S-wave Attenuation Research Articles

The volcanic and geothermally active Campi Flegrei caldera: an integrated multidisciplinary image of its buried structure

The Campi Flegrei caldera in southern Italy is one of the greatest geohazard areas on Earth. Evidence of an active magmatic and geothermal system is provided by ongoing ground uplift, with volcano-tectonic and long-period (LP) seismicity, the persistent degassing of ~1500 tonnes of CO2 per day, the presence of hot fumaroles at temperatures of 90–150 °C, brine-rich aquifers (with total dissolved solids up to 33 g l−1) and high thermal gradients in the crust (with temperatures reaching 420 °C at 3,050 m b.s.l.). Since the 1940s, more than 100 exploratory boreholes have been drilled in the area to depths of 80–3,100 m by the Azienda Geologica Italiana Petroli (AGIP) and the Societa Anonima Forze Endogene Napoletane (SAFEN). To date, however, no systematic reanalysis of the drilling data has been carried out, and the buried volcanic structure has not been updated using the most recent scientific results and previous findings. By integrating unpublished data from the AGIP and SAFEN reports with published information from geological, volcanological, petrological, petrophysical and geophysical studies, this paper presents an improved picture of the Campi Flegrei caldera that will be useful for volcanic hazard assessment and mitigation in the Naples area and for future research planning. The results suggest that intra-caldera activity has been influenced by how the magmatic system at depths greater than about 4 km has determined the transfer of magma, volatiles, and heat to the overlying geothermal system and, ultimately, to the surface. In particular, intriguing is that the most volcanically active central-eastern sector of the caldera, which is subject to intense bradyseismic ground movement and gas emission, coincides with a structurally delimited subsurface rock volume characterized by an uprising of the 100 °C isotherm, a deep water supply to the shallower aquifer, the early disappearance of secondary calcite, LP seismicity and high seismic S-wave attenuation. In this area, we also document evidence of repeated injection at depths of c. 1.5–3.0 km of isolated and small-volume batches of magma, where occurred their crystallization and degassing. Shallow intrusions and degassing of magma are thus identified as two of the key processes that drive unrest in Campi Flegrei.

Reference S-Wave Velocity Profile and Attenuation Models for Ground-Motion Prediction Equations: Application to Japan

Defining the reference rock or soil condition related to ground‐motion prediction is an important aspect of seismic‐hazard analysis. In a previous study by the authors, a method was proposed to establish a reference rock profile for Switzerland through the comparison of empirical amplification functions with shear‐wave velocity profiles at 27 selected sites of the Swiss National Seismic Network. The retrieved velocity profile served as reference for a regional ground‐motion prediction equation. However, a lacking piece of information remained: the anelastic attenuation for such a reference profile. Reference attenuation is essential to correctly model and interpret amplification at high frequencies. In the present study we extended our approach to simultaneously model both the reference shear‐wave velocity profile and the corresponding attenuation for Japan. We compared site‐specific attenuation measurements with quarter‐wavelength average velocities at 36 soil and rock sites from the Japanese KiK‐net strong‐motion network. The selected sites are characterized by a lack of observed resonance phenomena in order to avoid trade‐off between amplification and attenuation effects. We establish a parametric model through regression analysis. The resulting model gives us the possibility to estimate anelastic attenuation of a rock site with a given velocity profile and provides the base for host‐to‐target adjustments of real or modeled ground motion.

