William P. Clement

Seismic reflection and ground-penetrating radar imaging of a shallow aquifer

In 1995 and 1996, researchers associated with the US Air Force’s Phillips and Armstrong Laboratories took part in an extensive geophysical site characterization of the Groundwater Remediation Field Laboratory located at Dover Air Force Base, Dover, Delaware. This field experiment offered an opportunity to compare shallow‐reflection profiling using seismic compressional sources and low‐frequency ground‐penetrating radar to image a shallow, unconfined aquifer. The main target within the aquifer was the sand‐clay interface defining the top of the underlying aquitard at 10 to 14 m depth. Although the water table in a well near the site was 8 m deep, cone penetration geotechnical data taken across the field do not reveal a distinct water table. Instead, cone penetration tests show a gradual change in electrical properties that we interpret as a thick zone of partial saturation. Comparing the seismic and radar data and using the geotechnical data as ground truth, we have associated the deepest coherent event in...

Crosshole Radar Tomography in a Fluvial Aquifer Near Boise, Idaho

To determine the distribution of heterogeneities in the saturated zone of an unconfined aquifer in Boise, ID, we compute tomograms for three adjacent well pairs. The fluvial deposits consist of unconsolidated cobbles and sands. We used a curved-ray, finite-difference approximation to the eikonal equation to generate the forward model. The inversion uses a linearized, iterative scheme to determine the slowness distribution from the first arrival traveltimes. The tomograms consist of a layered zone representing the saturated aquifer. The velocities in this saturated zone range between 0.06 to 0.10 m/ns. We use a variety of methods to assess the reliability of our velocity models. Finally, we compare our results to neutron-derived porosity logs in the wells used for the tomograms. The comparison shows that the trends in porosity derived from the tomograms match the trends in porosity measured with the neutron probe.

Traveltime inversion of vertical radar profiles

Traveltimes of direct arrivals in vertical radar profiles VRPs are tomographically inverted to estimate the earth’s electromagnetic EM velocity between a surface transmitter and a downhole receiver. We determine the 1D interval velocity model that best fits the first-arrival traveltimes by using a weighted, damped, least-squares inversion scheme. We assess the accuracy of the velocity model using synthetic traveltimes from a known velocity-distribution model simulating an unconfined aquifer. The inverted velocity profile closely matched the velocity profile of the input model in the synthetic examples. Using vertical radar profile data from an unconfined aquifer near Boise, Idaho, we inverted traveltimes to obtain velocity estimates at the well location. The velocity change at a depth of 2.0 m corresponds well with the measured depth to the water table of 1.95 m, and at depths between 2 and 18 m, the velocities ranged between 0.06 and 0.1 m/ns. Our estimates approximately match the velocity distribution determined from neutron-derived porosity logs at depths greater than about 2 m. An important function of inverse methods is to assess quantitatively and qualitatively the uncertainty of inverted velocity estimates. We note that the velocity values in the upper and lower parts of the inverted model are not as well constrained compared to those between the depths of 4 and 13 m. From the model resolution and model covariance matrices of the real-data inversion,we determine theuncertainty in ourvelocity model, leading to more reliable interpretations of the subsurface.

Imaging complex structure in shallow seismic-reflection data using prestack depth migration

Prestack depth migration PSDM analysis has the potential to significantly improve the accuracy of both shallow seismicreflectionimagesandthemeasuredvelocitydistributions.InastudydesignedtoimagefaultsintheAlvordBasin, Oregon, at depths from 25‐1000 m, PSDM produced a detailed reflection image over the full target depth range. In contrast, poststack time migration produced significant migration artifacts in the upper 100 m that obscured reflection events and limited the structural interpretation in the shallow section. Additionally, an abrupt increase from 2500 to 3000 m/s in the PSDM velocity model constrained the interpretation of the transition from sedimentary basin fill to basementvolcanicrocks.PSDManalysisrevealedacomplex extensional history with at least two distinct phases of basin growthandamidbasinbasementhighthatformsthedivision betweentwomajorbasincompartments.

