Comparison of pneumatic impact and magnetostrictive vibrator sources for near surface seismic imaging in geotechnical environments

Abstract

The choice of an appropriate seismic source for a specific application is crucial for the success of seismic surveys. In this study, the utilization of two novel seismic sources is discussed for geoengineering applications in the bandwidth of 80–3000 Hz. These are a pneumatic impact and a magnetostrictive vibrator source mainly designed for seismic transmission and reflection applications. Seismic test surveys were carried out on Paleozoic sediment deposits to study a near surface fault zone at the southern rim of the Taunus mountain range in Germany and in crystalline rocks to investigate fracture zones along the Piora Adit of the Gotthard Base Tunnel in Switzerland, respectively. Both sources are evaluated with respect to their seismic wave energy, the sum of the squared amplitudes of first P- and S-arrivals and frequency content, signal to random noise ratio, as well as their suitability with regard to the specific site conditions and applications. Common Midpoint-stacked data of the Taunus survey and pre-stack depth migrated data of the Piora survey were analysed to assess the ability of the seismic sources to image geological structures and to give support for geotechnical aspects.For both test sites, the vibrator source proved to be more suitable to generate body waves than the impact source, which generates strong ground roll noise signals due to its non-controlled signal amplitude spectrum. In fact the control system of the vibrator source enabled improved adaption to varying ground conditions due to a controlled adjustment of the signal amplitude spectrum to optimize resolution and penetration for a given exploration depth.