Formation Shear Wave Velocity Research Articles

Comparing the performance of machine learning methods in estimating the shear wave transit time in one of the reservoirs in southwest of Iran.

Shear wave transit time is a crucial parameter in petroleum engineering and geomechanical modeling with significant implications for reservoir performance and rock behavior prediction. Without accurate shear wave velocity information, geomechanical models are unable to fully characterize reservoir rock behavior, impacting operations such as hydraulic fracturing, production planning, and well stimulation. While traditional direct measurement methods are accurate but resource-intensive, indirect methods utilizing seismic and petrophysical data, as well as artificial intelligence algorithms, offer viable alternatives for shear wave velocity estimation. Machine learning algorithms have been proposed to predict shear wave velocity. However, until now, a comprehensive comparison has not been made on the common methods of machine learning that had an acceptable performance in previous researches. This research focuses on the prediction of shear wave transit time using prevalent machine learning techniques, along with a comparative analysis of these methods. To predict this parameter, various input features have been employed: compressional wave transit time, density, porosity, depth, Caliper log, and Gamma-ray log. Among the employed methods, the random forest approach demonstrated the most favorable performance, yielding R-squared and RMSE values of 0.9495 and 9.4567, respectively. Furthermore, the artificial neural network, LSBoost, Bayesian, multivariate regression, and support vector machine techniques achieved R-squared values of 0.878, 0.8583, 0.8471, 0.847 and 0.7975, RMSE values of 22.4068, 27.8158, 28.0138, 28.0240 and 37.5822, respectively. Estimation analysis confirmed the statistical reliability of the Random Forest model. The formulated strategies offer a promising framework applicable to shear wave velocity estimation in carbonate reservoirs.

A new machine learning approach for estimating shear wave velocity profile using borelog data

The shear wave velocity is among the key parameters that are responsible for damage caused during an earthquake. Determining shear wave velocity is a costly and time-consuming direct geophysical method. In the present paper, a machine learning model has been developed to obtain the subsurface shear wave velocity profile without using direct methods. The bore log information and the subsurface shear wave velocity profile available at various stations of Japan’s Kyoshin network (K-NET) have been utilized for training this machine learning model. The parametric correlation study indicates that simple parameters like rock/soil type, the thickness of the layer in the model, and standard penetration test (SPT-N) value directly correlate with the medium’s shear wave velocity. A stacked ensemble machine learning model named VelProfES (an acronym for Velocity Profiler using Ensemble machine learning models) has been developed in this paper and has been utilized for effective prediction of the shear wave velocity profile using basic information from borelog data. The dataset used in the training and testing of the machine learning model consists of borelog and shear wave velocity information from 1101 stations. Of 1101 stations, 657, 283, and 71 stations have been utilized for training, testing, and validating the machine learning model. Training, testing, and validation of the developed machine learning model consist of parameters from 12351, 5279, and 1388 velocity layers. The problem of data imbalance based on soil type has been addressed using an additional 10510 layers of synthetic borelog data generated from conditional generative adversarial networks (CTGANs). A feature and model ablation study was conducted for the VelProfES model to provide substantiation for the model and feature selection choices. The predicted shear wave velocity profiles were compared at specific stations, focusing on average velocities at 5, 10, 15, and 20 depths. Further, the predicted values have been compared with the empirical relation of Sil and Haloi (2017) and a trained polynomial model. The machine learning model demonstrates close alignment between predicted and actual values across a broad spectrum of velocities, a contrast not observed in the empirical relation and polynomial model. The results show that the machine learning models and augmented data generated using CTGANs can efficiently minimize the error between actual and predicted subsurface shear wave velocity values.

