The use of geophysical tools such as magnetics and ground penetrating radar are becoming more prevalent in site characterization studies and other geologic research. Two case studies which illustrate this are described here. The first case study is in magnetics. The second case study deals with ground penetrating radar.

The re-use of existing sites is growing increasingly important for urban revitalization and prevention of urban sprawl. An extensive magnetic gradiometer survey was conducted using a Geometrics G-858 magnetometer on a previously occupied urban industrial site to locate objects in the subsurface prior to construction of the new Shreveport Convention Center in Shareveport, Louisiana.

To map the distribution of surface and buried magnetic material, two cesium vapor total-intensity magnetometers were mounted on an aluminum pole at 2.1m and 1.2m of elevation. The elevation difference between the magnetometers was used to indicate the depth of a buried object by observing the magnetic field "fall-off" between the anomalies generated when magnetic materials were present. A large "fall-off" signifies small near-surface metallic artifacts and a small "fall-off" indicates larger magnetic objects at greater depth.

From the data gathered, we were able to identify objects in the subsurface and determine where excavation would be difficult. We were also able to determine areas where caution should be exercised near buried utilities and void spaces. Magnetic surveying has been shown in this case to be a viable technique for urban site characterization prior to redevelopment.

The ground penetrating radar (GPR) study was conducted using a PulsEKKO 100 subsurface imaging radar with a 1000 volt pulser and a 25 MHz bi-static antennae arrangement on a temperate rock glacier located near Ouray, CO. A rock glacier is a mass of angular boulders and finer rock material in an ice matrix. The purpose of the study was to image the stratigraphy of the rock glacier and the substrate for implications of similar water-bearing structures on Mars. This joint study was funded by the National Aeronautics and Space Administration (NASA) and performed by the High Alpine Research Program (HARP) of both Texas A&M University and Oklahoma State University.

This study investigates the effect of mechanical stratigraphy, a fundamental characteristic of many fold and thrust belts, on ramp initiation and spacing. The causes for the sequential decrease in ramp spacing observed in some foreland fold and thrusts are examined. We present a finite element wedge model with stratigraphic heterogeneity that incorporates a pressure-dependent, elastic-perfectly plastic rheology. The study has two parts: (1) a mechanically homogeneous standard model is used to assess the effects of variations in yield stress, dilation angle and basal friction are studied to form a basis with which to evaluate (2) the effects of mechanical layering. We found that thrust ramp development always begins at the topographic inflection produced by the previous ramp but is preferentially localized in a forethrust after a sufficient amount of backstop displacement. Ramps initiate at different horizontal distances for different basal strengths in the layered models. The strongest member in a layered wedge that controls ramp location and ramp spacing. Plastic strain accumulation is noticeably diminished in existing ramps when new ramps are formed towards the foreland. However, for the 3-layer models, thrust ramps in the hinterland may be reactivated. Plastic strain accumulation continues in the hinterland much more so for the homogeneous models after more ramps are formed towards the foreland. Ramp spacing always decreases towards the foreland, predominantly a result of the wedge geometry. A plot of the spacing of the frontal ramps in layered models was is in good agreement with observed data of ramp spacing in some fold and thrust belts.

Potential arsenic toxicity in different geologic environments is dependent on total arsenic concentration and arsenic bioavailability. It is important to identify the geologic environments that may sequester arsenic because these systems can act as long-term sources for arsenic as well as retard transport and limit toxicity.

Bioavailability is defined as the readiness of a compound or element to be taken up by organisms (Gregorich et al., 2001), while potential bioavailability is possible uptake of a compound or element by organisms. The objective of this research is to quantify the potential bioavailability of arsenic in laboratory microcosms and in different geologic environments in the Nueces and San Antonio River Watersheds, Texas, using a chelating resin as an infinite sink.

