P-wave Velocity Measurements Research Articles

Prediction of Porosity and Density of Calcarenite Rocks from P-Wave Velocity Measurements

Petrophysical proprieties such as porosity, density, permeability and saturation have a marked impact on acoustic proprieties of rocks. Hence, there has been recently a strong incentive to use new geophysical techniques to invert such properties from seismic or sonic measurements for rocks characterization. The P-wave velocity, which is non-destructtive and easy method to apply in both field and laboratory conditions, has increasingly been conducted to determine the geotechnical properties of rock materials. The P-wave velocity of a rock is closely related to the intact rock properties, and been measuring the velocity in rock masses describes the rock structure and texture. The present work deals with the use of a simple and non-destructive technique, ultrasonic velocity, to predict the porosity and density of calcarenite rocks that are characteristic in historical monument. The ultrasonic test is based on measuring the propagation time of a P-wave in the longitudinal direction. Good correlations between P-wave velocity, porosity and density were found, which indicated them as an appropriate technique for estimating the porosity and density.

P-wave velocity changes in freezing hard low-porosity rocks: a laboratory-based time-average model

Abstract. P-wave refraction seismics is a key method in permafrost research but its applicability to low-porosity rocks, which constitute alpine rock walls, has been denied in prior studies. These studies explain p-wave velocity changes in freezing rocks exclusively due to changing velocities of pore infill, i.e. water, air and ice. In existing models, no significant velocity increase is expected for low-porosity bedrock. We postulate, that mixing laws apply for high-porosity rocks, but freezing in confined space in low-porosity bedrock also alters physical rock matrix properties. In the laboratory, we measured p-wave velocities of 22 decimetre-large low-porosity (< 10%) metamorphic, magmatic and sedimentary rock samples from permafrost sites with a natural texture (> 100 micro-fissures) from 25 °C to −15 °C in 0.3 °C increments close to the freezing point. When freezing, p-wave velocity increases by 11–166% perpendicular to cleavage/bedding and equivalent to a matrix velocity increase from 11–200% coincident to an anisotropy decrease in most samples. The expansion of rigid bedrock upon freezing is restricted and ice pressure will increase matrix velocity and decrease anisotropy while changing velocities of the pore infill are insignificant. Here, we present a modified Timur's two-phase-equation implementing changes in matrix velocity dependent on lithology and demonstrate the general applicability of refraction seismics to differentiate frozen and unfrozen low-porosity bedrock.

Correlating P-wave Velocity with the Physico-Mechanical Properties of Different Rocks

In mining and civil engineering projects, physico-mechanical properties of the rock affect both the project design and the construction operation. Determination of various physico-mechanical properties of rocks is expensive and time consuming, and sometimes it is very difficult to get cores to perform direct tests to evaluate the rock mass. The purpose of this work is to investigate the relationships between the different physico-mechanical properties of the various rock types with the P-wave velocity. Measurement of P-wave velocity is relatively cheap, non-destructive and easy to carry out. In this study, representative rock mass samples of igneous, sedimentary, and metamorphic rocks were collected from the different locations of India to obtain an empirical relation between P-wave velocity and uniaxial compressive strength, tensile strength, punch shear, density, slake durability index, Young’s modulus, Poisson’s ratio, impact strength index and Schmidt hammer rebound number. A very strong correlation was found between the P-wave velocity and different physico-mechanical properties of various rock types with very high coefficients of determination. To check the sensitivity of the empirical equations, Students t test was also performed, which confirmed the validity of the proposed correlations.

Combining impedance inversion and seismic similarity for robust gas hydrate concentration assessments – A case study from the Krishna–Godavari basin, East Coast of India

Gas hydrate saturations were calculated based on Archie's relation and rock-physics modeling utilizing log measurements of electrical resistivity and P-wave velocity through the gas hydrate stability zone (GHSZ) at two sites in the Krishna Godavari (KG) basin off the East Coast of India. Acoustic impedance inversion was then performed around the well sites for regional extrapolation of the borehole data. Well-log based gas hydrate concentration estimates and core data are in general agreement with the seismic impedance inversion results at the individual well sites. However, the correlation with seismic data and thus the confidence in the extrapolation decreases with distance from the well site. To address the general problem of unknown regional confidence limits in the extrapolation and aid in regional gas hydrate assessment analyses, a new approach is introduced by calculating the running-sum of the seismic similarity attribute across the gas hydrate stability zone. The running-sum of the similarity attribute can be used locally on a 2D seismic line or 3D seismic volume for defining the limit of well-data extrapolation around a given well site. By normalizing the running-sum of the similarity attribute from all available 2D seismic data in the KG basin, a regional map was generated yielding effective confidence limits for extrapolation of well-log data. Such maps of regional confidence limits can be used strategically in basin-wide gas hydrate assessments as they provide a measure of probability to find a given gas hydrate concentration, and may also offer a guide for defining a minimum regional spacing between well-sites to address the overall structural complexity of the basin (which is reflected in the similarity of the seismic data).

