Surface waves generated by underwater explosions offshore Scotland

Underwater explosions in the Kirkcaldy Bay region of the Firth of Forth, Scotland, recorded by the British Geological Survey's LOWNET array (Crampin et al.) generate seismograms which are characterized by a long dispersed wavetrain. On the assumption that these waves are caused by the shape of the dispersion curves, phase and group velocities are analysed to determine the structural parameters of a model of the Kirkcaldy Bay region to which they are most sensitive. The shear-wave velocity and thickness of the sea-bottom sediments are found to be most influential in determining the shape of these dispersion curves. Simple synthetic seismograms constructed for a solid model of the Kirkcaldy Bay region (formed by excluding the water and sediment layers) confirm that the extended wavetrain cannot be caused by the source. The source effects increase the duration of the seismogram by only a few seconds, compared to over 30 s for the effects of dispersion, and are thus not significant. It is demonstrated that, for surface wave studies intended to probe the deeper solid basement using underwater explosions, care must be exercised in selecting a shot location free from significant sediment thickness because these sediments effectively screen structure to between 10 and 100 times their own depth. On the other hand the Kirkcaldy Bay surface wave seismograms also illustrate the detail with which sediment structure may be determined for acoustic and geological studies; for example a 40-m thick sea-bed sedimentary layer with shear-wave velocity of about 0.2 km s−1 is resolved overlying a 1.4 km s−1 solid layer of 260 m thickness.