Velocities In Rocks Research Articles

Experimental study of the influence of water on elastic wave velocities in dunite and serpentinite (on the nature of the low-velocity zone in the upper mantle of the Earth)

Longitudinal wave velocities (V P ) in rocks were measured experimentally in dunite (olivinite) and serpentinite at a water pressure of 300 MPa and temperatures of 20–850°C. It is shown that the strong decrease in V P in dunite (by ~3 km/s) observed within the range of 400–800°C results from penetration of water into rock along microfractures and from the formation of hydrous minerals (mostly serpentine) along the boundaries of mineral grains as a result of water–olivine interaction. It is suggested that serpentinization or the formation of similar hydrous minerals in olivine-rich mantle rocks under the influence of deep fluids may result in the formation of zones of low-velocity elastic waves in the upper mantle at great depths (~100 km).

The Changes of P-Wave Velocity of Rock Samples Over Time

The main aim of this study was to determine the variation of the P- wave velocity of carbonate rocks over time. Samples of carbonate rocks like dolomite and limestone were carried out from three quarries. The study was done in May and November 2015. To test equipment Pundit Lab+ was used, which measure the transmission time of ultrasonic wave. On the base on the transmission time P- wave seismic velocities were calculated. It allows to compare the results obtained for one time interval and to calculate, using the Student's t test, if differences of P- wave seismic velocity values are significant.

Effects of Anisotropy for P-Wave Velocity on Locating Accuracy of Acoustic Emission Sources in Sandstone

The accurate measurement of wave velocity has a significant effect on the locating accuracy of acoustic emission sources, especially in health assessment and determination for rock structures. There commonly exists some deviation between the measured velocity value and the actual velocity value due to the anisotropy of rock velocity, which causes the large locating error. In this paper, the locating error caused by the anisotropy for P-wave velocity is discussed quantitatively. The locating experiments were carried out on the sandstone sample, whose length, width, and height are 100, 100, and 200 cm respectively. The mineral composition and micro structures for the sample were observed. AE sources were triggered by lead breaks and the arrival times were detected through eight sensors. Then, the sources can be located based on time difference locating method. Results show that the difference of wave velocity is large in different directions, which reaches 2400 m/s. The errors of wave velocity affect the locating accuracy seriously, which is up to 20.5%. A total of 135 groups of velocity values in different directions were measured. It is found that the locating accuracy of repeatedly measuring velocity through diagonal line is higher than accuracy of traditional used axial velocity. In addition, the source locating accuracy can be improved greatly by measuring and solving the average wave velocity in multiple directions. However, the average velocity value corresponds to the locating result is not the best. Tests clarify that the optimal velocity is higher than the average velocity. It is suggested that the wave velocity should be measured in multiple directions for several times when performing the laboratory acoustic emission (AE) sources locating experiments. The conclusions provide important theoretical guidance for the current traditional AE sources locating experiments with only average value of axial velocities.

Elasticity of calcium and calcium-sodium amphiboles

Measurements of single-crystal elastic moduli under ambient conditions are reported for nine calcium to calcium-sodium amphiboles that lie in the composition range of common crustal constituents. Velocities of body and surface acoustic waves measured by Impulsive Stimulated Light Scattering (ISLS) were inverted to determine the 13 moduli characterizing these monoclinic samples. Moduli show a consistent pattern: C33>C22>C11 and C23>C12>C13 and C44>C55∼C66 and for the uniquely monoclinic moduli, |C35|≫C46∼|C25|>|C15|∼0. Most of the compositionally-induced variance of moduli is associated with aluminum and iron content. Seven moduli (C11C12C13C22C44C55C66) increase with increasing aluminum while all diagonal moduli decrease with increasing iron. Three moduli (C11, C13 and C44) increase with increasing sodium and potassium occupancy in A-sites. The uniquely monoclinic moduli (C15C25 and C35) have no significant compositional dependence. Moduli associated with the a∗ direction (C11C12C13C55 and C66) are substantially smaller than values associated with structurally and chemically related clinopyroxenes. Other moduli are more similar for both inosilicates. The isotropically averaged adiabatic bulk modulus does not vary with iron content but increases with aluminum content from 85GPa for tremolite to 99GPa for pargasite. Increasing iron reduces while increasing aluminum increases the isotropic shear modulus which ranges from 47GPa for ferro-actinolite to 64GPa for pargasite. These results exhibit far greater anisotropy and higher velocities than apparent in earlier work. Quasi-longitudinal velocities are as fast as ∼9km/s and (intermediate between the a∗- and c-axes) are as slow as ∼6km/s. Voigt-Reuss-Hill averaging based on prior single crystal moduli resulted in calculated rock velocities lower than laboratory measurements, leading to adoption of the (higher velocity) Voigt bound. Thus, former uses of the upper Voigt bound can be understood as an ad hoc decision that compensated for inaccurate data. Furthermore, because properties of the end-member amphiboles deviate substantially from prior estimates, all predictions of rock velocities as a function of modal mineralogy and elemental partitioning require reconsideration.