Investigating the Stratigraphy of an Alluvial Aquifer Using Crosswell Seismic Traveltime Tomography

In this study, we investigate the use of crosswell P-wave seismic tomography to obtain spatially extensive information about subsurface sedimentary architecture and heterogeneity in alluvial aquifers. Our field site was a research wellfield in an unconfined aquifer near Boise, Idaho. The aquifer consists of a 20-m-thick sequence of alluvial cobble-and-sand deposits, which have been subdivided into five stratigraphic units based on neutron porosity logs, grainsize analysis, and radar reflection data. We collected crosswell and borehole-to-surface seismic data in wells 17.1 m apart. We carefully considered the impact of well deviation, data quality control, and the choice of inversion parameters. Our linearized inverse routine had a curved-ray forward model and used different grids for forward modeling and inversion.An analysis of the model covariance and resolution matrices showed that the velocity models had an uncertainty of 10 m/s, a vertical resolution of 1 m, and a horizontal resolution of 5 m. The velocity in the saturated zone varied between 2100 m/s and 2700 m/s. Inclusion of the borehole-to-surface data eliminated the Xshaped pattern that is a common artifact in crosswell tomography, and the increased angular coverage also improved the accuracy of the model near the top of the tomogram. The final velocity model is consistent with previous stratigraphic analyses of the site, although the locations of some of the unit boundaries differ by as much as 2 m in places. The results of this study demonstrate that seismic tomography can be used to image the sedimentary architecture of unconsolidated alluvial aquifers, even when the lithologic contrasts between units are subtle.

GEOPHYSICAL SURVEYS ACROSS THE BOISE HYDROGEOPHYSICAL RESEARCH SITE TO DETERMINE GEOPHYSICAL PARAMETERS OF A SHALLOW, ALLUVIAL AQUIFER

At the Boise Hydrogeophysical Research Site (BHRS), we are characterizing the hydrogeophysical parameters of a cobble-and-sand, unconfined aquifer using a wide variety of geophysical methods. Our goal is to develop methods for mapping variations in permeability by combining non-invasive geophysical data with hydrologic measurements. We are using seismic, ground penetrating radar, and electrical methods in a variety of configurations to provide images of and parameter distributions at the BHRS. Issues such as resolution, depth of penetration, and the ability to image the desired parameters will help determine the most effective methods. Supporting data sets from the BHRS include core analyses and geophysical logs from 18 wells at the site. We will use these data to verify our geophysical interpretations. The various geophysical methods and acquisition geometries, combined with the well control, will provide an outstanding data set to characterize the heterogeneity of the subsurface beneath this alluvial aquifer, and find ways to map permeability with geophysical information. INTRODUCTION The Boise Hydrogeophysical Research Site, or BHRS (Fig. l), is being developed to support the goal of using non-invasive geophysical methods to supplement hydrologic data to map the distribution of permeability in 3-D heterogeneous shallow alluvial aquifers. In particular, the BHRS is a research wellfield in a shallow, unconfined, coarse (cobble-and-sand), braided-stream, alluvial aquifer (Fig. 2a) that is broadly similar to many other alluvial aquifers comprised of correlated random fields of facies and hydraulic parameters. Eighteen wells were cored and installed at the BHRS in 1997 and 1998, including 13 wells in the central portion of the wellfield (Fig. 1). These wells provide: (a) a dense network with direct “hard” information such as core (Fig. 2b) and permeability measurements at the individual well scale; (b) access for a wide variety of geophysical and hydrologic testing in individual wells (1 -D), between wells (2-D), and in multi-well subsurface and surface-subsurface configurations (3-D); and (c) a configuration designed to support geostatistical analysis of geophysical or hydrologic parameters (Barrash and Knoll, in press; Barrash et al., 1999). The intention is to thoroughly characterize the wellfield as a control volume with “known” 3-D distributions of sedimentary facies, geophysical parameters, and hydrologic parameters. Then responses from geophysical methods, alone and together, can be correlated against known parameter distributions to develop techniques for mapping the 3-D distribution of permeability with non-invasive geophysical methods. Initial efforts will concentrate on three generally accessible geophysical methods: seismic, ground penetrating radar (GPR), and time-domain or transient electromagnetics (TEM). Also, a variety of borehole geophysical logs are being run in all wells. The general approach at the BHRS is to establish “control” with dense, high-resolution measurements at wells (l-D), use these as calibration or reference points for level runs and tomography measurements between wells (2-D), and use these as calibration or reference 399 D ow nl oa de d 08 /2 7/ 19 to 1 32 .1 78 .1 61 .1 54 . R ed is tr ib ut io n su bj ec t t o SE G li ce ns e or c op yr ig ht ; s ee T er m s of U se a t h ttp :// lib ra ry .s eg .o rg / sections for surface profiles (2-D) and multiwell experiments (3-D). We acknowledge that differences will exist: (a) because of measurement scales and data acquisition methods, (b) because of differences in actual parameters measured with similar geophysical methods, and (c) because of differences between information contained in structural images and parameter fields. The task of analyzing any given data set will be assisted by the availability of data sets from other methods, including many coincident data sets. Indeed, part of the overall research effort includes investigation of scaling, petrophysical relationships, and multivariate relationships.

