Passive Seismic Surveying
Passive Seismic Surveying is a geophysical method of subsurface imaging. It differs from traditional seismic in several key and cost-effective ways. The equipment is portable, about the size of a roll of toilet paper, and includes integrated memory and GPS for streamlined data collection. Operation of the equipment requires only a single person, and it is as easy as placing the instrument securely on the ground and allowing it to run for 10-20 minutes.
Unlike traditional seismic equipment that requires a large number of geophones, yards and yards of cabling and often small explosions to collect the necessary data, this method relies on background and anthropogenic noise to give us insight into what is buried beneath the surface of the earth. This is particularly interesting because noise is exactly what we are trying to minimize in our other data collection methods.
Westward has utilized this method in construction aggregate exploration. Like other geophysical methods, effective use of passive seismic relies on contrasting values between target and waste. With seismic, we are recording and evaluating velocities; how fast or how slow sound waves travel though the earth. For example, a good acoustical target would be solid bedrock, overlain by clayey soil, or a dense clay overlain by a sand and gravel deposit.
In a recent project, Westward evaluated this method for large-scale use at a site with scattered shallow deposits of high-value aggregate intermixed with sand and caliche. In that instance, the target aggregate proved to be a poor target when compared against the surrounding lithology, however, the method was successful in detecting the base of the weathered caliche overburden and the contact with the underlying stratigraphic unit.
This method requires a small degree of ground truthing, which can be done through recent exploratory efforts, examination of outcrops, test pits or even published water well records. As a portable and non-invasive method, it can be utilized in locations inaccessible by drill rig or heavy machinery, with applications in exploration and mine planning. Contact us at geology@westwardenv.com if you have any questions or would like to learn more.