P-wave velocities and anisotropy of typical rocks from the Yunkai Mts. (Guangdong and Guangxi, China) and constraints on the composition of the crust beneath the South China Sea

In order to provide constraints on the interpretation of seismic data of the crust beneath the South China Sea (SCS) and its continental margins, we have measured P-wave velocities and anisotropy as a function of hydrostatic confining pressure, up to 650MPa, for 31 representative samples (i.e., granite, diorite, felsic gneiss, mylonite and ultramylonite, amphibolite, schist, and marble) from the Yunkai Mts (Guangdong and Guangxi Provinces, China) that represent the crystalline basement beneath the continental margins of the SCS. The intrinsic velocity of each crack-free rock increases with increasing density (ρ) which is linearly dependent on the chemical composition: ρ increases with increasing MgO, CaO, FeO+Fe2O3, and Al2O3 contents, but decreases with increasing contents of SiO2andNa2O+K2O. Most of the rocks have small (<4%) or moderate (4–8%) seismic anisotropy because (1) the contribution of quartz to the bulk anisotropy opposes that of feldspar, and (2) the rocks only contain small amounts of amphibole and/or mica. The interpretation of 12 seismic transects suggests that the crust of the Cathaysia block (the southern part of South China) has a mafic-to-felsic layer thickness ratio (Rm/f) of 41–43% and the ratio shows a general increase from the continental margin to the central basin. The high velocity (7.0–7.6km/s) materials in the lower crust could be either the former lower crustal mafic rocks that were present before rifting, which have experienced less extensional thinning than the felsic upper crust, or the materials crystallized from mafic magma which underplated the lower crust from the partially molten upper mantle during rifting.

Changes to coal pores and fracture development by ultrasonic wave excitation using nuclear magnetic resonance

To maximize the extraction efficiency and quantity of coalbed methane, we improved reservoir permeability by fracturing coalbed methane reservoirs using ultrasonic wave excitation. Changes in number and diameter of pores in coal masses were determined by using nuclear magnetic resonance analysis of coal masses that had been fractured using ultrasonic waves. The development of coal fractures was monitored and analyzed using thermal imaging, a digital camera, and a measurement system for P-wave velocities in rocks. The development of fractures induced by ultrasonic waves, the pore diameters, and the number of pores in the coal masses increased, and the porosity increased by 111.8%. The entire fracturing process was divided into three stages; the initiation of micropores occurred earlier than that of mesopores, and was followed by macropore development and fracturing. P-wave tests and photographs showed that fracture networks formed inside or on coal mass surfaces under ultrasonic wave excitation, which improved the coal mass permeability significantly. Because pores that contain water provide initiation points for fractures that are caused by ultrasonic waves, the ultrasonic waves impact larger ranges and require less energy than with traditional fracturing measures. By promoting oil production using ultrasonic waves, a design concept for industrializing ultrasonic wave-based fracturing was proposed to improve coalbed methane extraction.

Novel determination of radon‐222 velocity in deep subsurface rocks and the feasibility to using radon as an earthquake precursor

AbstractA novel technique utilizing simultaneous radon monitoring by gamma and alpha detectors to differentiate between the radon climatic driving forces and others has been improved and used for deep subsurface investigation. Detailed long‐term monitoring served as a proxy for studying radon movement within the shallow and deep subsurface, as well as for analyzing the effect of various parameters of the radon transport pattern. The main achievements of the investigation are (a) determination, for the first time, of the radon movement velocity within rock layers at depths of several tens of meters, namely, 25 m/h on average; (b) distinguishing between the diurnal periodical effect of the ambient temperature and the semidiurnal effect of the ambient pressure on the radon temporal spectrum; and (c) identification of a radon random preseismic anomaly preceding the Nuweiba, M 5.5 earthquake of 27 June 2015 that occurred within Dead Sea Fault Zone.

