Unlike Engineering or Environmental  projects, geophysical groundwater projects tend to be more complex, yet require less resolution or precision in the measurements.  For example, detection of a fault at a depth of several hundred feet, does not require the precision in the measurements that is required to design a bridge foundation.  Therefore, the projects often cover broad areas that may include an entire valley or mountainside, and as such, the team must have a good understanding of the geologic structure.  Large-scale USGS maps can guide the geophysical program design, in most cases, but a greater understanding of the site conditions by our clients leads to the:  choice of the method(s), survey design, and data acquisition parameters.  These are all critical to success of the survey, except for the presence of water, of course.

In most circumstances, more than one method is required to effectively determine where to drill a test hole, or a production well.  In general, the detection of a fault, or series of faults, is not difficult (using seismic), but determining if water exists along those structures requires applying EM or ER survey techniques.  Seismic cannot alone determine the presence of water, and EM or ER are not effective at imaging structures.  We have the capability with this combined-method approach to target both the structure and presence of water. This approach then allows a prediction for the presence of (fresh) groundwater and anticipated depth to intersect structure(s) or water-bearing formations (i.e., aquifers).  In unconsolidated alluvial settings, the method selection is determined by the objective of finding the overall thickness of the soil deposits, or determining the presence of impermeable clay layers and/or lenses or permeable sand and gravels.

Aquifer Storage Recharge (ASR) is a burgeoning approach to storing groundwater in the porous alluvial deposits in the valley floor of large river drainages.  Olson has helped with defining bedrock characteristics in order to understand the lateral continuity of the surface, its character and quality, as well as aid in the design of the cut-off wall for: depth to the bedrock surface to ensure positive cutoff, avoid obstacles (e.g., landfill materials or indurated soil deposits), and continuity of known soil conditions. Geophysics is a valuable tool to ASR, as drill holes alone cannot cover the entire area impacted by the storage of water below the ground surface.

For groundwater investigations, Olson has worked with clientele ranging from home owners to hydrologic consulting companies, as well as municipalities.  Due to this wide range of clients, we tailor the scale of work and the complexity of the survey to meet the clients’ desire. For example, is the purpose of the groundwater investigation to produce a 10 to 15 gpm well for residential consumption, or construction of multiple wells for a high-yield groundwater well field?  Similarly, in unconsolidated / unconfined aquifer settings, meeting the need for a single well or aiding in the design of a shallow alluvial gallery is the advantage geophysical imaging provides hydrologists.  Understanding the yield you need from groundwater resources beneath a site dictates how Olson approaches the finding of drilling target(s).