Distributed acoustic sensing for shallow structure imaging using mechanical noise: A case study in Guangzhou, China

Distributed Acoustic Sensing (DAS) is a novel seismic detection technology that transforms fiber-optic cables into densely sampled seismic stations, showing immense potential in both urban seismology and the oil and gas industry. We deployed a ~ 182-m-long single-core fiber-optic cable in a campus under construction to record human activity and demonstrated the application for seismic imaging. The spectrogram analysis revealed significant differences in ambient noise between the daytime and nighttime at the construction site. During the daytime, the noise energy exhibited dominant frequencies ranging from 2 to 11 Hz, approximately one order of magnitude higher than that at night, which had frequencies ranging from 2 to 5 Hz. Beamforming analysis demonstrated that noise sources exhibited distinct directional characteristics and propagation velocities during daytime and nighttime periods. We extracted the cross-correlation functions between DAS channels from the mechanical noise generated by a large excavator and enhanced signal-to-noise ratio by using the phase-weighted stacking method. The multichannel analysis of surface waves (MASW) method was then employed to obtain 2D near-surface shear-wave velocity. Based on five 60-m-deep boreholes drilled along the DAS array, we established the correlation between shear-wave velocity information and shallow structure. We obtained the unbiased cross-correlation function in the presence of continuous mechanical noise along the DAS array direction, demonstrating the importance of site noise source distribution analysis in ambient noise tomography. Our research is informative for research related to the construction of accurate, high-resolution near-surface shear wave velocity models.

Application of a density-based spatial clustering algorithm for fully automatic picking of surface-wave dispersion curves

Surface waves can be used to enhance the characterization of the shallow subsurface in desert environments. A high-resolution shear-wave velocity model is typically obtained by inverting dispersion curves, which correspond to different propagation modes of the surface waves. A common approach to estimate the dispersion curves is to manually pick the magnitude maxima from the frequency-phase velocity spectra of the seismic data. This approach is inefficient, time consuming, highly subjective, and not feasible for large surveys. Automatic picking of dispersion curves has become a topic of interest recently in the oil and gas research community, where many of the developed algorithms were inherited from the fields of image processing and machine learning. By exploring in the area of unsupervised learning, we recently derived an algorithm and workflow for fully automatic picking of surface-wave dispersion curves by employing a density-based spatial clustering technique. Our approach has been tested on the SEG Advanced Modeling Corporation Arid model synthetic data set and a field data set acquired in a desert environment. The results of the synthetic tests show that the estimated dispersion curves match the true dispersion curves with high accuracy, and they can be inverted for shear-wave velocities, successfully recovering the shallow near-surface features. The application of the method to field data provides high-resolution geology-consistent shear-wave velocity information that can be converted into a compressional-wave velocity model in agreement with uphole observations.

Refraction Microtremor method for delineation of layers and lenses, and assessing liquefaction potential within an alluvial setting - Morobe Province, Papua New Guinea

Refraction Microtremor (ReMi) is a relatively new method in the geophysics industry. ReMi provides high resolution seismic shear wave velocity models up to 200m depth and as such has the potential of being an efficient method for assessing the soil liquefaction potential in seismically active regions.This paper presents a geophysical investigation carried out as part of a geotechnical feasibility study for a proposed Tailings Storage Facility (TSF) in the Morobe Province of Papua New Guinea. The primary objective of the investigation was to use geophysical methods to obtain subsurface parameters to assess the liquefaction potential within an interbedded and lensed clay/gravel alluvial setting. ReMi together with down hole and cross hole seismic methods were used to generate shear wave velocity information of multiple layers with depth, and in particular to define seismic velocity inversions.ReMi data was acquired using two array setups specifically targeting the top 100m of subsurface material and the top 50m of subsurface material at increased layer resolution. The data was inverted to produce shear wave velocity soundings which were correlated with the cross-hole and down-hole seismic methods, and with bore hole Standard Penetration Tests (SPT). The soundings were compiled to generate high resolutions shear wave velocity sections, analysis of which proved pertinent in defining the interconnectivity of the lensed clay/gravel and shear wave velocity variations for the calculation of liquefaction potential thresholds.

