Thin superficial layer and lateral heterogeneities in southern Sweden using short-period Rayleigh-wave dispersion

Abstract

SUMMARY Using short-period Rayleigh-wave dispersion data recorded along refraction lines, the shear-velocity background structure of the uppermost part of the crystalline in the southern Baltic Shield was found. On a regional scale the dispersion was grouped into dispersion regions and inverted to shear-velocity models down to about 2-3 km depth. The inferred P-velocity models are given. The models separated naturally into categories in close agreement with the large-scale geology of the area investigated. The regional shear-velocity structure varied strongly over the area and the uppermost few hundred metres of the crust. At deeper levels the structure showed a normal variation. The variation in these uppermost layers could be explained by the variation of seismic velocities with pressure in pre-stressed rock, due to partial closure of crack porosity. As there was no significant sedimentary cover in the area, this superficial layer could be attributed to the weathering layer of crystalline rocks. On a local characteristic horizontal scale of about 6 km, the lateral heterogeneities were studied using the dispersion in the 1.5-2.5 Hz band. With a perturbation method the local variations were mapped into the background shear-velocity structure along the EUGENO-S profile IV array. The local shearvelocity models were constrained to be increasing functions of depth and to satisfy the values and slopes of the known regional dispersion. Most of the 48 models could be mapped using a reference model constructed from the profile IV average model. The models defined a strongly heterogeneous weathering layer with a thickness of 0.4-0.5 km. The uppermost crustal shear background velocities correlated in detail with the local surface geology. The velocities formed two distinct populations composed essentially of granites (population I) and granitic gneisses (population 11) with PI = 3.19 f 0.08 and P,, = 2.84 i 0.15 km s-', respectively. These velocities correspond to an in situ crack-porosity density of 0.1-0.5%.