Cross-hole seismic technique for assessing in situ rock mass conditions around a tunnel

The cross-hole seismic technique was used to identify the extent of the Excavation Damage Zone (EDZ) is in a test tunnel and in a tunnel under construction. For the cross-hole test, a new electro-mechanical impact-based transmitter was developed and tested. The transmitter was specially designed to generate appropriate impact energy and fit into a 45-mm rock bolt borehole. In the test tunnel, 79P-wave rays were measured. The interval between the boreholes was 1.5m. From the measured P-wave velocity, 2D tomography was constructed and the average velocity gradient curves at the offsets of 0.21, 0.57, 0.93, and 1.29m were drawn. Given that the wave velocity gradient curves show spatial variation due to the quality of the rock mass, upper and lower bounds that delimit the P-wave velocity were drawn. The extent of the EDZ was determined as the depth at which the average velocity is lower than the lower bound. By applying this approach, the depth of the EDZ at the test tunnel was determined to be about 1.1m from the tunnel surface. The cross-hole seismic measurement was compared with the condition of the extracted rock cores. A gray-scale image of the rock core was compared to the P-wave velocity profile. According to the image, disintegrated rock cores were present up to a depth of 1.1m. From the seismic data and a visual inspection, a similar result was found. The same approach was applied in a tunnel under construction. From the site, the observed depth of the EDZ was about 0.9m. In addition, because of the developed transmitter can generate S-waves, P- and S-wave velocities were measured and compared. The results show that S-waves can be successfully applied to identify an EDZ. The S-wave profile shows less scatter compared to a P-wave measurement.

Role of Geophysical Testing in Geotechnical Site Characterization

The lecture attempts to highlight the insights late Victor De Mello provided on some key areas. Considering the increasing role of the geophysical methods in the geotechnical site characterization, the writer focuses on the use of in-hole geophysical methods when assessing, both in field and in laboratory, the parameters depicting the soil state and its stiffness at small strain. With this aim the writer draws the attention to seismic transversal (S) and longitudinal (P) body waves generated both in field, during in-hole tests, and in laboratory using piezocristals. Within this framework the following issues are discussed: • Stiffness at very small strain as obtainable from the S and P velocities. • Difference between fully from near to saturated soils from the measured P-wave velocity. • Evaluation of undisturbed samples quality based on the comparison of S-waves velocities measured in field and in laboratory respectively. • Evaluation of porosity and void ratio from measured P and S waves velocity. • S-wave based evaluation of the coarse grained soils susceptibility to cyclic liquefaction.

Effect of Thermal Treatment on the Dynamic Fracture Toughness of Laurentian Granite

The effect of thermal treatment on the dynamic fracture toughness of Laurentian granite (LG) was investigated in this work. Notched semi-circular bend (NSCB) LG specimens are heat treated at temperatures up to 850 °C. The micro-cracks in the rock samples induced by thermal treatment are examined by scanning electron microscope (SEM). The microscopic observations are consistent with the subsequent P-wave velocity measurements, which shows that the P-wave velocity decreases with the treatment temperature monotonically when the temperature is higher than 250 °C. Dynamic fracture toughness measurements are then carried out on these samples with the dynamic load exerted by a modified split Hopkinson pressure bar (SHPB) system. The relationship between fracture toughness and treatment temperature is investigated. Experimental results show that fracture toughness increases with the loading rate but decreases with the treatment temperature. However, when the heating temperature is below 250 °C and above 450 °C, the dependence of dynamic fracture toughness on the temperature is different from other temperatures, which can be explained by the physical processes at the microscopic level of the rock due to heating. At treatment temperatures below 250 °C, the thermal expansion of grains leads to an increase in the toughness of the rock. At treatment temperatures above 450 °C, the sources of weakness such as grain boundaries and phase transition of silicon are depleted, and as a result the decrease in fracture toughness is not as significant as other treatment temperature ranges.