Direct Inversion Algorithm for Shear Velocity Profiling in Dipole Acoustic Borehole Measurements

The evaluation of radial heterogeneities within formations plays an important role in oil and gas exploration and exploitation. Dipole acoustic well logging data are widely used to invert radial shear velocity profiles, for which efficient dispersion analysis algorithms and radial profile inversion algorithms constitute fundamental issues. A direct parametric inversion method is, therefore, proposed in this letter, and a phase-based dispersion method is employed to calculate the dispersion curves of measured dipole acoustic data. The accuracy and computational efficiency of the inversion can be simultaneously guaranteed by using the proposed dispersion analysis method. An exponential radial profile function with three parameters, namely, the relative change in the shear modulus, the decaying parameter of the exponential function, and the radial extent of the perturbation zone, is used to describe alterations in formation shear wave velocities. Formations with different alteration characteristics can be expressed effectively using this proposed exponential function. The cost function is then calculated in terms of the three exponential function parameters and the real data dispersion curve. After the three parameters are inverted by finding the minimum value of the cost function, the radial shear wave velocity profile can be expressed using the exponential function. Finally, processing results using synthetic data confirm the efficiency of the proposed algorithm.

Single-station ellipticity retrieval and its joint inversion with dispersion curve, for a borehole test site

To obtain the shear wave velocity profile for engineering application to near-subsurface, many geophysical techniques are used. The dispersion curve alone is unable to provide deeper shear wave velocity information for deep soil deposit; therefore, a joint inversion of dispersion curves with the horizontal over vertical (H/V) (i.e., apparently ellipticity) curve is recommended. The H/V curve obtained from the microtremor recording contains a major part of Love wave contribution to the noise wavefield horizontal component. Due to this presence of Love wave fraction, the H/V curve does not completely replicate the ellipticity of Rayleigh wave. In this study, we try to compare the Rayleigh wave ellipticity obtained from the borehole velocity model to the H/V curve obtained at the same locality from seismic ambient noise recording. Two different techniques available for the Love effect minimization are tested and compared with the borehole ellipticity. Finally, the joint inversion of H/V and dispersion curve is made, which shows great correspondence with the previous measurement at the site.

Shear-wave high-resolution seismic reflection in Ottawa and Quebec City, Canada

Two examples of multicomponent shear (S)-wave, shallow, seismic reflection profiling from urban en-vironments in eastern Canada are presented to examine the benefits of shear-wave reflection data and the latest developments in acquisition methodology, as well as our evolving understanding of the complex nature of seismic-wave propagation. In “soft” soils characterized by low shear-wave velocities, high-resolution shear-wave reflection sections can be obtained, with the highest-resolution data related to horizontal or vertical components of motion. Multicomponent recording provides the capacity to record valuable shear-wave velocity information for earthquake amplification and engi-neering investigations in urban environments.

Converted waves in a shallow marine environment: Experimental and modeling studies

Seismic waves converted from compressional to shear mode in the shallow subsurface can be useful not only for obtaining shear-wave velocity information but also for improved processing of deeper reflection data. These waves generated at deep seas have been used successfully in hydrocarbon exploration; however, acquisition of good-quality converted-wave data in shallow marine environments remains challenging. We have looked into this problem through field experiments and synthetic modeling. A high-resolution seismic survey was conducted in a shallow-water canal using different types of seismic sources; data were recorded with a four-component water-bottom cable. Observed events in the field data were validated through modeling studies. Compressional waves converted to shear waves at the water bot-tom and at shallow reflectors were identified. The shear waves showed distinct linear polarization in the horizontal plane and low velocities in the marine sediments. Modeling results indicated the presence of a nongeometric shear-wave arrival excited only when the dominant wavelength exceeded the height of the source with respect to the water/sediment interface, as observed in air-gun data. This type of shear wave has a traveltime that corresponds to the raypath originating not at the source but at the interface directly below the source. Thus, these shear waves, excited by the source/water-bottom coupled system, kinematically behave as if they were generated by an S-wave source placed at the water bottom. In a shallow-water environment, the condition appears to be favorable for exciting such shear waves with nongeometric arrivals. These waves can provide useful information of shear-wave velocity in the sediments.