Detection of gas and water using HHT by analyzing P- and S-wave attenuation in tight sandstone gas reservoirs

A direct detection of hydrocarbons is used by connecting increased attenuation of seismic waves with oil and gas fields. This study analyzes the seismic attenuation of P- and S-waves in one tight sandstone gas reservoir and attempts to give the quantitative distinguishing results of gas and water by the characteristics of the seismic attenuation of P- and S-waves. The Hilbert–Huang Transform (HHT) is used to better measure attenuation associated with gas saturation. A formation absorption section is defined to compute the values of attenuation using the common frequency sections obtained by the HHT method. Values of attenuation have been extracted from three seismic sections intersecting three different wells: one gas-saturated well, one fully water-saturated well, and one gas- and water- saturated well. For the seismic data from the Sulige gas field located in northwest Ordos Basin, China, we observed that in the gas-saturated media the S-wave attenuation was very low and much lower than the P-wave attenuation. In the fully water-saturated media the S-wave attenuation was higher than the P-wave attenuation. We suggest that the joint application of P- and S-wave attenuation can improve the direct detection between gas and water in seismic sections. This study is hoped to be useful in seismic exploration as an aid for distinguishing gas and water from gas- and water-bearing formations.

The 2012 May 20 and 29, Emilia earthquakes (Northern Italy) and the main aftershocks: S-wave attenuation, acceleration source functions and site effects

We used strong-motion records from the 2012 May 20 and 29 Emilia-Romagna earthquakes (Mw 6.1 and 5.9, respectively) and four aftershocks with magnitudes ranging between 4.9 and 5.5 to analyse the S-wave spectral amplitude decay with distance and estimate acceleration source functions and site effects. The data set consists of six earthquakes, 44 stations and 248 records with hypocentral distances in the range 10 < r < 100 km. We rotated the accelerograms to calculate transverse and radial components of the acceleration spectrum. We found non-parametric attenuation functions that describe the spectral amplitude decay of SH and SV waves with distance at 60 different frequencies between 0.1 and 40 Hz. These attenuation functions provide an estimate of the quality factor Q at each frequency analysed. Assuming that geometrical spreading is 1/r for r ≤ rx and 1/(rx r)0.5 for r > rx with rx = 60 km and normalizing at 15 km (the recording distance where the attenuation functions start to decay), we find that the average Q for SH waves can be approximated by QSH = 82 ± 1 f 1.2±0.02 and by QSV = 79 ± 1 f 1.24±0.03 for SV waves in the frequency range 0.10 ≤ f ≤ 10.7 Hz. At higher frequencies, 11.8 ≤ f ≤ 40 Hz, the frequency dependence of Q weakens and is approximated by QSH = 301 ± 1 f 0.36±0.04 and QSV = 384 ± 1 f 0.28±0.04. These results indicate that the S-wave attenuation is radially isotropic at local distances in the epicentral area. Nevertheless, we used these attenuation parameters separately to correct the radial (with QSV) and transverse (with QSH) components of the acceleration spectra and to separate source and site effects using a non-parametric spectral inversion scheme. We found that the source function of the main event and the bigger aftershocks show enhanced low frequency radiation between 0.4 and 3.0 Hz. We converted the source functions into far-field source acceleration spectra and interpreted the resulting source spectra in terms of Brune's model. The stress drops obtained range between approximately 0.9 and 2.9 MPa. Although all the recording stations used are located in the Po Plain, the site functions obtained from the spectral inversion show important amplification variability between the sites. We compared these site functions with the average horizontal to vertical spectral ratios calculated for each station, and we found consistent results for most stations.

The effect of methane hydrate morphology and water saturation on seismic wave attenuation in sand under shallow sub-seafloor conditions

A better understanding of seismic wave attenuation in hydrate-bearing sediments is needed for the improved geophysical quantification of seafloor methane hydrates, important for climate change, geohazard and economic resource assessment. Hence, we conducted a series of small strain (<10−6), seismic frequency (50–550Hz), laboratory resonant column experiments on synthetic methane hydrate-bearing sands under excess-water seafloor conditions. The results show a complex dependence of P- and S-wave attenuation on hydrate saturation and morphology. P- and S-wave attenuation in excess-water hydrate-bearing sand is much higher than in excess-gas hydrate-bearing sand and increases with hydrate saturation between 0 and 0.44 (the experimental range). Theoretical modelling suggests that load-bearing hydrate is an important cause of heightened attenuation for both P- and S-waves in gas and water saturated sands, while pore-filling hydrate also contributes significantly to P-wave attenuation in water saturated sands. A squirt flow attenuation mechanism, related to microporous hydrate and low aspect ratio pores at the interface between sand grains and hydrate, is thought to be responsible for the heightened levels of attenuation in hydrate-bearing sands at low hydrate saturations (<0.44).

