Seismic studies of the upper crust in central England.

The first arrival time-term analyses of four seismic refraction experiments using mainly quarry blasts as seismic sources, are presented. The analysis of the recordings from a 5 km aperture seismic array situated on the outcropping Precambrian 'Charnian' rocks of Charnwood Forest, has revealed lateral and vertical variations in the p wave velocity beneath the array. The oldest of the outcropping rocks, the Blackbrook Series, were found to have an average velocity of 5.4 km s-1, while the younger Maplewell Series have a velocity of 5.65 km s-1. A refractor, with a velocity of approximately 6.4 km s-1 has been observed below a depth of 2 km. Three, 25 to 50 km long refraction lines radiating from Charnwood Forest were subsequently undertaken. These seismic lines all mapped basement refractors with velocities of between 5.64 and 5.76 km s-1, which are considered to be either Charnian or Cambrian. Regions of basement with a dissimilar velocity have been identified on the eastern and western flanks of Charnwood and are postulated to be continuations of the Mountsorrel Granodiorite, and South Charnwood and/or South Leicestershire diorite outcrops respectively. Horst block structures have been mapped to the west and north west of Charnwood in positions that correlate with positive gravity anomalies. The mapped graben structures may contain considerable thicknesses of Carboniferous and/or Lower Palaeozoic rocks. An investigation was also carried out into the time-term method as applied to linear or near linear seismic profiles. The refractor topography is considered as having a structural wavelength ?. The errors in the solution parameters that are a consequence of this seismic method being only an approximate method for all but the simplest structures are found to depend on the structural amplitude, and the relationship of ? to the length of the seismic line and the critical distance.