Multi-Component Seismic Acquisition In Engineering And Environmental Investigations

Shear waves and compressional waves are sensitive to different physical properties of earth materials. In addition<br>shear waves exhibit unique phenomena associated with anisotropic material which can be used to examine<br>variations in earth properties with direction. These facts have been exploited to advantage in the oil and gas<br>industry for over 10 years. In the engineering and environmental fields, similar advantages can be demonstrated.<br>The advanced utility of multi-component seismic will be illustrated using a number of examples from both the<br>shallow and deeper fields.<br>Fracture maDDin2. In bedrock controlled hydrology, fracture and shear zones often represent the dominant<br>pathways for groundwater flow and contaminant migration. On entering such anisotropic zones, shear waves split<br>into two distinct waves with different velocities and polarization directions. The split shear waves (s-wave<br>birefringence) can be used to determine not only the strike of fractured zone but can also be used as a tool to map<br>areas of increased fracturing.<br>ManDing stratiPraDhv in unconsolidated sediments. In unconsolidated sediment sequences, important objectives<br>often are mapping clay and sand strata. In particular, the continuity of clay layers as barriers or aquitards to<br>contaminant migration is a principal objective in characterization and remediation investigations. As noted by Clark<br>et al. (1994) and Dobecki (1995) shear wave reflection surveys can, for this purpose, under certain circumstances,<br>demonstrate significant advantages over compressional wave reflection surveys, such as; higher seismic impedance<br>contrasts at clay-sand interfaces, insensitivity to water table and better resolution for detecting “thin strata.<br>However, these benefits are not always realized.<br>In shallow investigations only a small part of the potential information available in shear wave investigations is<br>typically acquired and analyzed. The full amount of information available in multi-component data has been<br>illustrated by Crampin (1985) and by Hasbrouck (1987) for the shallow field. By recording a full nine component<br>(9C) data set or some subset thereof, properties associated with shear wave birefringence can be utilized. For<br>example, the difference between Sh, or cross line, and Sv, or inline, shear wave travel times at the far offsets on a<br>surface reflection survey can reveal the magnitude of the layer anisotropy in unconsolidated sediment sequences.<br>These additional properties and the information contained within them are reviewed in further detail.