Dr. Nigel Crook uses a sledge hammer to hit a seismic source that generates controlled seismic energy used to perform both reflection and refraction seismic surveys.
Article by: Jeremy Strohmeyer | Senior Project Manager | hydroGEOPHYSICS
As engineering and consulting professionals we grapple with the challenges of designing projects on top of and across unknown subsurfaces. Geologic features, visual inspections, infrastructure, and historical documents can only give us clues to what is happening beneath our feet. Geophysical tools (such as Seismic Methods) can offer an inexpensive way to quantitatively understand large areas of the subsurface.
Seismic Methods are some of the most widely used exploration techniques in the geotechnical industry. Seismic investigations have been used to improve the understanding of numerous geological, geotechnical, and hydrogeological projects. A variety of techniques can be employed including seismic refraction, seismic reflection, and multi-channel analysis of surface waves (MASW).
How it works –
The seismic refraction method is based on the measurement of the travel time of seismic (sound) waves refracted at the interfaces between subsurface layers of different velocity. A seismic wave is introduced into the subsurface via a shot point using explosives (blank shotgun cartridge), hammer blow, or an accelerated weight drop. Energy radiates out from the shot point, either traveling directly through the upper layer (direct arrival), or traveling down to and then laterally along higher velocity layers (refracted arrivals) before returning to the surface. This energy is detected on the surface at a series of receivers (geophones) spaced at regular intervals. A seismograph records the travel time for the energy to travel between source and receivers. Seismic reflection data collection is very similar, except that we are instead analyzing the time it takes for the seismic waves to reflect off subsurface interfaces with contrasting acoustic impedance (product of the seismic velocity and density). MASW data are collected similarly to seismic refraction and reflection surveys, but for this method we are concerned with the surface wave energy, also known as ground roll. The fundamental property utilized in MASW is dispersion, or change in phase velocity with frequency, which allows us to model the shear wave velocity of the subsurface.
The seismic refraction method is well suited to determining depth to bedrock and bedrock structure, based on the contrast in seismic velocity between unconsolidated alluvial and bedrock materials. Due to the dependence of seismic velocity on the elasticity and density of the material through which the energy is passing, seismic refraction surveys provide a measure of material strengths and can consequently be used as an aid in assessing rippability, rock quality, and depth to bedrock.
Seismic Methods are ideally suited to investigating layered media and we employ a variety of techniques to perform both large- and small-scale surveys. HGI has conducted surveys in a variety of situations from large-scale seismic refraction surveys in very rugged terrain to small-scale MASW surveys in dense urban areas. Seismic Methods are applicable to many areas including: depth-to-bedrock surveys, rock rippability studies, stratigraphic mapping, void detection, water table mapping, seismic hazard determination, and fault mapping.
Follow the link below for more information on Seismic Methods, and, while you are at it, why not connect with us on LinkedIn and give us a like/follow on Facebook! If you have a project where seismic imaging could benefit you or your client, give us a call and we will work to design your specialized survey. We are passionate about what we do and we take pride in giving an honest assessment of the possible applications or limitations of geophysical technologies. Cheers!
Link to hydroGEOPHYSICS Seismic Methods page
About the Author: Jeremy Strohmeyer | hydroGEOPHYSICS
Jeremy Strohmeyer is a Senior Project Manager for hydroGEOPHYSICS, Inc (HGI). He has a broad range of experience with surface, underwater, and borehole near-surface geophysical methods. Jeremy has a particular interest in seismic methods, including: refraction, reflection, and surface wave methods.
Jeremy is a licensed geologist in the states of Arizona, Kansas, Missouri, and North Carolina. He has participated in committees and leadership positions in several organizations, including EEGS, AEG, SEG, SAME, and ITE. Jeremy holds a BS in Geology & Geophysics and an MS in Geology & Geophysics from the University of Missouri-Rolla.
Jeremy’s HGI Webpage | Jeremy’s LinkedIn Page
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