Seismic imaging of the shallow subsurface: shear‐wave case histories

Abstract

Obtaining high‐resolution images of the geology and hydrogeology of the subsurface in the depth range from ground level to 50 m is one of the major challenges of modern geophysics. The methods which are commonly used (such as compressional‐wave surveys and ground‐penetrating radar) often suffer from adverse effects caused by the near‐surface conditions, changes in water saturation and various sources of noise. This paper demonstrates some of the advantages offered by the use of shear‐wave seismology and by the combination of shear‐ and compressional‐wave seismic methods in shallow subsurface investigations. Multicomponent shallow seismic tests were carried out at four different sites to examine the effectiveness of different acquisition geometries under a variety of near‐surface geological conditions. Near‐surface conditions encountered at the sites included thick clays, clay/sand sequences overlying Chalk, mudstone overlying granodiorite bedrock and landfill material. Under all conditions, shear‐wave data acquisition was found to have advantages over compressional‐wave acquisition for the investigation of the shallow subsurface. Shear head waves, being unaffected by water saturation, achieved penetration to greater depths at a site in Crewkerne, Dorset where compressional head‐wave penetration was limited to the near‐surface layers. Better vertical resolution was achieved at shallow depths using shear‐wave reflection energy at a landfill site. Shear‐wave reflections from shallow interfaces were in some cases less affected by noise compared with the equivalent compressional‐wave reflections. Combinations of shear‐ and compressional‐wave data recording allowed the measurement of a Poisson's ratio log and gave indications of seismic anisotropy at two sites where dipping clay layers were present.

High-resolution shear-wave seismic reflection as a tool to image near-surface subrosion structures – a case study in Bad Frankenhausen, Germany

“…As a result this survey configuration enables straightforward data processing. The slower seismic velocities of SH waves produce images of higher resolution than those produced when using P waves (e.g., Dasios et al, 1999;Inazaki, 2004). A further advantage is the autonomous suppression of surface Love waves, which occurs if the first subsurface layer is of higher seismic velocity than the second layer, which is often the case on paved or compacted roads.…”

Results From Seismic Data and Implications for Soil Dynamics Near Hospital Dr. Antonio Patricio De Alcalá, Cumaná, Sucre State, Venezuela

“…Dasios et al, 1999). La saturación de agua no sólo limita la profundidad de penetración de las ondas P, sino que reduce también el contraste de velocidades entre las diferentes litologías.…”

High-resolution shear wave reflection seismics as tool to image near-surface subrosion structures – a case study in Bad Frankenhausen, Germany

“…The slower seismic velocities of SH waves enable images of higher resolution than using P waves (Dasios et al, 1999;Inazaki, 2004). A further advantage is the autonomous suppression of surface Love waves, which occurs if the first subsurface layer is of higher seismic velocity than the second layer, which is often the case on paved or compacted roads.…”