Reservoir properties prediction in the West Baram Delta through data integration constrained by rock physics

We present the methodology and results that allowed conditioning the geological model in Siwa Field in Malaysia's Sarawak Basin using seismic-derived properties. Synergic integration of advanced petrophysical techniques, rock physics analysis/modeling, and seismic inversion, combined with a detailed structural modeling and depth conversion, allowed us to successfully incorporate the seismic-driven petrophysical properties and achieve a more complete description of the reservoir than could be done through conventional statistical techniques based on wells and seismic maps. The high structural and stratigraphic complexity, lack of shear-wave velocity information, and high levels of noise in the data represented the biggest challenges to deriving seismic properties based on amplitude variation for conditioning the reservoir model.

Dynamic Periods and Building Damage at Charleston, South Carolina during the 1886 Earthquake

Fundamental dynamic periods of Quaternary deposits beneath the peninsula of Charleston, South Carolina, are characterized spatially using an updated isopach map of Quaternary thickness, characteristic small-strain shear wave velocity information, a 1:24,000 geologic map, and a simple approximating equation. The updated isopach map is developed from subsurface information from 266 investigation sites. Estimates of fundamental periods for the Quaternary sediments primarily range between 0.3 and 0.7 s. These periods are lower end estimates of actual ground periods, based on a comparison with modeled response-spectra ratios. Estimates of fundamental periods range from 0.1 to 0.4 s for over 95% of the buildings present in 1886. Thus, the overlap between the range of building periods and the range in periods corresponding to high spectral ratios is not great. This finding agrees with the observation of Marciano and Elton that damage was independent (or only slightly dependent) of building height.

Fast Stacking and Phase Corrections of Shear Wave Signals in a Noisy Environment

Hardware, software, and analysis of transient response of sources and receivers are presented for a piezoelectric bender element system designed to measure shear wave velocities in noisy environments. Signal-to-noise ratio is improved by signal stacking, wherein data vectors from many pulses are summed. A new fast-stacking algorithm enables signal quality to be improved much more rapidly than conventional stacking. Conventional stacking is accomplished by repeatedly sending an excitation pulse to a source, waiting for the signal and secondary reflections to pass the receiver and then introducing a subsequent excitation pulse. Using conventional stacking, it is important to wait for the signal and secondary reflections to die out before exciting subsequent pulses. In the new fast-stacking algorithm, a varied interval between consecutive pulses is used so that high quality signals can be obtained even if consecutive pulses are excited in rapid succession. Transient behavior of soil-bender interaction was characterized using closed-form analytical solutions of single-degree-of-freedom oscillators, numerical solutions using a beam-on-springs method, and measurements from an array of bender elements in a sand model. The time delay caused by soil-bender interaction was calculated to be half of the natural period of the bender element, and this theoretical time delay was supported by experimental data. This system makes it feasible to rapidly collect accurate shear wave velocity information so that transient changes in shear wave velocity can be monitored even if background noise is large.

Application of Linear-Array Microtremor Surveys for Rock Mass Classification in Urban Tunnel Design

Urban conditions, such as existing underground facilities and ambient noise due to cultural activity, restrict the general application of conventional geophysical techniques. At a tunnelling site in an urban area along an existing railroad, we used the refraction microtremor (REMI) technique (Louie, 2001) as an alternative way to get geotechnical information. The REMI method uses ambient noise recorded by standard refraction equipment and a linear geophone array to derive a shear-wave velocity profile. In the inversion procedure, the Rayleigh wave dispersion curve is picked from a wavefield transformation, and iteratively modelled to get the S-wave velocity structure.The REMI survey was carried out along the line of the planned railway tunnel. At this site vibrations from trains and cars provided strong seismic sources that allowed REMI to be very effective. The objective of the survey was to evaluate the rock mass rating (RMR), using shear-wave velocity information from REMI. First, the relation between uniaxial compressive strength, which is a component of the RMR, and shear-wave velocity from laboratory tests was studied to learn whether shear-wave velocity and RMR are closely related. Then Suspension PS (SPS) logging was performed in selected boreholes along the profile, in order to draw out the quantitative relation between the shear-wave velocity from SPS logging and the RMR determined from inspection of core from the same boreholes. In these tests, shear-wave velocity showed fairly good correlation with RMR. A good relation between shear-wave velocity from REMI and RMR could be obtained, so it is possible to estimate the RMR of the entire profile for use in design of the underground tunnel.