Elastic anisotropy of core samples from the Taiwan Chelungpu Fault Drilling Project (TCDP): direct 3-D measurements and weak anisotropy approximations

The study of seismic anisotropy has become a powerful tool to decipher rock physics attributes in reservoirs or in complex tectonic settings. We compare direct 3-D measurements of P-wave velocity in 132 different directions on spherical rock samples to the prediction of the approximate model proposed by Louis et al. based on a tensorial approach. The data set includes measurements on dry spheres under confining pressure ranging from 5 to 200 MPa for three sandstones retrieved at a depth of 850, 1365 and 1394 metres in TCDP hole A (Taiwan Chelungpu Fault Drilling Project). As long as the P-wave velocity anisotropy is weak, we show that the predictions of the approximate model are in good agreement with the measurements. As the tensorial method is designed to work with cylindrical samples cored in three orthogonal directions, a significant gain both in the number of measurements involved and in sample preparation is achieved compared to measurements on spheres. We analysed the pressure dependence of the velocity field and show that as the confining pressure is raised the velocity increases, the anisotropy decreases but remains significant even at high pressure, and the shape of the ellipsoid representing the velocity (or elastic) fabric evolves from elongated to planar. These observations can be accounted for by considering the existence of both isotropic and anisotropic crack distributions and their evolution with applied pressure.

Geophysical monitoring and reactive transport modeling of ureolytically-driven calcium carbonate precipitation.

Ureolytically-driven calcium carbonate precipitation is the basis for a promising in-situ remediation method for sequestration of divalent radionuclide and trace metal ions. It has also been proposed for use in geotechnical engineering for soil strengthening applications. Monitoring the occurrence, spatial distribution, and temporal evolution of calcium carbonate precipitation in the subsurface is critical for evaluating the performance of this technology and for developing the predictive models needed for engineering application. In this study, we conducted laboratory column experiments using natural sediment and groundwater to evaluate the utility of geophysical (complex resistivity and seismic) sensing methods, dynamic synchrotron x-ray computed tomography (micro-CT), and reactive transport modeling for tracking ureolytically-driven calcium carbonate precipitation processes under site relevant conditions. Reactive transport modeling with TOUGHREACT successfully simulated the changes of the major chemical components during urea hydrolysis. Even at the relatively low level of urea hydrolysis observed in the experiments, the simulations predicted an enhanced calcium carbonate precipitation rate that was 3-4 times greater than the baseline level. Reactive transport modeling results, geophysical monitoring data and micro-CT imaging correlated well with reaction processes validated by geochemical data. In particular, increases in ionic strength of the pore fluid during urea hydrolysis predicted by geochemical modeling were successfully captured by electrical conductivity measurements and confirmed by geochemical data. The low level of urea hydrolysis and calcium carbonate precipitation suggested by the model and geochemical data was corroborated by minor changes in seismic P-wave velocity measurements and micro-CT imaging; the latter provided direct evidence of sparsely distributed calcium carbonate precipitation. Ion exchange processes promoted through NH4+ production during urea hydrolysis were incorporated in the model and captured critical changes in the major metal species. The electrical phase increases were potentially due to ion exchange processes that modified charge structure at mineral/water interfaces. Our study revealed the potential of geophysical monitoring for geochemical changes during urea hydrolysis and the advantages of combining multiple approaches to understand complex biogeochemical processes in the subsurface.

Compressional wave velocity measurements through sandy sediments containing methane hydrate

An experimental apparatus was built to measure P-wave velocity (vP) of sandy sediment during hydrate formation from brine and free methane gas. The influences of hydrate saturation, initial brine saturation, conversion ratio of water to hydrate, and grain size of sand upon vP were investigated. The experimental results demonstrate that vP strongly depends on both hydrate saturation and initial brine saturation, whereas the influence of the grain size of sand is unremarkable. During the formation of hydrate for different experimental runs at identical initial brine saturation, vP increases with the increase of hydrate saturation; while it decreases dramatically with the increase of initial brine saturation for a given hydrate saturation. The correlation between vP and conversion ratio of water into hydrate was studied, and it was found that the relation between vP and the conversion ratio of water to hydrate is approximately the same for different experimental runs, regardless of how big the differences in initial saturation of brine and the size of sand grain are. vP was calculated for four different hydrate distribution models. The results suggest that low conversion ratio of water to hydrate, hydrate mainly acts as a load component of a dry frame matrix, whereas at higher conversion ratio of water to hydrate it partly acts as a cement that coats grains at grain contacts, and thereby leads to a dramatic increase in vP. Compared with the hydrate saturation, the conversion ratio of water to hydrate has a large role in distributing hydrate in the pores of sediments.

