This paper presents those results from the 1974 Lithospheric Seismic Profde in Britain (LISPB) which relate to the compressional velocity structure of the crust and uppermost mantle beneath Northern Britain. A combination of interpretation techniques suitable for modelling laterally inhomogeneous media, including twodimensional ray-tracing and timeterm analysis, has resulted in a detailed seismic cross-section across the Caledonian orogenic belt. The main features of this section are a possible horizontal discontinuity in the Pre-Caledonian' basement, a change in the relationship between the lower crust and the uppermost mantle from north to south and a considerable thickening of the crust beneath the Caledonian fold belt. These results place considerable constraints upon tectonic models for the evolution of the Caledonides in particular in their implication of differing crustal structures north and south of the Southern Uplands and their indication of the primary significance of the Southern Uplands Fault. 44 mantle beneath Northern Britain. D. Barn ford et al .This paper presents LISPB results for the Pvelocity structure of the crust and uppermost 2 The LISPB experiment: data and travel-time correlationsThe LISPB experiment has been fully described in Bamford e l al. (1976) and Kaminski et al. (1976); that part of the experiment relevant to studies of the crust and uppermost mantle beneath Northern Britain in shown in Fig. I(a).In brief, during 1974 July and August, 60 German and British seismic stations (recording three components of ground motion on magnetic tape) occupied at different times the three segments ALPHA, BETA and GAMMA. Shots were fired at the various shotpoints to build up a series of reversed and overlapping crustal profiles (Fig. I@)) with observations out to at least 180 km distance, that is, sufficient for penetration to the Moho. In addition, a local earthquake (at KEQ - Fig. l(a)) was well recorded whilst the stations were occupying segments ALPHA and BETA and a single test profile had been completed in 1973 August using land shotpoint 2 and recording slightly to the east of GAMMA (Fig. l(a)).

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The planning, execution and preliminary results of a major Anglo-German explosion seismic project are presented in this, paper I of a series. This Lithospheric Seismic Profile in Britain (LISPB) was planned as a reversed 1000 km line between two major sea-shot points off Cape Wrath in Scotland and one in the English Channel; additional sea-shots and intermediate land-shots were fired to give reversed and overlapping crustal coverage (to 180400 km distance) along the line. In all, 29 shots were fired and 60 mobile magnetic tape stations recorded three-components of ground 'motion. The resulting 14 crustal and three long-range profiles have observations at intervals of typically 2-4 km. Recordings have been digitized and four examples of filtered, computer-plotted record sections are presented to illustrate data quality. In a preliminary analysis, phase correlations are discussed and some models presented; the latter especially are more relevant to future interpretations than to geological or tectonic problems. However, significant variations in crustal thickness and in the nature of the crust-mantle transition do seem to occur beneath the British Isles.

Interpretation of upper crustal data obtained during the LISPB seismic experiment reveals the velocity structure of the pre-Caledonian basement in northern Britain. Lewisian-like basement with a relatively high seismic velocity (> 6.4 km/s) extends from the Caledonian foreland into the Midland Valley and probably terminates at the Southern Uplands fault. To the south, beneath northern England, the basement has a lower velocity (< 6.3 km/s). We suggest that a horizontal discontinuity may exist in the pre-Caledonian basement between the Southern Uplands fault and the Stublick Line though we cannot yet determine the exact nature of this discontinuity.

Seismic velocity anisotropy is predicted for cracked rocks containing either a naturally‐occurring or an artificially‐induced preferred orientation of cracks. Methods developed for the study of velocity anisotropy in the uppermost mantle have been applied to the in situ measurement of P‐wave velocity anisotropy in fractured Carboniferous Limestone in Northwest England. At three different sites, first‐arrival travel‐time data has been obtained using conventional refraction equipment and a weight‐drop source. Velocity anisotropy of 15–29% is identified with directions of maximum and minimum velocities that can be broadly related to previously mapped orientations of joints.