposed in the sea-cliff These depo sits have been studied intensely over the past century. However, previous w orkers have made little reference to the total extent or architecture of th e Quaternary deposits that lie below present sea level and nothing is known about the rockhead geometry, A programme of high-resolution shallow seismi c surveys are described, including 3 km of refraction and 1 km of shallow r eflection profiling. Refraction results show that the Quaternary drift cove r is variable in composition and thickness with a maximum thickness of 80 /- 8 m. However, results indicate that rockhead within the embayment lies c onsistently at an elevation of around - 32 m OD. The spatial distribution o f refractor velocities is discussed. High-resolution shallow reflection sur veys identify three major Quaternary sequences below present sea-level that are consistent across the area. This study demonstrates that combined seis mic refraction and reflection studies are very useful in the rapid analysis of Quaternary stratigraphy and spatial depth to rockhead variations in are as of thick drift.