Crosshole and downhole seismic surveys in the UK Trias and Lias

Abstract

Abstract Crosshole and downhole seismic surveying methods are being used increasingly in geotechnical investigations to evaluate dynamic properties of soils and rocks when earthquake effects on structures have to be considered. The paper describes two case histories for which crosshole and downhole seismic surveying has been carried out. It was found that the compressional and shear wave velocities are related to depth and have a close correlation with lithology. For both sites, crosshole measurements had higher values than those obtained from downhole methods. This might be construed as evidence for anisotropy. Both sites are interbedded, one with mudstones and limestones and the other with mudstones and siltstones. It is therefore suggested that the crosshole velocities are exaggerated in the vicinity of stronger beds in horizontally layered ground. The derivation of dynamic soil properties from the results of seismic surveys is discussed using the velocity data obtained from the two sites. The sensitivity of dynamic Poisson’s ratio to low V p / V s ratios is examined and the sensitivity of dynamic shear modulus to velocity variations is demonstrated.