Rayleigh wave and detection of low-velocity layers in a stratified half-space

The excitation and propagation of the guided waves in a stratified half-space and a Rayleigh wave exploration method in shallow engineering seismic exploration are studied in this paper. All the modes of the guided waves are calculated by the bisection method in the case where the low velocity layers are contained in a stratified half-space. Cases when the formation shear wave velocity gradually decreases from the top to the bottom layers are also studied. The dispersion curves obtained in actual Rayleigh wave exploration are usually noncontinual zigzag curves, but the dispersion curves given by the elastic theory for given modes of the guided waves are smooth and continual curves. In this paper, the mechanism of zigzag dispersion curves in Rayleigh wave exploration is investigated and analyzed thoroughly. The zigzag dispersion curves can give not only the possible positions of the low-velocity layers but also the other information on the formation structure (fractures, oil, gas, etc.). It is found that the zigzag dispersion curves of the Rayleigh wave are the result of the leap of the modes and the existence of low velocity layers in a stratified half-space. The effects of the compressional wave velocity, shear wave velocity, and density of each layer on zigzag dispersion curves and the relationship of the low velocity layers to zigzag dispersion curves are also investigated in detail. Finally, the exploration depth of the Rayleigh wave is discussed. The exploration depth of the Rayleigh wave is equal to the wavelength multiplied by a coefficient that is variable and usually given by the work experience and the formation properties of the local work area.

Surface and downhole shear wave seismic methods for thick soil site investigations

Shear wave velocity–depth information is required for predicting the ground motion response to earthquakes in areas where significant soil cover exists over firm bedrock. Rather than estimating this critical parameter, it can be reliably measured using a suite of surface (non-invasive) and downhole (invasive) seismic methods. Shear wave velocities from surface measurements can be obtained using SH refraction techniques. Array lengths as large as 1000 m and depth of penetration to 250 m have been achieved in some areas. High resolution shear wave reflection techniques utilizing the common midpoint method can delineate the overburden-bedrock surface as well as reflecting boundaries within the overburden. Reflection data can also be used to obtain direct estimates of fundamental site periods from shear wave reflections without the requirement of measuring average shear wave velocity and total thickness of unconsolidated overburden above the bedrock surface. Accurate measurements of vertical shear wave velocities can be obtained using a seismic cone penetrometer in soft sediments, or with a well-locked geophone array in a borehole. Examples from thick soil sites in Canada demonstrate the type of shear wave velocity information that can be obtained with these geophysical techniques, and show how these data can be used to provide a first look at predicted ground motion response for thick soil sites.

Acoustic Logging through Casing to Detect Hydrocarbons and Determine Porosity in the Wilmington Field, CA: ABSTRACT

The Wilmington Field, located in the Los Angeles Basin, CA, is composed of relatively unconsolidated turbiditic sands waterflooded for more than 40 years. As is common in this and other oil fields in California and elsewhere, considerable bypassed oil remains in place. The water-oil ratio from one well selectively completed in high-oil saturation sands is significantly lower than the water-oil ratio in adjacent wells. We have begun a 2-year test program to identify sands with high remaining oil saturations by logging old cased wells using a high power low frequency acoustic logging tool as part of a project co-funded by the Department of Energy (DOE PON PS22-94BC14972). Formation compressional-wave velocity is obtained from monopole data. Formation shear-wave velocity is obtained by analyzing dipole wave modes. In some wells, however, problems associated with poor cement-casing and cement-formation bond, casing eccentricity in the well, and tool eccentricity in the casing make it difficult to detect the dipole mode. Where good data has been obtained, compressional-wave velocities determined in the same cased hole from logs recorded by two different companies agree quite well, as do open- and cased-hole compressional and shear-wave velocities. Porosities determined through casing using shear-wave velocities match conventional open-hole logmore » values. Saturations determined from the velocities and their ratio are similar to those calculated using Archie's Law. Relationships between porosities, saturations, and velocities are consistent with theoretical expectations. The results indicate that it is possible to determine porosity and saturation through casing using acoustic methods even in relatively unconsolidated sands such as those found in the Wilmington Field, provided sufficiently good monopole and dipole waveforms can be obtained.« less