P wave velocity across a noncoaxial ductile shear zone and its associated strain gradient: Consequences for upper crustal reflectivity

In order to simulate a normal incidence reflection profile across a noncoaxial ductile shear zone, we determined P wave velocities of samples cut parallel and normal to mylonite foliation along a closely spaced profile (≈ 27 cm long) through a transition zone and its associated strain gradient. The ductile shear zone, developed within an aplitic leucogranite, was sampled from a kilometer-wide ductile transcurrent fault in the northern French Massif Central. Strain analysis indicates that the sample experienced heterogeneous and progressive simple shear deformation; shear strain (γ) systematically increases from zero in the undeformed protolith to ≈ 30 in the mylonite. The transition zone thickness (T) is about 30 cm, and the mylonite thickness (M) is about 10 cm. The amount of quartz and mica increases relative to feldspar toward the mylonite, indicating that a mineralogical composition change accompanied mylonitization. Mica and quartz developed a strong crystallographic preferred orientation (CPO). In the least strained domain, seismic anisotropy is low and mean Vp is 6 km/s at 600 MPa. Anisotropy increases up to 10% and Vp decreases up to 5.35 km/s for propagation normal to the mylonite foliation through the transition zone. This systematic velocity change correlates with the increasing γ through the transition zone and can be directly related to the CPO of mica and the increase in volume percent mica within the mylonite zone. These results indicate that velocity and anisotropy gradients may, in some cases, be associated with ductile shear zones and that mylonite boundaries may not represent first-order discontinuities. The reflectivity of a ductile shear zone depends on the thickness of the transition zone relative to the seismic wavelength (λ) and on the T/M ratio. Synthetic seismograms show that for a given seismic wavelength the reflectivity decreases when the transition zone thickness increases and when the ratio T/M increases. We show that layers with second-order boundaries (velocity gradients in transition zones) are only seismically detectable within a narrow thickness range. Extrapolation to thicker shear zones is based on the assumption that the strain gradient thickness relative to shear zone thickness is, to a first approximation, scale independent. In granitic domains, ductile shear zones with similar geometrical and petrophysical features to the example studied here will be detected on deep seismic profiles only if their width is between 20 and 400 m. Development of ductile shear zones with strain gradients of the appropriate thickness to enhance reflectivity is favored under low-temperature conditions in the granitic upper crust. Indeed, low-temperature strain gradient may explain the high seismic reflectivity of the upper crust in the Scandinavian Caledonides, whereas high-temperature strain gradient may explain, in part, the relative transparency of the European Variscan upper crust.