S-waves and the near surface: A time-lapse study of S-wave velocity and attenuation in the weathering layer of an Alberta heavy oil field

This study used data from a three-month continuous reservoir monitoring experiment in Peace River, Alberta, Canada to measure spatial and temporal variations in near-surface S-wave velocity (VS) and attenuation (QS) in the weathering layer. The permanently buried sources generate a strong refracted S-wave that was recorded on buried 3C receivers. A method to perform receiver-side up-down separation and extract primary and ghost S-wavefields is presented. These wavefields are then used to measure near-surface VS and QS for the near-surface layer above the buried receivers, which is the top 12 m in this case. The measured VS values range between 180 and 220 m/s and QS values between 8 and 22. Maps of VS and QS show a robust correlation (low VS with low QS) and a clear spatial variation that can be associated with soil type. Both properties increase slowly over a three-month period in one section of the survey area while remaining constant in another. The cumulative increase in VS and QS is 10%.

Source and Path Parameters for Recorded Earthquakes in Tehran Province, Iran

The goal of this study is to estimate source and path attenuation parameters of recorded earthquakes in Tehran province, Iran so it can be used for earthquake prediction purposes. For this, we have used four well recorded earthquakes by Iranian Strong motion Network (ISMN).. By assuming a simple geometric spreading we have estimated Q factor for S-wave attenuation. The method we have used here is “Double Spectral Ratio Method” which is simple but reliable and has applied to many cases to estimate Qs factor in Japan. We propose the linear function of Q(f)=25.03f for attenuation of S-waves in Tehran province which is applicable to Iran plateau too. This is smaller than the Q factors that are normally being calculated in Japan but close to the ones in Iran. We have also estimated different source parameters, such as Seismic moment (M0), corner frequency (fc) and Brune stress drop (Δσ) for each earthquake. We have fitted an ω model to the spectra of the motion at unity distance by Andrews method (1986). Spectra at unity distance is calculated by removing site and path effects from the spectra of the recorded motion. The attenuation path factor that is used for this purpose is the same one that is estimated in this study.

PS検層による地盤のS波の減衰定数の評価に関する検討&lt;br/&gt;─鉛直アレー観測による減衰定数との比較─

原位置の計測から地盤のS波の減衰定数を合理的に評価する方法を検討するため，ほぼ水平な地質構造を持つ第四紀地盤テストフィールドにおいて，PS検層 (ダウンホールPS検層) に基づく減衰定数の評価に関する実証試験を行った。実証試験では，主として鉛直アレー観測による減衰定数の評価結果に対比させる形でPS検層の適用性を検討した。その結果，両手法から評価されるS波の減衰定数は，PS検層が適用対象とするおよそ10 Hzより高周波数側でほぼ一致し，評価手法の違いによる差はほとんどないことが示された。PS検層で得られるやや高周波数領域におけるS波の減衰定数を基に，周波数一定型の減衰定数の関数モデルを付与することにより，精度の高い地震動評価ができることが期待される。

Temporal changes in attenuation ofSwaves through a fault zone in a South African gold mine

SUMMARY We investigated the temporal changes in the seismic attenuation of a fault zone using nearsource recordings of S waves from repeating microearthquakes that occurred both before and after M ∼2 earthquakes in the Bambanani gold mine, South Africa. Because the source locations and the mechanisms of repeating earthquakes can be regarded as identical, the attenuation change can be estimated using the spectral ratios for repeating earthquake pairs. We found an increase in the S-wave attenuation parameter Q −1 S on the vertical component which is positively correlated with frequency, corresponding to times before and after the M ∼2 earthquakes. This increase can be explained by scattering attenuation, with a typical scale of damage in the fault zone of ∼ 3m .

