Seismic refraction tomography is an alternative to conventional seismic refraction analysis methods. While the limitations and potential pitfalls of conventional refraction methods are wellknown the same is not true for refraction tomography. As refraction tomography becomes more widely used, the need to know and understand its capabilities as well as its limitations becomes more critical. In this study we created eight representative models for use in evaluating three commercially available codes as well as refraction tomography in general. These models range from simple two-layer or dipping-layer problems to more complicated models designed to represent features of karst terrains. We demonstrate quantitatively and qualitatively that all three codes perform at a similar level, although each has strengths and weaknesses. Refraction tomography performs well in many situations where conventional methods fail, e.g., where lateral or vertical gradients compose a significant component of the velocity structure.

Borehole flowmeters that measure horizontal flow velocity and direction of groundwater flow are being increasingly applied to a wide variety of environmental problems. This study was carried out to evaluate the measurement accuracy of several types of flowmeters in an unconsolidated aquifer simulator. Flowmeter response to hydraulic gradient, aquifer properties, and well‐screen construction was measured during 2003 and 2005 at the U.S. Geological Survey Hydrologic Instrumentation Facility in Bay St. Louis, Mississippi. The flowmeters tested included a commercially available heat‐pulse flowmeter, an acoustic Doppler flowmeter, a scanning colloidal borescope flowmeter, and a fluid‐conductivity logging system. Results of the study indicated that at least one flowmeter was capable of measuring borehole flow velocity and direction in most simulated conditions. The mean error in direction measurements ranged from 15.1° to 23.5° and the directional accuracy of all tested flowmeters improved with increasing hydraulic gradient. The range of Darcy velocities examined in this study ranged 4.3 to 155 ft/d. For many plots comparing the simulated and measured Darcy velocity, the squared correlation coefficient (r2) exceeded 0.92. The accuracy of velocity measurements varied with well construction and velocity magnitude. The use of horizontal flowmeters in environmental studies appears promising but applications may require more than one type of flowmeter to span the range of conditions encountered in the field. Interpreting flowmeter data from field settings may be complicated by geologic heterogeneity, preferential flow, vertical flow, constricted screen openings, and nonoptimal screen orientation.

This paper reports on experiments and simulations of subsurface flow from a slotted acrylic tube deployed in a sand-tank flow chamber for two different purposes. In the first instance, the slotted tube is used to represent a single fracture intersected by an uncased well. In the second instance, the slotted tube is used to represent a multislot well screen within a porous medium. In both cases, the scanning colloidal borescope flowmeter (SCBFM) measures ground water velocity within the well by imaging colloids traveling through a well to measure their speed and direction. Measurements are compared against model simulations. For the case of a slotted tube representing a single fracture, SCBFM and model results agree with respect to the flow direction and to within a factor of 1.5 for the speed near the well's center. Model and experimental agreement lend confidence that for an uncased well drilled in a fractured-rock medium, a calibrated SCBFM could be used to identify and quantify flowing features. Next, the SCBFM was deployed in a four-column multislotted casing with slots aligned with the flow direction. Another numerical model was developed to estimate the flow field within this well screen to evaluate the potential usefulness of employing the SCBFM in a screened well to estimate flow speed and direction in the surrounding porous medium. Results indicate that if the slots are not aligned with the flow, the SCBFM may only provide order-of-magnitude speed measurements and direction measurements with an uncertainty of approximately +/-25 degrees .

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Characterization of material changes with depth (profiles) in many landfill sites can be problematic for some conventional geophysical methods. Localized anomalies within the landfill can complicate mapping of underlying layers, and layered-model techniques are inappropriate for imaging laterally discontinuous landfills. Recently-developed geophysical hardware and software tools provide the opportunity to image the vertical structure of a landfill and its geologic setting. In May, 2000 a sequence of geophysical data sets were acquired at a landfill site at Camp Roberts, CA to test the benefits of new hardware and software for characterizing the three-dimensional boundaries of the landfill and the geologic setting. Conventional magnetic and electromagnetic measurements provided a backdrop for these new methods. A Geometrics G-858 magnetic gradiometer equipped with a real-time GPS positioning system was used to map the areal extent of the landfill. Resistivity, seismic refraction, and electromagnetic data were acquired along profile lines to characterize the vertical extent of the landfill and geology. Seismic refraction data were processed with conventional time-delay methods, and with newer tomographic methods. The multielectrode resistivity data were compared with data acquired with the capacitively-coupled OhmMapper system The landfill boundaries that are defined in map view by the magnetic data are supported in profile by the seismic refraction data and multielectrode resistivity data. The seismic data are most effective in identifying trench locations when a tomographic inversion is used, instead of a conventional delay-time approach to interpretation. This shows a localized high-velocity zone that coincides with the trench boundaries that are defined by the magnetic data. The multielectrode resistivity data show a disruption of layering where trenching has occurred. Both the seismic data and the multielectrode resistivity data provide evidence that the shallow geology is laterally discontinuous and heterogeneous. The high electrical conductivity of the near surface imposed limitations on the penetration depth of both the OhmMapper and multielectrode resistivity systems. The multielectrode system was better suited for penetrating this zone than was the OhmMapper.

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Seismic refraction tomography programs were evaluated with data collected over karst sites in Illinois and Kentucky, and with synthetic data. Tomographic models were evaluated for accuracy by comparison with borehole logs, with other geophysical models, and with models from more established refraction programs. Evaluation criteria included reproducibility of the tomographic models, computer time required, ease of data input and output, and sensitivity of the programs to noisy or missing data. Two separate tomography programs imaged a hydrogeologically important buried bedrock knob along intersecting lines in Bourbonnais, Illinois, beneath approximately 4 m of cover. Gross characteristics of the knob were similar between the models, but the depth and imaged shape of the knob varied. Comparison between tomographic images and borehole control at Ft. Campbell Kentucky indicate good agreement between borehole bedrock depths and tomographic model estimates. Layers in the sediment overburden, and possibly saturated zones, were also imaged. Tomographic model instability typically occurs when too many layers or velocity elements are used. This instability manifests itself in the development of high or low velocity artifacts in the lower third of the model that look similar to karstic features such as "pinnacles" and "cutters." Also, tests with synthetic models show that inversion algorithms may actually produce an asymmetrical velocity distribution of no geological significance.

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