Reflectivity modeling of a ground-penetrating-radar profile of a saturated fluvial formation

Major horizons in radar reflection profiles may correlate with contacts between stratigraphic units or with structural breaks such as fault surfaces. Minor reflections may be caused by clutter or, in some cases, may indicate material properties or structure within stratigraphic units. In this study, we examine the physical basis for major and minor reflections observed in a shallow, unconfined, fluvial aquifer near Boise, Idaho, U. S. A. We compare a 2D profile from a surface ground-penetrating-radar reflection transect with the 1D modeled reflection profiles at three wells adjacent to the surface-reflection profile. The 1D models are based on dielectric constant and electrical conductivity values from borehole logs and vertical radar profile data. Reflections at the water table/capillary fringe, at the base of a sand-filled channel, and at the base of two sand-rich lenses in a cobble-dominated unit are recognizable in the surfacereflection profile and in all 1D reflectivity models. Less prominent reflections in stratigraphic units occur in both the surface-profile model and the reflectivity model. Although such minor reflections are not correlated easily, general similarities in their presence and location indicate that sometimes the reflections may be useful for recognizing internal facies structure or character.

3-D GPR IMAGING OF COMPLEX FLUVIAL STRATIGRAPHY AT THE BOISE HYDROGEOPHYSICAL RESEARCH SITE

A series of three-dimensional (3-D) ground-penetrating radar (GPR) data sets were acquired over the central wellfield area at the Boise Hydrogeophysical Research Site (BHRS). The survey region is 30 m x 18 m and encompasses 13 wells. The goal of the surveys is to image the complex fluvial (cobble-and-sand) stratigraphy around the wellfield. These images will be used to construct 3-D models of the sedimentary architecture and to help constrain fine-scale models of hydrologic and geophysical parameters at the site. The data sets were acquired using 25 MHz, 50 MHz, 100 MHz and 200 MHz antennas. Depth of penetration ranges from -9.6 m for the 200 MHz data to -22 m for the 25 MHz data. Processing significantly improves the reliability and interpretability of the images. The images suggest that the deposit can be subdivided laterally and vertically into several distinct units or radar architectural elements; these elements are typically separated by erosional bounding surfaces. Horizontal bedding, cross-bedding and channel structures are clearly evident in the 100 MHz and 200 MHz data, and a clay layer that underlies the cobble-and-sand aquifer at -20 m depth is successfully imaged in the 25 MHz and 50 MHz data. The water table, at a depth of l-2 m, is imaged in the 100 MHz and 200 MHz data. Time slices and vertical cuts through the data volumes are used to identify the shape and orientation of the different architectural elements, and to accurately locate important hydrostratigraphic boundaries. These data are being used to construct a 3-D model of the hydrogeologic zonation of the aquifer. Hydrologic and geophysical parameter values associated with each zone will be determined from additional field measurements (e.g., hydraulic tests in wells, crosshole radar and seismic tomography, transient electromagnetics, and well logs). The 3-D GPR surveys provide valuable information about the location, scale and geometry of different stratigraphic units at the BHRS.

VSP traveltime inversion: Near-surface issues

P-wave velocity information obtained from vertical seismic profiles (VSPs) can be useful in imaging subsurface structure, either by directly detecting changes in the subsurface or as an aid to the interpretation of seismic reflection data. In the shallow subsurface, P-wave velocity can change by nearly an order of magnitude over a short distance, so curved rays are needed to accurately model VSP traveltimes. We used a curved-ray inversion to estimate the velocity profile and the discrepancy principle to estimate the data noise level and to choose the optimum regularization parameter. The curved-ray routine performed better than a straight-ray inversion for synthetic models containing high-velocity contrasts. The application of the inversion to field data produced a velocity model that agreed well with prior information. These results show that curved-ray inversion should be used to obtain velocity information from VSPs in the shallow subsurface.