Estimation of earthquake parameters in the Alborz seismic zone, Iran using generalized inversion method

Generalized inversion of the S-wave amplitude spectra from the strong-motion network data in the Alborz, Iran has been used to estimate simultaneously source parameters, site response and S-wave attenuation (Qs). To obtain an optimum inverse solution, and also for decreasing the uncertainty level, a frequency-dependent site amplification as a constraint, was imposed to five reference site responses. This constraint removes the undetermined degree of freedom in the inversion. Furthermore, for removing the trade-off between geometrical spreading and anelastic attenuation, a geometrical spreading factor was adopted from the Motazedian [20] study. A point source model has been calibrated against the resulting source terms and consequently source parameters, like corner frequency, moment magnitude and high frequency fall off coefficient, for each event has been determined separately. Also, based on the available data and their connectivity two sub-regions including western and eastern parts of Alborz located east and west of 52.5°E have been considered to see that if there is any possible systematic difference in their seismic source characteristics. The average stress drops obtained are about 182 and 116bars, respectively for eastern and western Alborz. Another result of the study is the site responses, which have been determined for all of 81 stations individually. Though soil nonlinearity was detected at the Ab-bar station (experienced strong ground shaking, i.e., PGA>0.5g) near to the epicenter of Manjil M7.4 earthquake of June 20, 1990, but an analysis of residuals showed generally a weak influence of soil nonlinearity (i.e., dependence of amplification on shaking level); probably because of the relatively weak levels of acceleration in our database. Finally, the shear wave quality factor (i.e., Qs) has been determined as a function of frequency represented by a linear equation in logarithmic scale. To evaluate the outcomes of the current study, the results have been compared with similar studies wherever it was available. The results of the current study are of utmost importance for seismic hazard assessment of the metropolitan area of Tehran, where 15 million people live, one-fifth of the population of Iran.

Pore fluid effects on S-wave attenuation caused by wave-induced fluid flow

We studied seismic attenuation of P- and S-waves caused by the physical mechanism of wave-induced fluid flow at the mesoscopic scale. Stress relaxation experiments were numerically simulated by solving Biot’s equations for consolidation of 2D poroelastic media with finite-element modeling. The experiments yielded time-dependent stress-strain relations that were used to calculate the complex moduli from which frequency-dependent attenuation was determined. Our model consisted of periodically distributed circular or elliptical heterogeneities with much lower porosity and permeability than the background media, which contained 80% of the total pore space of the media. This model can represent a hydrocarbon reservoir, where the porous background is fully saturated with oil or gas and the low-porosity regions are always saturated with water. Three different saturation scenarios were considered: oil-saturated (80% oil, 20% water), gas-saturated (80% gas, 20% water), and fully water-saturated media. Varying the dry bulk and shear moduli in the background and in the heterogeneities, a consistent tendency was observed in the relative behavior of the S-wave attenuation among the different saturation scenarios. First, in the gas-saturated media the S-wave attenuation was very low and much lower than in the oil-saturated or in the fully water-saturated media. Second, at low frequencies the S-wave attenuation was significantly higher in the oil-saturated media than in the fully water-saturated media. The P-wave attenuation exhibited a more variable relative behavior among the different saturation degrees. Based on the mechanism of wave-induced fluid flow and on our numerical results, we suggest that S-wave attenuation could be used as an indicator of fluid content in a reservoir. Additionally, we observed that impermeable barriers in the background can cause a significant increase in S-wave attenuation. This suggests that S-wave attenuation could also be an indicator of permeability changes in a reservoir due to, for example, fracturing operations.