Seismic amplification in a fractured rock site. The case study of San Gregorio (L'Aquila, Italy)

The village of San Gregorio (SG), eight kilometres away from L'Aquila (central Italy), was severely damaged by the April 6, 2009 L'Aquila earthquake (MW 6.1). A coseismic fracture zone was mapped along SW-dipping fault segments crossing SG, which is situated at the base of a carbonate relief bounded by the Aterno river alluvial plain. An interdisciplinary approach was used to investigate the seismic response of the area based on geological-structural, geophysical and seismic analyses. We integrated our data with available information from the recent microzonation studies. SG is partly built on alluvial fan deposits constituted by cemented gravel, and partly on jointed carbonate bedrock. An extensive survey of noise measurements showed strong and polarized peaks in the horizontal-to-vertical spectral ratios (H/V), both on soft and rock sites in the 2–10 Hz frequency band. Further, we checked the stability with time of H/V ratios at three sites of SG. An analysis on local earthquakes confirmed the results of noise measurements. To understand the influence of rock mass jointing condition on site effects, we performed structural surveys on carbonate bedrock. We also evaluated the propagation velocities at rock sites using seismic active and seismic dilatometer test (SDMT) surveys. Our analysis showed low values of compressional (VP) and shear wave (VS) velocities of the outcropping rock, where we also observed strong H/V spectral peak and high-density rock fracturing.

Comparison of laboratory and field measurements of P and S wave velocities of a peridotite rock

In this study, surface seismic measurements in the field at a rock outcrop (peridotite) and laboratory high frequency seismic measurements of spherical rock samples were compared. Both the field and the laboratory velocities of P- and S- waves were determined using pulse-transmission technique. Special conditions at the field site enabled multi-directional measurement in three mutually orthogonal planes. The anisotropy of seismic P- and S- wave propagation was estimated. Special measurements using neutron diffraction were carried out making it possible to establish the influence of mineral crystallographic preferred orientations (CPOs) of bulk rock sample on elastic wave propagation and its anisotropy. It was determined that the deep peridotite rocks exhibit weak anisotropy. A good directional correspondence for different seismic waves (field/laboratory) even for 3D velocity calculated based on neutron diffraction was found.

Region‐Specific Assessment, Adjustment, and Weighting of Ground‐Motion Prediction Models: Application to the 2015 Swiss Seismic‐Hazard Maps

We present a strategy for the region‐specific assessment, adjustment, and weighting of ground‐motion prediction models, with application to the 2015 Swiss national seismic‐hazard maps. The models are provided within a logic‐tree framework adopted for the probabilistic seismic‐hazard analysis (PSHA). Through this framework, we consider both aleatory and epistemic uncertainties in ground‐motion prediction in Switzerland, a region of low‐to‐moderate seismicity and consequently data poor in terms of strong‐motion records. We use both empirical models developed using global strong‐motion data and stochastic simulation models calibrated to local seismicity, characteristic wave‐propagation effects, and site conditions. The empirical models were adjusted to account for (a) the selected hazard reference rock velocity model (using V S ‐ κ adjustments) and (b) the median instrumental ground‐motion data at low magnitudes. The use of a carefully calibrated simulation model and V S ‐ κ adjusted empirical ground‐motion prediction equations allowed us to precisely define the reference‐rock profile, upon which subsequent analyses, such as microzonation and site‐specific hazard, can be applied without uncertainty related to the reference condition. We implemented partially nonergodic aleatory uncertainties in ground‐motion prediction through the single‐station sigma approach. This strategy, complemented with the known reference rock condition, leads to significant reductions in the contribution of uncertainty in ground‐motion characterization to PSHA in Switzerland. However, the application of the methodological framework outlined herein extends to any region, particularly those of low‐to‐moderate seismicity. Online Material: Tables of adjustment factors to convert selected ground‐motion prediction equations (GMPEs) to the Swiss rock reference and figures showing trellis plots of all adjusted GMPEs with uncertainty estimates.

