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Focus of a National Program

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The Focus of a National Geophysical Hydrology Program: A CUAHSI/Geophysics/HMT Central FacilityGround penetrating radarDC ResistivityMagnetometersGravimeter (1 unit) (digital, self-leveling; .005 mgal precision)A Workshop on a National Program for Geophysical Hydrology____________________________National Program for Geophysical HydrologyDraft Version: Tuesday, June 17, 2003- 1 -CUAHSI: Focus of a National Program for the Geophysical Characterization ofSurface and Subsurface Hydrology (A Proposal)Need for a New Level of Coordination Among Geophysicists and Hydrologists1Surface geophysics is the only hydrological tool that allows one to remotely and non-invasivelycharacterize the physical properties of materials in situ, as well as to define subsurface featurescontrolling the volumetric distribution of water, and the direction and magnitude of its flow. Inaddition, if boreholes are available, then critical constraints on physical properties in the thirddimension are provided by downhole, hole-to-hole and hole-to-surface geophysicalmeasurements. While, over the last several decades, enormous advances have been made in theacquisition, processing and interpretation of geophysical data, in order to address the increasingneeds of society to quantitatively characterize, develop, manage, and allocate our nation’s watersupplies, we require a substantially enhanced level of coordination, communication andapplication of technology that only a national organization like CUAHSI can provide2.Information that Hydrologists Need. The contemporary intersection of hydrology and nearsurface (or “environmental”) geophysics is a field of increasing relevance to the stewardship offresh water. The success of many hydrological field investigations is often a consequence of theprudent choice of the most appropriate geophysical “tools” by field-savvy personnel, along witha comprehensive reality check against the ground-truth of field samples, drilling and in situhydraulic testing. Fundamental questions about a field site that are readily resolved by acombination of geophysical methods are a) Is a site, mantled with a thin veneer of soil, underlaindirectly by bedrock, or by a relatively thick section of unconsolidated sand and gravel? − if theformer applies, b) Can one delineate the fabric of, or fracture zones in, the bedrock? − whereas ifthe latter applies, c) What is the stratigraphy and soil water content of the unsaturated (vadose)zone? − d) How deep is the watertable? − e) What is the porosity of the saturated unconsolidatedmaterial? − f) What is the configuration of the underlying bedrock? − and g) What is theevidence for impoundment features, channel deposits, and aquitards within the saturated zone?The “Toolbox” of Geophysical Hydrology. Two or more geophysical methods arecomplementary in that, for example, GPR is excellent for high resolution stratigraphic studies inrelatively shallow, resistive media, but often benefits from DC resistivity, electromagnetics (timeor frequency domain) and/or seismic data to identify the physical nature of a particular“reflector” or “refractor” at depth. If the environment is electrically conductive, and one needsinformation from depths greater than a few tens of meters, then perhaps they might selectseismic, large multi-electrode array DC resistivity, time domain electromagnetics and/or gravitymeasurements. Surface geophysics complements geologic sampling from boreholes because thelatter observations are usually limited in coverage – a borehole may provide a quality sample;but only for a limited volume. Geophysical data, at the surface or from boreholes, caneffectively extrapolate information away from the vicinity of a borehole, and interpolateinformation between boreholes. If, in addition, monitoring or producing water wells are availablefor slug or pumping tests, then one can directly determine the hydraulic properties of the target 1 For a more extended discussion of this topic, with more details on how geophysics can resolve importanthydrological questions, see Link 1. Many of the recommendations in the geophysics section were compiled froma recent ad hoc poll of a representative group of 26 geophysicists (see Link 5).2 An update of an old saw: “If a CUAHSI didn’t exist, the geophysicists would have to invent one.”____________________________National Program for Geophysical HydrologyDraft Version: Tuesday, June 17, 2003- 2 -area (i.e. the hydraulic conductivity and storativity or specific yield), and subsequently“calibrate” their geophysical analysis to substantially improve the robust interpretation of thesite.The “Ultimate Tool. Any combination of field procedures or instrumentation is only as good asthe geophysicist or the geophysical team designing and interpreting the survey. Thus the mostessential need is a core of knowledgeable, experienced individuals who can train and servicepotential users among both the hydrology and the larger geophysics communities. The need forinstrumentation, and the personnel to support its application, requires a new level ofcoordination among academics, government labs and industry.The Focus of a National Geophysical Hydrology Program:A CUAHSI/Geophysics/HMT Central Facility3Required Functions. A recent ad hoc poll of 26 near surface geophysicists4 indicated that thetype of coordination, expertise and instrumentation described by the CUAHSI/HMTF initiativeis not only an opportunity to serve hydrologists, but to serve geophysicists, as well − those nothaving access to certain facilities at their own institutions − provided the centralized facility: a) Serves as a clearing-house for resource information, by maintaining an inventory of expertsand equipment from around the country (in academia, government labs and industry)available for supporting and/or collaborating on specific hydrological studies;b) Serves as a coordinating entity for visiting scientists, collaborations and training at itscentral facility as well as at the facilities of its distributed network, test sites and study areas;c) Maintains and continually upgrade a pool of equipment (with technical support staff) thatcan be loaned on an as-needed basis for limited times for specific projects.The Central Facility as a Clearing-House for Distributed Resources. While the mostimportant aspect of the CUAHSI/HMTF will be the center’s technical support staff, the staffalone cannot be expected to comply with all


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