Three-dimensional attenuation structure of intrinsic absorption and wide-angle scattering of S waves in northeastern Japan

SUMMARY Intrinsic absorption and random velocity inhomogeneities are important medium properties for studies on wave propagation at high frequency (>1 Hz) in the lithosphere. Recent studies have shown that the spatial distribution of random velocity inhomogeneities can be estimated by an inversion analysis of peak delay times, where the peak delay time is the time lag between direct wave onset and its maximal amplitude arrival. This study estimates the 3-D structure of the S-wave attenuation quality factor Q−1, after we corrected the maximal amplitude attenuation due to multiple forward scattering on the basis of the Markov approximation for parabolic wave equation. Even though this estimate of Q−1 includes both intrinsic absorption and wide-angle scattering, the contribution of wide-angle scattering can be identified by taking into account random inhomogeneities estimated in the peak delay time analysis. The estimated Q−1 structure in northeastern Japan shows weak attenuation (Q−1(f) ∼ 10−2.5f−0.5) on the forearc side of the volcanic front, and strong attenuation (Q−1(f) ∼ 10−2.3f−0.3) beneath the Quaternary volcanoes and near the collision zone between the Honshu and Kuril arcs. According to the Born approximation, the estimated Q−1 at 0–40 km depth consists mainly of the intrinsic absorption if the characteristic scale of random inhomogeneities is 5 km or longer. Beneath the Quaternary volcanoes of northeastern Japan, Q−1(f) shows weak frequency dependence and the power spectral density functions of random velocity inhomogeneities are characterized by a weak spectral gradient that means rich in small-scale inhomogeneities. These results demonstrate the importance of the frequency dependence of wave propagation in the lithosphere. That is, the scale dependence of velocity fluctuation or small-scale inhomogeneities is an important property of the lithosphere underlying Quaternary volcanoes.

Cross-hole electromagnetic and seismic modeling for CO2 detection and monitoring in a saline aquifer

The injection of CO2 in saline aquifers and depleted hydrocarbon wells is one solution to avoid the emission of that greenhouse gas to the atmosphere. Carbon taxes can be avoided if geological sequestration can efficiently be performed from technical and economic perspectives. For this purpose, we present a combined rock-physics methodology of electromagnetic (EM) and seismic wave propagation for the detection and monitoring of CO2 in crosswell experiments.First, we obtain the electrical conductivity and seismic velocities as a function of saturation, porosity, permeability and clay content, based on the CRIM and White models, respectively. Then, we obtain a conductivity–velocity relation. This type of relations is useful when some rock properties can be more easily measured than other properties. Finally, we compute crosswell EM and seismic profiles using direct modeling techniques. P- and S-wave attenuation is included in the seismic simulation by means of White's mesoscopic theory. The modeling methodology is useful to perform sensitivity analyses and it is the basis for performing traveltime EM and seismic tomography and obtain reliable estimations of the saturation of carbon dioxide. In both cases, it is essential to correctly pick the first arrivals, particularly in the EM case where diffusion wavelength is large compared to the source–receiver distance.The methodology is applied to CO2 injection in a sandstone aquifer with shale intrusions, embedded in a shale formation. The EM traveltimes are smaller after the injection due to the higher resistivity caused by the presence of carbon dioxide, while the effect is opposite in the seismic case, where water replaced by gas decreases the seismic velocity.

Attenuation of P, S and Coda Waves in the NW-Himalayas, India

The frequency-dependent characteristics of P- and S-wave attenuation in the upper crust of NW Himalayas have been estimated using local earthquakes for a frequency range of 1.5 to 18 Hz. A total of 43 local events of magnitude 2.1 - 4.8, mostly from the vicinity of Main Boundary Thrust (MBT) and Main Central Thrust (MCT) have been used in the analysis. The extended coda normalization methods were applied to estimate the quality factors for P- waves (QP) and S-waves (QS) and the single back-scattering model has been used earlier (Kumar et al. [1]) to determine the quality factor for coda waves (QC). The observed quality factors QP and QS is strongly frequency dependent and the estimated average frequency dependent relation is given by QP = (97 ± 3)f (1.06 ± 0.06) and QS = (127 ± 6)f (0.96±0.06) respectively for P- and S-waves. A comparison of QS estimated in this study and QC previously reported shows that QC > QS for entire frequency range. This indicates the enrichment of coda waves and the importance of scattering attenuation to the attenuation of S-waves in the study region infested with faults and fractures. The ratio QS/QP is found to be greater than unity for the entire frequency range indicating that the body waves from source to station paths crossed a crustal volume with dry and rigid rocks. The frequency dependent relations developed in this study can be very useful to ground motion modeling which in turn is required in the seismic hazard assessment of the region.