Pulverized fault rocks and damage asymmetry along the Arima-Takatsuki Tectonic Line, Japan

We present field and laboratory data on pulverized rocks at the Hakusui-kyo outcrop of the Arima-Takatsuki Tectonic Line (ATTL), which is a dextral strike slip fault with ~ 17 km displacement juxtaposing granite to the south against rhyolite to the north. The majority of slip at the surface is localized to a clay-rich gouge fault core 8–10 cm in width, surrounded by a coarsening outwards fault breccias up to 3 m wide. Fault damage is highly asymmetric with respect to the slipping zone. The granite south of the fault has a pulverized damage zone up to 200 m wide, while the rhyolite to the north has only about 3 m wide non-pulverized fault breccia. The degree of pulverization in the granite decreases approximately logarithmically with normal distance from the slip zone. The highly fractured pulverized rocks exhibit several distinct textural characteristics. In thin section, grains appear to be highly comminuted but the original grain shapes and margins are recognizable. Microfractures tend to be tensile in no preferred orientation. Grain fragments display little to no rotation and lack evidence of in-situ shear. Consequently, at macroscale the rocks appear to preserve original granitic textures, despite being highly fractured and friable. The observed pulverization and rock damage asymmetry are most consistent with generation mechanism involving ruptures on a bimaterial interface with statistically preferred propagation direction, leading to damage primarily on the side with higher seismic velocity at depth. This is supported by laboratory measurements of P-wave ultrasonic velocities on intact samples which indicate that the granites have consistently higher velocity than the rhyolite with increasing confining pressure.

Constraints on the Permeability Structure of Alluvial Aquifers From the Poro-Elastic Inversion of Multifrequency P-Wave Sonic Velocity Logs

We have explored the possibility of obtaining first-order permeability estimates for saturated alluvial sediments based on the poro-elastic interpretation of the P-wave velocity dispersion inferred from sonic logs. Modern sonic logging tools designed for environmental and engineering applications allow one for P-wave velocity measurements at multiple emitter frequencies over a bandwidth covering 5 to 10 octaves. Methodological considerations indicate that, for saturated unconsolidated sediments in the silt to sand range and typical emitter frequencies ranging from approximately 1 to 30 kHz, the observable velocity dispersion should be sufficiently pronounced to allow one for reliable first-order estimations of the permeability structure. The corresponding predictions have been tested on and verified for a borehole penetrating a typical surficial alluvial aquifer. In addition to multifrequency sonic logs, a comprehensive suite of nuclear and electrical logs, an S-wave log, a litholog, and a limited number laboratory measurements of the permeability from retrieved core material were also available. This complementary information was found to be essential for parameterizing the poro-elastic inversion procedure and for assessing the uncertainty and internal consistency of corresponding permeability estimates. Our results indicate that the thus obtained permeability estimates are largely consistent with those expected based on the corresponding granulometric characteristics, as well as with the available evidence form laboratory measurements. These findings are also consistent with evidence from ocean acoustics, which indicate that, over a frequency range of several orders-of-magnitude, the classical theory of poro-elasticity is generally capable of explaining the observed P-wave velocity dispersion in medium- to fine-grained seabed sediments.

Laboratory Measurements of the Effects of Methane/Tetrahydrofuran Concentration and Grain Size on the P-Wave Velocity of Hydrate-Bearing Sand

An experimental apparatus was developed to measure P-wave velocity (VP) of gas-hydrate-bearing sediment. Tetrahydrofuran (THF) was added to quicken the hydrate formation in the porous media and to synthesize hydrate-bearing sediments with uniform distribution. Methane acted as a free gas to participate in the hydrate formation. Five experimental runs were performed to examine the influence of sediment grain size and THF concentration on VP. The P-wave velocity and the amplitude for the first arrival wave signal were collected in real time during hydrate formation process. The experimental data showed that VP increases monotonically with the increase of hydrate saturation in the sediment pore space and finally tends to be a constant value. This final VP value increases with the increase of initial THF content, but the effect of sand grain size on VP is inconclusive. The variations of amplitude for the first arrival wave signal with elapsed time during hydrate formation illustrates that the amplitude increa...