To assess the applicability of chelating resins to estimate potential arsenic bioavailability in the field, iron-loaded DOWEX M4195 resin was used to extract arsenic from solutions and sediments (pond sediment, river sediment, and ephemeral stream sediment). The average percentage of arsenic sorbed from solution was 66% ± 0.16. Competition studies between arsenate, phosphate, and vanadate suggest there is moderate competition, reducing overall arsenic sorption to the resin in the presence of competing ions. Iron-loaded resin was then exposed to sediment samples spiked with increasing amounts of arsenic over 15, 30, 60 and 90 days. Results of the sediment study showed 1) increased arsenic sorption to the resin over time, 2) small variations of potential bioavailable arsenic among geologically different sediments, and 3) evidence of arsenic sequestration.

Field devices that housed iron-loaded resin were used to extract potentially bioavailable arsenic from sediment in six different geologic environments (i.e. lake, river, perennial stream, ephemeral stream, pond, and wetland) in the watersheds over a twenty-eight day period. The wetland (15.7 mmol As/g wet resin) and perennial stream sediments (11.0 mmol As/g wet resin) represented the maximal and minimal calculated potential bioavailability, respectively. However, the potentially bioavailable index calculated from mmol As/g wet resin extracted from field environments and mmol As/ g sediment in digested samples showed sequestration would be high in the wetland environment and high bioavailability in the perennial stream and river environments.

The Miocene Wonocolo Formation in the North Madura area, East Java Basin, contains numerous isolated carbonate platforms that are broadly distributed across a ~3000 sq km area of the Indonesian back-arc region. The Wonocolo platforms provide an interesting test for comparing the different growth histories of closely spaced individual platforms, where eustatic history was the same for all the platforms, but where subtle differences in other extrinsic factors influenced their growth. A grid of 2D seismic data and information from several wells across the region were used to map all Wonocolo platforms across the study area.

Five growth phases are recognizable in the platforms, based on seismic facies analysis and internal seismic-stratigraphic relationships. Platforms from the western part of the study area are larger in plan view than age-equivalent platforms to the east and record a complex history of platform initiation, backstepping, progradation, coalescence into larger composite platforms, and termination. Although all five growth phases are also recognizable in some Wonocolo platforms from the eastern part of the study area, the eastern platforms are different in that they: 1) are much smaller in plan view, 2) are spaced farther apart, 3) tend to have steeper platform margins, 4) have largely aggradational stratal geometries, 5) are slightly thicker overall than the western platforms, and 6) the tops of the platforms are at greater burial depths than the tops of the western platforms.

Most of these differences in platform morphology and growth history can be attributed to slightly faster subsidence rates in the eastern part of the study area. Faster subsidence rates in the eastern part of the study area from 12 - 6 Ma (the age range for the Wonocolo platforms) are probably related to differential surface loading by the Indonesian volcanic arc.

Several seismic properties of Lake Maracaibo are unique and difficult to understand. However, studies show that the two principal factors that affect the seismic data are likely to be the low compressional and shear wave velocities generated by the gassy sediment in the mud layer, and the high attenuation of the compressional and shear waves. This mud layer sediment is heavy and is not suspended in the water. Furthermore, it is compacted enough to support shear stresses, and therefore has a finite shear wave velocity.

In theory, the gas content of the sediment reduces the compressional wave velocity by an order of magnitude below the values for water saturated sediments, but in Lake Maracaibo, several studies show that gassy sediment reduces the compressional wave velocity from 1500m/s to between 300m/s and 700m/s. This situation produces high attenuation of the compressional waves that are traveling through the sediment in the first 100~m. However, the results of seismic modeling show that this velocity has to be between 700m/s and 900m/s in order to get reasonable results, at least in the study area.

The results show that there are very important lithological differences between the zones with and without gassy sediments in the mud layer. The best match between the raw data and the synthetic seismogram was found when an embedded rigid shale layer was located within the mud layer, in the first 100m. Unrealistic results were produced when the rigid shale layer was removed in the modeling. This rigid layer produces a destructive interference in the stoneley wave that could be observed in the seismic data and the synthetic seismogram.