Convertion Shear Wave Velocity to Standard Penetration Resistance

Multichannel Analysis Surface Wave (MASW) measurement is one of the geophysics exploration techniques to determine the soil profile based on shear wave velocity. Meanwhile, borehole intrusive technique identifies the changes of soil layer based on soil penetration resistance, i.e. standard penetration test-number of blows (SPT-N). Researchers across the world introduced many empirical conversions of standard penetration test blow number of borehole data to shear wave velocity or vice versa. This is because geophysics test is a non-destructive and relatively fast assessment, and thus should be promoted to compliment the site investigation work. These empirical conversions of shear wave velocity to SPT-N blow can be utilised, and thus suitable geotechnical parameters for design purposes can be achieved. This study has demonstrated the conversion between MASW and SPT-N value. The study was conducted at the university campus and Sejagung Sri Medan. The MASW seismic profiles at the University campus test site and Sejagung were at a depth of 21 m and 13 m, respectively. The shear wave velocities were also calculated empirically using SPT-N value, and thus both calculated and measured shear wave velocities were compared. It is essential to note that the MASW test and empirical conversion always underestimate the actual shear wave velocity of hard layer or rock due to the effect of soil properties on the upper layer.

Compressional wave dispersion due to rock matrix stiffening by clay squirt flow

AbstractThe standard Biot‐Gassmann theory of poroelasticity fails to explain strong compressional wave velocity dispersion experimentally observed in 12 tight siltstone with clay‐filled pores. In order to analyze and understand the results, we developed a new double‐porosity model of clay squirt flow where wave‐induced local fluid flow occurs between the micropores in clay aggregates and intergranular macropores. The model is validated based on the combined study of ultrasonic experiments on specimens at different saturation conditions and theoretical predictions. The presence of a sub‐pore‐scale structure of clay micropores contained in intergranular macropores, where the fluid does not have enough time to achieve mechanical equilibrium at ultrasonic frequencies and thus stiffens the rock matrix, provides a suitable explanation of the experimental data. Moreover, the model provides a new bound for estimating the compressional wave velocity of tight rocks saturated with two immiscible liquids. The theoretical predictions indicate that the velocity variation between gas‐ and liquid‐saturated specimens is predominantly induced by the clay squirt stiffening effect on the rock matrix and not by fluid substitution. The effect contributes more than 90% to the variation in the porosity range of 0–5%. Thus, clay squirt flow dominates the relationships between compressional wave velocity and pore fluid in tight rocks.

Frequency-dependent wave velocities in sediments and sedimentary rocks: Laboratory measurements and evidences

The pioneering work of Mike Batzle and his colleagues has provided a fundamental understanding of mechanisms behind dispersion and attenuation of elastic waves in fluid-saturated rocks. It also has made way for a realization that these phenomena need to be accounted for in a better way when interpreting seismic and sonic data from the field. Laboratory experiments have formed the basis for new insight in the past and will continue to do so. Here, examples of experimental observations that give direct or indirect evidence for dispersion in sand, sandstone, and shale are presented. Ultrasonic data from compaction tests show that Biot flow is the most likely dispersion mechanism in pure unconsolidated sand. Strong shale dispersion has been identified through low-frequency and low-strain quasistatic measurements and through a novel technique based on static loading and unloading measurements. In shale and sandstone containing clay, there is evidence for water weakening. A comparative study shows an example where the stress dependences of P- and S-wave velocities at seismic frequencies exceed those measured by traditional ultrasonic methods.

Quantitative evaluation of rock brittleness and fracability based on elastic-wave velocity variation around borehole

Abstract Brittleness and fracability are two important rock properties in hydraulic fracturing of unconventional reservoirs. Based on the variation of compressional and shear velocity around borehole using acoustic measurement, an effective technique is developed to estimate these parameters to guide reservoir-fracturing. During drilling, when the rock is broken, a significant amount of drilling induced cracks will occur in the formation around the borehole, reducing elastic wave velocity around borehole and causing the wave velocity variation away from borehole. The radial variation of compressional and shear velocities of formation rocks surrounding a borehole were respectively obtained from P-wave travel time tomography and dipole shear-wave dispersion inversion. By integrating the two variation profiles along the radial direction, the brittleness-fracability index is obtained to estimate the brittleness and fracability of formation rocks. The index shows fairly good consistency and correlation with rock brittleness and fracability, which demonstrates the practicability and effectiveness of the proposed approach. Well log data analysis examples are presented to demonstrate the effectiveness of our technique.