Estimation of shear-wave interval attenuation from mode-converted data

Interval attenuation measurements provide valuable information for reservoir characterization and lithology discrimination. We extend the attenuation layer-stripping method of Behura and Tsvankin to mode-converted (PS) waves with the goal of estimating the S-wave interval attenuation coefficient. By identifying PP and PS events with shared ray segments and applying the [Formula: see text] method, we first perform kinematic construction of pure shear (SS) events in the target layer and overburden. Then, the modified spectral-ratio method is used to compute the effective shear-wave attenuation coefficient for the target reflection. Finally, application of the dynamic version of velocity-independent layer stripping to the constructed SS reflections yields the interval S-wave attenuation coefficient in the target layer. The attenuation coefficient estimated for a range of source-receiver offsets can be inverted for the interval attenuation parameters. The method is tested on multicomponent synthetic data generated with the anisotropic reflectivity method for layered VTI (transversely isotropic with a vertical symmetry axis) and orthorhombic media.

Estimation of site amplification, attenuation and source spectra of S-waves in the East-Central Iran

Generalized inversion of the S-wave amplitude spectra from the strong-motion network data in the East-Central Iran has been used to estimate simultaneously source parameters, site response and the S-wave attenuation ( Q s ). In this regard, 190 three-component records were used corresponded to 40 earthquakes with the magnitudes M3.5–M7.3. These earthquakes were recorded at 42 stations in the hypocentral distance range from 9 to 200 km. The inverse problem was solved in 20 logarithmically equally spaced points in the frequency band from 0.4 to 15 Hz. The frequency-dependent site amplification was imposed, as a constraint, on two reference site responses in order to remove the undetermined degree of freedom in the inversion and obtain a unique inverse solution. Also, a geometrical spreading factor was assumed for removing the trade-off between geometrical spreading and anelastic attenuation. Different source parameters, such as seismic moment ( M 0), seismic energy ( E s ), corner frequency ( f c ) and Brune stress drop (Δ σ), were estimated for each event by fitting an ω 2 model to the spectra obtained from the inversion. The stress drop values of earthquakes, obtained in this research, are in good agreement with those of other studies. Also average site response values were correlated to the average shear wave velocities in the uppermost 30 m, in high and low frequency bands. The peak frequencies of site amplifications, estimated by the generalized inversion method, where in good agreement with those of horizontal to vertical (H/V) spectral ratios for the S-wave portion of records. However, no perfect matching in amplitude was obtained due to the deficiencies of the H/V ratio technique. By supposing a free shape for Q factor, a frequency dependent function was found, the logarithm of which could be approximated by a linear function, Q( f)=151 f 0.75. The uncertainties of model parameters have been evaluated by covariance matrix of least-square fit. The residuals were also analyzed in order to assess the validity of the model. The analysis of residuals with respect to magnitude and distance indicates that they are distributed normally with approximately zero mean. The robustness of the results has been studied concerning their sensitivities to the omission of different datasets, selected randomly from original database. The results obtained here can be used in predicting ground-motion parameters applying stochastic methods.

