Groundwater hydrogeology can be a challenging subject, even when water flows through a fairly homogeneous medium of grains with interconnected flow paths, but when water moves down via gravity through various strata at some point it typically reaches a level beneath the earth where the rock is primarily interconnected crystals with little or no interconnected flow paths (no hydraulic connectivity – think granite or hard metamorphic rock). One feature of the hard rocks, however, are fractures that provide preferential pathways for water. The photo shows a fractured rock face as an example of how fractures appear in a cross section.
The fractures are typically formed because the rock has had a change in overlying pressure, or possibly some tectonic stress from faulting or folding. The use of geophysical surveys can assist in providing a more comprehensive understanding of the location, orientation and extent of fractures in rock as well as hydrogeologic behavior. A variety of geophysical tools can be useful for these investigations, including both surface geophysical instruments (non-invasive) and downhole geophysical logging which requires a borehole. Specifically, some of the most useful instrumentation includes:
- Seismic surveys (MASW and refraction)
- Electrical resistivity tomography (ERT)
- Acoustic and optical televiewers
- Fluid temperature
- Single point resistance (SPR)
The first two methods listed above are surface geophysical methods that do not require a borehole. Seismic testing measures how sound velocity varies through different materials in the subsurface and provides quantitative (velocity) data that can be used to make inferences into the density of soil and rock formations, the depth of a competent rock unit, and the presence of fractures. Seismic refraction is most useful in determining the depth to rock across a well-defined bedrock unit with a clear transition between overburden and bedrock. Fractures or down-dipping bedrock surfaces can be identified by seismic refraction testing. Multi-Channel Analysis of Surface Waves (MASW) provides an alternative to seismic refraction and can be useful in more complex geologic environments where the bedrock contact is less well-defined. MASW also provides density information within the bedrock formation itself, allowing for analysis of weak zones and possible fractures that may exist below the initial bedrock contact depth.
Electrical resistivity testing utilizes an array of electrodes at the ground surface that send and receive electrical current that is injected into the subsurface. Variations in soil type and bedrock are manifested by increases or decreases in resistivity (the inverse of conductivity). ERT can be useful in identifying the depth and integrity of a rock formation. Its direct relationship to conductivity also allows for hydrogeologic analysis of the subsurface. Zones that may exhibit increased porosity and/or water content, such as fractures within a saturated bedrock formation, will generate low resistivity anomalies that can be delineated and analyzed.
Pyramid commonly uses a multi-method surface geophysical approach for comprehensive geologic and hydrogeologic analysis of fractured rock and hydrogeology. Performing both a seismic survey and an ERT survey can help to correlate interpreted rock fractures with zones of increased water content/porosity to accurately examine focused groundwater flow and the integrity of a bedrock formation.
In addition to the surface methods described above, downhole geophysical logging can provide even higher resolution information at specific locations within a fracture rock formation. An acoustic televiewer can be sent down a borehole and, using a magnetically-oriented acoustic beam, can measure the aperture and orientation of fractures and lithologic contacts. Typically this instrument is combined with an optical televiewer, which simply records a digital image of the borehole to view these features. Fluid temperature within the borehole can also be measured with downhole instrument to analyze possible water flow pathways that are typically manifested by rapid changes in the fluid temperature. And single point resistance measurements provide a more qualitative analysis of variability in resistance in the borehole with depth that can be a function of entering and exiting fracture zones or other variables (water quality, etc.).
A combination of some or all of the above geophysical tools can provide a rapid and cost-effective approach to understanding fractured rock and hydrogeologic conditions at a project site. Contact us today to help you better understand the geologic complexity of your project site!