Modeling dynamic elastic properties of compacting chalk reservoirs using an integrated rock-physics workflow: A case study in the Ekofisk Field, Norway

An integrated rock-physics modeling workflow is discussed that aims at predicting the effective elastic moduli and density of a producing reservoir in response to pressure, porosity, and fluid-saturation changes, and that is suitable for application to compacting chalk reservoirs. A simple extended form of Nur's modified Voigt model is used to build the dry-rock elastic modulus and porosity relationship. Granular medium contact theory is used to model pressure change, and Gassmann's equation is used to model the fluid effect. A dynamic-compaction model from reservoir simulation is used to model porosity change due to compaction. The rock-physics model is calibrated at well locations from the Ekofisk Field in the North Sea; however, the absence of time-lapse logging makes the 4D calibration challenging. Effective fluid properties are calculated dynamically with the Fluid Acoustics for Geophysics (FLAG) software program, using inputs from the reservoir simulator and PVT measurements. For low water saturation (&lt; 20%) and high-porosity (&gt; 30%) rocks, the model predictions show very good correlation with measured data. The model underpredicts the sonic velocities for rocks with low porosity and high water saturation.

Correlations of Uniaxial Compressive Strength and Modulus of Elasticity with Point Load Strength Index, Pulse Velocity and Dry Density of Limestone and Sandstone Rocks in Sulaimani Governorate, Kurdistan Region, Iraq.

The Uniaxial Compressive Strength (UCS) of rocks is important parameter in design of geotechnical engineering. If the rocks are affected by weathering agents, there will be difficult to obtain intact samples and to determine UCS in laboratory.Hence, the use of another engineering index of rocks as an alternative for determining UCShave been studied and investigated by researchers. The Pulse Velocity and Point Load tests are used as a quick, easy and non-expensive means of obtaining rock strength indexes. This paper presents an experimental study for correlating between Uiaxial Compressive Strength (UCS) and Point Load Index (PLI), Pulse Velocity (Vp), Dry Density andModulus of Elasticity (E) of limestones and sandstone rock samples. 80 specimens of limestones were obtained from Pila Spi and Lower Fars Formations and 46 specimens of sandstone were obtained from Tanjero ormation. Based on the results, for the first time in this region, a new good correlation is introduced to estimate the UCS and Efrom Pulse Velocity, Point Load Index and Dry Density.

The elastic tensor of monoclinic alkali feldspars

The full elastic tensors of two K-rich monoclinic alkali feldspars, Or 83 Ab 15 sanidine and Or 93 Ab 7 orthoclase, have been determined by using the Impulse Stimulated Light Scattering technique to measure surface acoustic wave velocities. The new data confirm that alkali feldspars exhibit extreme elastic anisotropy, so the bounds of their isotropic average properties span a wide range. The measured adiabatic moduli are, for Or 83 Ab 15 and Or 93 Ab 7 , respectively, K Reuss = 54.7(7), 54.5(5) GPa; K Voigt = 62.9(1.1), 64.4(0.6) GPa; G Reuss = 24.1(1), 24.5(1) GPa; and G Voigt = 36.1(5), 36.1(7) GPa. The small differences in moduli between the samples suggests that variations in composition and in state of Al, Si order only have minor effects on the average elastic properties of K-rich feldspars. The new measurements confirm that the earliest determinations of elastic wave velocities of alkali feldspars, widely used to calculate wave velocities in rocks, resulted in velocities systematically and significantly too slow by 10% or more.

On the evolution of the elastic properties of organic-rich shale upon pyrolysis-induced thermal maturation

The evolution of the elastic properties of organic-rich shale as a function of thermal maturity remains poorly constrained. This understanding is pivotal to the characterization of source rocks and unconventional reservoirs. To better constrain the evolution of the elastic properties and microstructure of organic-rich shale, we have studied the acoustic velocities and elastic anisotropy of samples from two microstructurally different organic-rich shales before and after pyrolysis-induced thermal maturation. To more physically imitate in situ thermal maturation, we performed the pyrolysis experiments on intact core plugs under applied reservoir-magnitude confining pressures. Iterative characterization of the elastic properties of a clay-rich, laminar Barnett Shale sample documents the development of subparallel to bedding cracks by an increase in velocity sensitivity to pressure perpendicular to the bedding. These cracks, however, are not visible through time-lapse scanning electron microscope imaging, indicating either submicrometer crack apertures or predominant development within the core of the sample. At elevated confining pressures, in the absence of pore pressure, these induced cracks close, at which point, the sample is acoustically indistinguishable from the prepyrolysis sample. Conversely, a micritic Green River sample does not exhibit the formation of aligned compliant features. Rather, the sample exhibits a largely directionally independent decrease in velocity as load-bearing, pore-filling kerogen is removed from the sample. Due to the weak alignment of minerals, there is comparatively little intrinsic anisotropy; further, due to the relatively directionally independent evolution of velocity, the evolution of the anisotropy as a function of thermal maturity is not indicative of aligned compliant features. Our results have indicated that horizons of greater thermal maturity may be acoustically detectable in situ through increases in the elastic anisotropy of laminar shales or decreases in the acoustic velocities of nonlaminar shales, micritic rocks, or siltstones.