Estimation of stress-dependent anisotropy from P-wave measurements on a spherical sample

Our aim is to understand the stress-dependent seismic anisotropy of the overburden shale in an oil field in the North West Shelf of Western Australia. We analyze data from measurements of ultrasonic P-wave velocities in 132 directions for confining pressures of 0.1–400 MPa on a spherical shale sample. First, we find the orientation of the symmetry axis, assuming that the sample is transversely isotropic, and then transform the ray velocities to the symmetry axis coordinates. We use two parameterizations of the phase velocity; one, in terms of the Thomsen anisotropy parameters α, β, ɛ, δ as the main approach, and the other in terms of α, β, η, δ. We invert the ray velocities to estimate the anisotropy parameters α, ɛ, δ, and η using a very fast simulated reannealing algorithm. Both approaches result in the same estimation for the anisotropy parameters but with different uncertainties. The main approach is robust but produces higher uncertainties, in particular for η, whereas the alternative approach is unstable but gives lower uncertainties. These approaches are used to find the anisotropy parameters for the different confining pressures. The dependency of P-wave velocity, α, on pressure has exponential and linear components, which can be contributed to the compliant and stiff porosities. The exponential dependence at lower pressures up to 100 MPa corresponds to the closure of compliant pores and microcracks, whereas the linear dependence at higher pressures corresponds to contraction of the stiff pores. The anisotropy parameters ɛ and δ are quite large at lower pressures but decrease exponentially with pressure. For lower pressures up to 10 MPa, δ always is larger than ɛ; this trend is reversed for higher pressures. Despite the hydrostatic pressure, the symmetry axis orientation changes noticeably, in particular at lower pressures.

Evaluation of intrinsic velocity-pressure trends from low-pressure P-wave velocity measurements in rocks containing microcracks

Dependent on the ‘intrinsic’ effects on the crystal lattice of the rock constituents and the diminishing ‘extrinsic’ effects of pores and microcracks, elastic wave velocity versus pressure trends in cracked rocks are characterized by non-linear velocity increase at low pressure. At high pressure the ‘extrinsic’ influence vanishes and the velocity increase becomes approximately linear. Usually, the transition between non-linear and linear behaviour, the ‘crack closure pressure’, is not accessible in an experiment, because actual equipment is limited to lower pressure. For this reason, several model functions for describing velocity—pressure trends were proposed in the literature to extrapolate low-pressure P-wave velocity measurements to high pressures and, in part, to evaluate the ‘intrinsic’ velocity—pressure trend from low-pressure data. Knowing the ‘intrinsic’ velocity trend is of particular importance for the quantification of the crack influence at low pressure, at high pressure, the ‘intrinsic’ trend describes the velocity trend as a whole sufficiently well. Checking frequently used model functions for suitability led to the conclusion that all relations are unsuitable for the extrapolation and, if applicable, the estimation of the ‘intrinsic’ velocity trend. However, it can be shown that the ‘intrinsic’ parameters determined by means of a suitable model function, the zero pressure velocity and the pressure gradient depend on maximum experimental pressure in a non-linear way. Our approach intends to obtain better estimates of particular parameters from observed non-linear behaviour. A converging exponential function is used to approximate particular trends, assuming that the point of convergence of the function represents a better estimate of the zero pressure velocity and the pressure gradient, respectively. Whether the refined ‘intrinsic’ velocity trend meets the ‘true intrinsic’ velocity trend within acceptable errors cannot be proven directly due to missing experimental data at very high pressure. We, therefore, conclude that our approach cannot ensure absolutely certain ‘intrinsic’ velocity trends, however, it can be shown that the optimized trends approximate the ‘true intrinsic’ velocity trend better as all the other relations do.

Electrical conductivity and P-wave velocity in rock samples from high-temperature Icelandic geothermal fields

Measurements of electrical conductivity and P-wave velocity of seven rock samples were made in the laboratory under inferred in situ conditions. The samples were collected from smectite and chlorite alteration zones in boreholes from the Krafla and Hengill, Iceland, geothermal areas. The measurements were done in the 25–250 °C range, with pore pressure and confining pressure equal to inferred in situ hydrostatic and lithostatic pressures, respectively. Conductivity increases linearly with temperature over the 30–170 °C range; that rise is considerably smaller above 170 °C. Time-dependent effects on conductivity occur above approximately 100 °C. These effects may be related to ion exchange between the clay minerals or the Stern layer, and the pore fluid. The temperature coefficient of conductivity is found to be considerably higher than attributed to pore fluid conduction alone, indicating interface conduction in an electrical double layer on the mineral-water interface in the pores. The results also show that there is no distinction in electrical conduction mechanism in the smectite and chlorite alteration zones; both are dominated by interface conductivity under in situ conditions. The sharp decrease in conductivity at the top of the chlorite alteration zone, commonly observed in resistivity surveys in high-temperature geothermal systems, is most likely due to the lower cation exchange capacity of chlorite compared to that of smectite.