In this research, the attenuation quality factor Q, an intrinsic property of rock, will be studied. Common attenuation mechanisms include grain sliding, viscous flow of pore fluid or gas, viscous relaxation and other features. Additionally, it will be shown that other factors can be proposed to account for the attenuation of compressional and shear waves in Lake Maracaibo sediments. They include: the viscous losses between the particles and the fluid immediately above the mud layer, and the solid friction losses between the particles, the mud layer and the gassy sediment. This research shows that the attenuation in the mud layer in the zones with gassy sediment and without gassy sediment are very different from each other, and that the compressional wave attenuation is larger in the mud layer zone with gassy sediment than in the mud layer zone without gassy sediment. Finally, the research shows that the ringing is an important phenomenon associated with the low velocity in the mud layer and that this ringing has more frequency content in the zone without gassy sediment than in the zone with gassy sediment.

The interpretation of 3D seismic data from Ship Shoal South Addition Blocks 349-358, Gulf of Mexico shows a complex interaction between salt, faults, and sedimentary strata.

Reconstruction of the geometry of early Pliestocene (about 3.65 Ma) through recent salt and associated sediments reveals the evolution of a supralobal basin in the study area. The basin depocenter shifted from the northeastern part to the center of the study area through time. A small, bulb-shaped, salt-stock structure occurs in the northwest, and a salt sheet structure is present in the southeastern part of the study area. Those structures are part of a pennant-shaped structure bounded by counter regional faults trending northeastward.

Salt movements created instability and triggered extensive faulting of the overlying strata. Three-dimensional reconstruction suggests that salt blocked the sediment during the early Pleistocene. The sediment was diverted around the salt high on both east and west sides of the salt body to the southwest and southeast.

Stratigraphic interpretation of the interval between 1.35 Ma and 1.95 Ma led to the identification of a highstand systems tract (HST), a transgressive systems tract (TST), and two lowstand systems tracts (LST). The strata are developed normally in the depocenter area, whereas the strata at the basin margin were deformed by salt movement and faulting.
Each systems tract is uniquely associated with a certain seismic facies. Three seismic facies were identified associated with LST, TST, and HST. Additionally, seismic sections reveal channel geometries in the LST. Seismic attribute analysis elucidates facies distribution in the systems tracts.

Because of its ability to move, to divert sediment, to create instability, and to block sediment transport pathways, salt exercises the main control on the sedimentary processes in the study area.

The effect of mean ductility, interlayer thickness, and magnitude of shortening on fracture development in bedded rock was investigated by shortening multilayer cylinders (5 cm dia.) 4 to 14% normal to layering in a triaxial apparatus. Multilayers were constructed by stacking two 1.4-cm thick layers of Berea Sandstone (relatively strong and brittle) with interlayers of Indiana Limestone (relatively weak and ductile). Thickness of the interlayer between the sandstone was 30%, 100%, or 150% the thickness of the sandstone layer. Mean ductility was varied by shortening at confining pressures (Pc) of 5, 25, 50, and 100 MPa. Sandstone layers fracture at all conditions. Fractures have preferred orientation symmetric to the cylinder axis, and display systematic spacing. At the lowest Pc and mean ductility, fractures in the sandstone are dominantly opening mode (joints) and mixed mode fractures oriented at high angles to layer boundaries. At greater Pc and mean ductility, fractures are dominantly shear mode (faults) and display conjugate geometry. Average dihedral angle of the conjugates increases from 16 to 67 degrees with increase in mean ductility. Maximum fracture density in the sandstone occurs at intermediate mean ductility and maximum interlayer thickness. Fractures propagate from the sandstone into the limestone and may link across the limestone interlayer as shortening is increased. Linkage is enhanced with decreasing mean ductility and interlayer thickness, and increasing shortening. At high mean ductility, fractures are confined to the sandstone layers. Limestone deforms by faulting and compactive cataclastic flow at low and high mean ductility, respectively. Faults in limestone are more variable in orientation and display larger dihedral angles than in the sandstone. Fracture mode and orientation are consistent with Mohr-Coulomb failure, and a spatially heterogeneous stress state where the most tensile stress occurs in the sandstone. Types of fracture networks in multilayer sequences with moderate ductility contrast vary from joints and faults, refracted faults, to faults and flow with increasing mean ductility. Fracture spacing depends on layer and interlayer thickness, mean ductility and ductility contrast, and magnitude of shortening.