Origin of high-velocity anomalies beneath the Siberian craton: A fingerprint of multistage magma underplating since the Neoarchean

Despite the violent eruption of the Siberian Traps at ~250 Ma, the Siberian craton has an extremely low heat flow (18–25 mW/m2) and a very thick lithosphere (300–350 km), which makes it an ideal place to study the influence of mantle plumes on the long-term stability of cratons. Compared with seismic velocities of rocks, the lower crust of the Siberian craton is composed mainly of mafic granulites and could be rather heterogeneous in composition. The very high Vp (>7.2 km/s) in the lowermost crust can be fit by a mixture of garnet granulites, two-pyroxene granulites, and garnet gabbro due to magma underplating. The high-velocity anomaly in the upper mantle (Vp = 8.3-8.6 km/s) can be interpreted by a mixture of eclogites and garnet peridotites. Combined with the study of lower crustal and mantle xenoliths, we recognized multistage magma underplating at the crust-mantle boundary beneath the Siberian craton, including the Neoarchean growth and Paleoproterozoic assembly of the Siberian craton beneath the Markha terrane, the Proterozoic collision along the Sayan-Taimyr suture zone, and the Triassic Siberian Trap event beneath the central Tunguska basin. The Moho becomes a metamorphism boundary of mafic rocks between granulite facies and eclogite facies rather than a chemical boundary that separates the mafic lower crust from the ultramafic upper mantle. Therefore, multistage magma underplating since the Neoarchean will result in a seismic Moho shallower than the petrologic Moho. Such magmatism-induced compositional change and dehydration will increase viscosity of the lithospheric mantle, and finally trigger lithospheric thickening after mantle plume activity. Hence, mantle plumes are not the key factor for craton destruction.

Investigating the dependence of shear wave velocity on petrophysical parameters

The dependence of the shear wave velocity (Vs) of water-saturated reservoir rocks on petrophysical parameters was investigated. Two general regression neural network (GRNN) models were developed to predict Vs of sandstones, shaly sands, and carbonate rocks as a function of the compressional velocity (Vp), grain density (ρg), clay content, porosity (ϕ), permeability (k), and the cementation exponent (m) at a fixed effective stress and frequency. A set of 59 sample measurements of clean and dirty sandstones and carbonate rocks was used to train and test the GRNN models.The first model predicts Vs as a function of ϕ, k, ρg, m, and the clay content expressed as a percentage. This GRNN model was trained using two data sets consisting of 80% and 70% of the data, respectively. The truncated portions of the data were used as blind test sets to validate the GRNN model. The analysis reveals that the porosity, clay content, grain density, permeability, and cementation exponent are essential variables for capturing the variance of the shear wave velocity. Porosity appears to be the most important variable for estimating Vs, and the grain density is the second most important variable. The GRNN model was able to estimate Vs from both the training and the blind test data sets within an average absolute error of approximately 6%. Models from the literature that predict Vs as a function of porosity and clay content appear to be specific to particular lithologies, such as sandstones or carbonates. Modeling Vs by including parameters such as grain density, cementation exponent, and permeability in addition to porosity and clay content appears to capture the dominating physics that affects Vs. By including this comprehensive set of input parameters, we have developed a generalized model to predict the shear wave velocity Vs value for all types of lithology, such as carbonates and clean and dirty siliciclastics.A second GRNN model predicts Vs as a function of only Vp. This model was also trained using two data sets that consisted of 80% and 70% of the data, respectively. Similar to the first model, the truncated portions of the data were used as blind test sets to validate this GRNN model. The second GRNN model was able to estimate Vs as a function of Vp from the training sets within an average absolute error of approximately 4% while the average absolute error was 3% for the blind test data sets. The compressional velocity alone appears to be sufficient to predict the shear wave velocity.