Overview: Matching Geophysical Measurements at the Surface to Geologic Conditions in the Subsurface
In order to optimize the characterization of a particular site - be it for developing groundwater resources for a public water supply or determining foundation characteristics for an industrial complex - one needs to be aware of subsurface conditions. And the more economically, and reliably, that this knowledge can be provided, the more economical will be the overall cost for installing and maintaining, or developing, the site. The focus of the following discussion is on how noninvasive geophysics can be used to improve the quality of knowledge of subsurface condtions at a site, which, ultimately, will provide essential guidelines for its long term management. The bottom panel on the attached schematic (with the heading "The Geologic Environment") underscores certain of the issues addressable with geophysical investigations.
Preliminary site assessment. Whether a site can be economically developed might be based on whether the subsurface material is bedrock or unconsolidated sediment.
Final site selection. Having determined that a property is generally suitable for development, where precisely should the relevant installations be located? If this involves the development of a public water well field, where should the wells be drilled? If this involves the construction of a commercial building, what are the foundation materials, and is there a better place to locate the building than another?
Scoping the actual installation plan. If excavation or drilling will be required, what is the cost of excavating trenches, holding ponds or utility lines? Can one use a simple backhoe, or will one need to employ a tractor-mounted jack hammer (hoe-ram), or even blasting?
Detailed characterization of the subsurface environment. Is it important to know the depth of the watertable? What is the integrity of the subsurface unsaturated zone above the watertable? Are subsurface materials competent?
Delineating subsurface infrastructure that is potentially hazardous to the driller or to site development. Have all buried utility lines, water mains, underground storage tanks, storage drums, etc. been identified in the immediate area of excavation, test and/or production drilling?
Specific Role of Noninvasive Geophysics
General. Environmental geophysical investigations involve applying one or more of the following techniques to noninvasively characterize the earth's subsurface relative to the above design concerns. Current methods of choice, relevant to geothermal applications, are ground penetrating radar (GPR), seismic, gravity, resistivity, electromagnetic and magnetic methods. Because even under the best of conditions, geophysical tools "see through a glass darkly", there is substantial synergy in combining two or more complementary methods to study the same area. GPR is excellent for high resolution, but often benefits from DC resistivity and/or seismic data to identify the physical nature of a particular "reflector" at depth. Seismic refractions could suggest a planar discontinuity at depth, but DC resistivity data might be needed for the interpreter to differentiate between bedrock and the groundwater table.
Applications to Groundwater and Hydrogeology. Surface geophysics is used to non-invasively determine the depth to the water table for unconfined aquifers, to characterize the distribution of soil moisture in the unsaturated zone, and to delineate the depth, thickness, and lateral dimensions of groundwater aquifers. In a follow-up mode, geophysics might define features that control aquifer recharge or discharge, such as underground channels in unconsolidated sediments, fracture zones in bedrock, impoundment features and so forth.
Applications to Environmental Assessment & Remediation. Surface geophysics can be used for identifying the presence of subsurface infrastructure such as buried utility lines, water mains, underground storage tanks and storage drums, that one needs to avoid in any drilling or excavation activity. In addition, geophysics can assess the presence and potential impact of subsurface hazardous materials and chemical contamination that need to be remediated to return a site to its intrinsic value. In developing the essential "base-line" information at a site, the use of "blind" test borings and trenching should be discouraged - inadvertently tearing up a utility service, or puncturing even one drum of certain common, but extremely toxic, products, could enormously complicate the site development, and astronomically escalate the cost of a project.
Geophysics is the only, investigatory technique that is noninvasive of the subsurface.
Reconnaissance Surveys versus Detailed Site Characterization:
Three Phases of Environmental Investigations
Phase 1: Reconnaissance of Geological and Hydrogeological Setting
Early in assessing the potential of a site, a preliminary geophysical investigation would serve a two-fold purpose:
To determine the effectiveness of various geophysical techniques in addressing the range of conditions and assessment issues specific to the hydrogeologic setting and resource needs of the client.- To contribute to the technical "screening" of prospective sites by (non-invasively) identifying
- General subsurface conditions; differentiating between whether there is bedrock or a thick soil deposit directly beneath the surface.
- For unconsolidated material (sands, gravels and clays), determine the thickness of sediment, the nature of the unsaturated zone, depth to the watertable, aquifer thickness, and/or depth to bedrock
- For areas of bedrock adjacent to surface, determine mechanical competency of bedrock, delineate fracture patterns, determine depth to watertable.
- Preliminary assessment of subsurface water quality (total dissolved solids, etc.)
- Qualitative bounds on the lateral extent of aquifers.
Consider the hypothetical case of a substantial thickness of unconsolidated sediments, with a watertable on the order of 2 to 10 meters deep. Having detected the presence of a groundwater reservoir that might be potentially developed, geophysics can then characterize its
- depth,
- hydraulic properties,
- thickness,
- lateral dimensions.
In addition, geophysics might define features which control their recharge or discharge such as
- underground channels in unconsolidated sediments,
- fracture zones in bedrock (directional patterns, etc.),
- impoundment features,
- predrilling estimates of the hydraulic characteristics of the aquifer, etc.
Surface geophysics could be essential in optimizing the design of the engineering plan for developing a site.
For developing groundwater supplies:
Beside providing the information in Phases 1 and 2 above, geophysics can be used for deciding
- the location of investigatory exploration test wells and, eventually,
- the location of production wells when a site is finally developed.
- to site the actual location where such samples are taken, then
- to extrapolate such information away from the sample point (i.e. boring or trench), and eventually
- to interpolate information between multiple sampling sites.
For siting a commercial structure:
While test cores, drilling and trenching ultimately provide the most definitive information regarding a site, surface geophysics is often needed