Global variation of body-wave attenuation in the upper mantle from teleseismic P wave and S wave spectra

[1] We constrain the spatial variation of P-wave (tP*) and S-wave (tS*) attenuation by inverting 190,000 teleseismic P- and S-wave spectra up to 0.8 Hz. These spectra are derived from 250 deep earthquakes recorded at 880 broadband global and regional network stations. The variance and ratios of tP* and tS* values are consistent with PREM's upper mantle velocity and Q structures and conventional tP* and tS* values. High attenuation is resolved beneath stations in tectonically active regions characterized by high heat flow. Low attenuation marks stable continental regions. The maps of tP* and tS* correlate well with the variations of tS* computed and inferred from (1) the most recent surface-wave Q model and (2) a thermal interpretation of shear-wave velocity tomography. This indicates that maps of body- and surface-wave attenuation reflect intrinsic attenuation and variable temperature in the mantle.

Attenuation, source parameters and site effects in the Irpinia–Basilicata region (southern Apennines, Italy)

We derive S-wave attenuation characteristics, earthquake source parameters and site amplification functions at seismic stations used for earthquake early warning in the Irpinia–Basilicata region, using non-parametric spectral inversion of seismograms from 49 local events with M L = 1.5–3.1. We obtain relatively low Q values (Q 0 = 28 at a frequency of 1 Hz) in conjunction with a strong frequency-dependence (close to linear). The source spectra can be satisfactorily modeled using the omega-square model, with stress drops ranging between 0.01–2 MPa, and in the narrow magnitude range available for analysis, the source spectra seem to scale self-similarly. The local magnitude M L shows a linear correlation with moment magnitude M W, however with a systematic underestimation by about 0.5-magnitude units. The results obtained in this work provide important insights into the ground-motion characteristics that are required for appropriate seismic hazard assessment and are of practical relevance for a suite of applications, such as the calibration of ground-motion prediction equations or the correction for site amplification in earthquake early warning and rapid calculation of shake-maps for seismic emergency management.

Pressure effects on the joint elastic-electrical properties of reservoir sandstones

We conducted a laboratory study of the joint elastic-electrical properties of sixty-three brine-saturated sandstone samples to assess the likely impact of differential pressure (confining minus pore fluid pressures) in the range 8–60 MPa on the joint interpretation of marine seismic and controlled-source electromagnetic survey data. The samples showed a wide range of petrophysical properties representative of most sandstone reservoirs. We found that a regression equation comprising both a constant and an exponential part gave a good fit to the pressure dependence of all five measured geophysical parameters (ultrasonic P- and S-wave velocity, attenuation and electrical resistivity). Electrical resistivity was more pressure-sensitive in clay-rich sandstones with higher concentrations of low aspect ratio pores and micropores than in clean sandstones. Attenuation was more pressure-sensitive in clean sandstones with large open pores (macropores) than in clay-rich sandstones. Pore shape did not show any influence on the pressure sensitivity of elastic velocity. As differential pressure increases, the effect of the low aspect ratio pores and micropores on electrical resistivity becomes stronger than the effect of the macropores on attenuation. Further analysis of correlations among the five parameters as a function of pressure revealed potentially diagnostic relationships for geopressure prediction in reservoir sandstones.

Joint elastic-electrical properties of reservoir sandstones and their relationships with petrophysical parameters

We measured in the laboratory ultrasonic compressional and shear-wave velocity and attenuation (0.7–1.0 MHz) and low-frequency (2 Hz) electrical resistivity on 63 sandstone samples with a wide range of petrophysical properties to study the influence of reservoir porosity, permeability and clay content on the joint elastic-electrical properties of reservoir sandstones. P- and S-wave velocities were found to be linearly correlated with apparent electrical formation factor on a semi-logarithmic scale for both clean and clay-rich sandstones; P- and S-wave attenuations showed a bell-shaped correlation (partial for S-waves) with apparent electrical formation factor. The joint elastic-electrical properties provide a way to discriminate between sandstones with similar porosities but with different clay contents. The laboratory results can be used to estimate sandstone reservoir permeability from seismic velocity and apparent formation factor obtained from co-located seismic and controlled source electromagnetic surveys.