High-resolution seismic reflection surveying of shallow marine and estuarine environments

High-frequency seismic reflection profiling is a well-established and often used technique in marine investigations. Traditionally seismic data are viewed as two-dimensional time sections. Given closely spaced profile lines, it is possible to produce posted two-way travel time maps of sub-surface reflectors which, when plotted as isometric displays, clearly show the three-dimensional spatial morphology of the sub-surface topography. With borehole control, such information can be used to provide a series of images which indicate temporal as well as spatial relationships of sub-surface reflectors. With the high-resolution afforded by high-frequency methods, detailed information on palaeo-environments can be reconstructed. Using the geophysical database as a basic framework, other aspects of the same environment can be examined in considerable detail. To demonstrate the effectiveness of these procedures, examples will be given from Plymouth Sound where a series of nested buried rock valleys has been mapped in detail from Sparker and Boomer surveys which have been interpreted in the light of newly-acquired borehole information. Isometric plots of the various sub-surface interfaces show how the channels have developed as sea level has risen over the last ca. 10,000 years. Different sedimentological facies can be resolved within the channel system thus providing information about the processes involved in their formation. Methods of improving the resolution and subsequent geological interpretation of high-resolution seismic reflection surveys are being developed for shallow marine and estuarine environments such as those found in Plymouth Sound.