Estimation of Atterberg limits and bulk mass density of an expansive soil from P-wave velocity measurements

This brief technical note reports the relationship between P-wave velocity and the Atterberg limits and bulk mass density of an expansive soil from the Derince region of Turkey. Reasonably good correlations were found, which were improved when the relationship was between P-wave velocity divided by water content.

Laboratory measurements of P-wave and S-wave velocities across a surface analog of the continental crust–mantle boundary: Cabo Ortegal, Spain

The Paleozoic Cabo Ortegal Complex of NW Spain provides an exposed analog of the continental crust–mantle transition. It is composed of an overturned section that, at its base, begins with felsic gneisses, followed upward by eclogites, intermediate and mafic granulties, and ultramafic rocks. Peak metamorphic conditions reached c. 800 °C and 1.7 GPa in the Middle to Late Devonian. Fourteen samples were analysed for P-wave and S-wave velocities, as well as density at the High Pressure Lab at Dalhousie University, Canada. Seismic velocities were measured at pressures of 10 to 600 MPa at a temperature of 20 °C. When possible, measurements were made parallel and perpendicular to banding and parallel to the lineation. The major element composition of each sample was measured by XRF at the University of Barcelona, Spain. Samples display a broad range of P-wave and S-wave velocities (6.2 to 8.2 km/s and 3.2 to 4.6 km/s at 600 MPa, respectively) that generally increase with density (2.7 to 3.4 g/cm 3) and reflect an overall increase from middle to lower crustal velocities in the felsic gneisses and intermediate to mafic granulites to mantle velocities in the eclogites and ultramafic rocks. The seismic Moho (P-wave velocity > 7.6 km/s) is reached at the mappable contact between the gneisses and the eclogite, whereas the compositional Moho, or crust–mantle transition occurs at the transitional contact between the mafic granulites and peridotites. Between 200 and 600 MPa, P-wave anisotropy ranges from between 2% and 8%, whereas S-wave anisotropy ranges from < 1% to around 8%, according to rock type. Poisson's ratios calculated from the laboratory measurements are within the range of those determined from field experiments elsewhere. P-wave reflection coefficients between the various lithologies range from 0.21 to 0.08. These laboratory data provide a calibration for the physical properties and the nature of reflectivity of the in-situ lower continental crust and upper mantle transition.

Evidence of dilatant and non-dilatant damage processes in oolitic iron ore:P-wave velocity and acoustic emission analyses

Uniaxial and triaxial compression experiments were performed on oolitic iron ores to investigate damage processes. Most of these experiments included four indirect measurements of damage evolution, that is, P-wave velocity and maximum amplitude received during pulse transmission experiments, elastic properties (apparent Young´s modulus and apparent Poisson´s ratio) and acoustic emission (AE) monitoring. The mechanical behaviour deduced from strain measurements is dilatant for some samples and non-dilatant for the other samples. However, variations in elastic properties indicate damage processes for all samples. AE source mechanism analysis shows two different microscopic damage processes: (1) for dilatant rock, the development of axial extensive microcracks as well as their interaction and coalescence lead to the formation of shear macroscopic discontinuities; (2) for non-dilatant oolitic iron ore, both compressive and shear micromechanisms take place and interact with macroscopic fractures. A particular consistency between the four types of measurements employed was observed.

Application of Seismic Refraction Tomography in Karst Terrane

Seismic refraction tomography field data were collected on several bridge foundation sites in Pennsylvania, in close proximity to geotechnical boring locations. Profiles determined from these field measurements were plotted against drilling data, and these comparisons revealed the ability of seismic wave velocities to differentiate overburden soil from rock. In addition, foundation construction data were collected at each of the sites and compared with refraction test results determined prior to construction. In particular, top of rock revealed by an excavation, and pile tip elevations at driving refusal, were compared with refraction test results. From these data it appears that seismic wave tomograms can characterize the soil/rock interface, and that it is possible to predict expected design pile lengths based upon a measured P-wave velocity tomogram. It can be concluded from these site comparisons that geophysical techniques such as seismic refraction tomography can provide important additional information to site characterization for bridge foundations in karst terrane. However, these techniques should not be viewed as a replacement, but should be conducted during design stage site investigation to aid selection of borehole locations and other testing needs.