Interaction of flexural modes of a borehole and a cylinder

A low-frequency dipole source excites flexural waves in cylindrical structures. Flexural waves in rods and pipes assume vanishingly small phase velocities at low frequencies. On the other hand, the flexural waves of a borehole surrounded by an infinite formation asymptotically approach formation shear wave velocity at low frequencies. These two individual lowest-order flexural modes interact with each other in a composite structure where a rod or pipe is concentrically placed in a borehole surrounded by an infinite formation with liquid in the annulus. This interaction results in two coupled modes of the composite structure. Depending on how strong this interaction is, these two coupled modes may resemble or drastically differ from the two individual modes of the rod (or pipe) as well as the borehole. The interaction is stronger at frequencies where the two individual modes are closer in phase velocity. Results of experimental and theoretical studies will be reviewed and compared.

Full waveform inversion of P waves for Vs and Qp

We present an indirect method of determining shear wave velocities from full waveform acoustic logs based on the inversion of the spectral ratio of the P wave trains at two source‐receiver separations. This method simultaneously inverts for the formation shear wave velocity and compressional wave attenuation. The P wave response is calculated by means of branch‐cut integration. This method is useful in “soft” formations where the shear wave velocity is lower than the acoustic velocity of the borehole fluid and thus there are no refracted shear wave or pseudo‐Rayleigh wave arrivals. The method is shown to give good estimates of formation shear wave velocity in both synthetic and field data. The inversion algorithm is sensitive to local minima; care must be taken to avoid them.

Shear wave logging in semi-infinite saturated porous formations

Recent theoretical and experimental studies have demonstrated the interest of using nonaxisymmetric sources, such as dipole and quadrupole, to record shear wave events in any kind of formation (fast or slow). Nonaxisymmetric sources excite surface waves whose Airy phase is predominant in the records at intermediate frequencies (≂3 to 6 kHz). They are referred to as the flexural and screw modes, respectively, for the dipole and the quadrupole sources. Their dispersion and attenuation are studied in a fluid-filled borehole embedded in a homogeneous saturated porous formation. The two-phase medium is modeled using Biot’s theory modified in accordance with the homogenization theory. Calculation of synthetic full waveform logs is also performed using the discrete wavenumber method. Whatever the formation, the most reliable information that can be extracted from low-frequency parts of the wavetrains is the formation shear wave velocity and attenuation. It is of great interest in the inversion of Stoneley wave velocity and attenuation for the evaluation of in situ permeability. In the presence of fast formations, the attenuation of the Airy phase of both the flexural and screw modes is sensitive to the in situ permeability. The same conclusion holds for the slow formations only when they are saturated by a fluid of low viscosity and high compressibility.

Shear wave velocity and attenuation in ocean bottom sediments from acoustic log waveforms

In boreholes where formation shear wave velocity is lower than borehole fluid velocity neither refracted shear waves nor pseudo‐Rayleigh waves can propagate. When frequency response of the sonde does not extend to low frequencies (e.g. 2 kHz) Stoneley waves are not excited. In such cases refracted P and leaking PL modes become dominant phases on a full waveform acoustic log. The P wave velocity can be determined from the first arrivals. Then, using synthetic seismograms and a waveform matching technique, formation shear wave velocity and attenuation can be estimated. This method is demonstrated using data from DSDP Site 612 in the Baltimore Canyon Trough area of the Atlantic margin.