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SUMMERY Understanding the influence of pore fluids on acoustic characteristics of rocks containing pores of low aspect ratio, that is, cracks is crucial for both laboratory and field observations of seismic exploration. Unlike the petrophysical aspects of conventional rock species, those of tight dolomites are relatively more complex and poorly understood. To address this problem, we investigated the ultrasonic velocity of dry and saturated tight dolomite in the effective pressure range of 2–60 MPa and analysed the variation in shear modulus upon fluid saturation. Our results showed that while undergoing the change in form from the dry state to being saturated with water, the rock may weaken or harden, and the shear modulus may correspondingly either decrease or increase. Shear modulus hardening was more apparent in dolomites, with microcracks mostly dispersed between and within particles of a high proportion of soft pores with a wide distribution range of aspect ratio. This study used the Walsh–Hudson model to satisfactorily simulate the hardening of shear modulus in fluid-saturated tight dolomites. The suppression of squirt flow that is responsible for the increased shear modulus of fluid-saturated tight dolomites tested at sufficiently high frequencies. As the change in shear modulus affects the prediction of longitudinal and shear wave velocities, using Gassmann's theory to fluid substitution in unconventional reservoirs such as tight dolomite may yield inaccurate results. Therefore, it may be necessary to consider variation in shear modulus while selecting the appropriate simulation model.

Compared with the seismic wave fields, the influence of porosity and permeability to seismoelectric coupling may be more significant. The seismoelectric effect mainly forms the coseismic signals (CS) and interface response (IR). The influences of the porosity and permeability on the two types of seismoelectric field are still not clear. In this paper, we numerically analyze the response characteristics of the CS and IR with the dependency of porosity and permeability, respectively. The differences in the sensitivity of the CS and the IR fields to porosity and permeability are also determined. The results show that the change of permeability is mainly sensed by the IR field, while the CS has almost no obvious response to it: both the CS and IR fields are sensitive to the change of porosity, but the sensitivity of the IR field is higher and with the increase of porosity, the discrepancy between the sensitivity of the CS and IR fields to porosity changes increases. The signal amplitude anomaly will be caused in both P-wave fields and seismoelectric fields (IR and CS) at the geological body where the porosity changes, but the change of permeability has little impact on the P-wave and only induces an obvious IR amplitude anomaly in seismoelectric fields. This indicates that the seismoelectric effect can better reveal the connectivity characteristics of the geological body than the seismic wave. This investigation can help to distinguish the sensitivity of seismoelectric response to porosity and permeability more clearly.

SUMMARY For many geophysical problems it is important to understand the influence of clay on the elastic behaviour of the rocks. However, this is difficult to measure because the complex petrophysical characteristics of tight sandstones make it challenging to control the clay parameters in natural samples. In this study, we synthesized nine tight sandstones with different clay types and content. Ultrasonic measurements and theoretical simulations were then used to analyse the influence of clay on the elastic modulus of brine saturated samples. We found that the shear modulus of smectite-rich samples decreased drastically after saturation, while the decrease observed in kaolinite-rich samples was relatively low. We propose that the reduction in surface energy caused by surface-particle clay hydration is a common mechanism that leads to shear softening in both kaolinite-rich and smectite-rich samples. However, the contact deformation caused by cation hydration of smectite is the primary mechanism leading to greater shear softening of tight sandstones containing smectite. Although the differential Kuster–Toksöz model is based on idealized pore shapes, a dual-porosity scheme can be used to explain and simulate the shear softening of artificial clay-rich tight sandstones.

SUMMARY Seismic anisotropy can occur in rocks that have complicated internal structures and thin layering. Wave-induced fluid flow (WIFF) is one of the major causes of elastic wave dispersion and anisotropy. The principle goal of this paper is to combine the effects of WIFF and layer-induced anisotropy in orthorhombic (OTR) models that are often used in the seismic industry nowadays to describe azimuthal and polar anisotropy. We derive the effective frequency-dependent anisotropy parameters based on the Chapman model that accounts for the WIFF mechanism. First, we summarize two major problems to establish the link between the frequency-dependent seismic anisotropy and the multiple sets of fractures with different scales and orientations. Then we specify the multiple mesoscale fractures to be vertical and orthogonal so as to simplify the rock physics model to be an ORT medium. We also give the explicit expressions for the effective stiffness and the Thomsen-style parameters (vP0, vS0, ϵ1, ϵ2, γ1, γ2, δ1, δ2, δ3). Finally, we derive the effective frequency-dependent anisotropy parameters for ORT multiple layers using the Backus averaging under the approximation of weak contrast between layers. We also investigate the influence of frequency, fracture parameters (density and scale), effective porosity and volume fraction on the Thomsen-style parameters.

SUMMARYEffective medium theory plays an important role in the characterization of fractured reservoirs. The main goal of this paper is to assess the accuracy of three classical effective medium theories (compliance-based non-interaction approximation, Hudson theory and anisotropic self-consistent approximation) and investigate their applicable frequency range. To evaluate the crack density limitation of these models, we construct a series of physical models with crack density varying from 0 to 12 per cent. To satisfy the long-wavelength approximation of effective medium theory, we consider a crack diameter and aperture of 3 and 0.12 mm, respectively. To study their appropriate high-frequency laboratory condition, we use the ultrasonic transmission method to measure the elastic wave velocities at frequencies of 500, 250 and 100 kHz. The experimental results are then compared with the effective medium theories to discuss their accuracy. We also highlight and compare the accuracy of these theories for the inversion procedure to quantify the crack density.

The 3D pre-stack seismic data from a physical modeling experiment were employed to investigate the effect of reservoir scales on AVO (amplitude variation with offset or incidence angle). Eight cuboid samples simulating cavernous reservoirs with different widths and the same thickness and elastic parameters were set within a 3D model. 3D seismic data acquisition and processing were conducted. To get the AVO responses of the samples with different widths, trough amplitudes corresponding to the sample tops at different incidence angles were extracted from the pre-stack angle gathers. Amplitude calibration for transducer radiation patterns was conducted on the extracted amplitudes at different angles. AVO analysis was conducted to quantitatively demonstrate the effect of sample scales on AVO characteristics. The effect of sample width was weak when the width was less than 60 m, which was 3/5 of the wavelength. When the width was larger than 60 m, both AVO intercept and gradient gradually increased with the sample width. The AVO gradient peaked at 150 m, which was 1.5 times the wavelength. Cross-plot analysis of AVO intercept and gradient showed the samples were aligned in a straight line when the sample width was less than twice the seismic wavelength. The result in this study partially verified the conclusions of reservoir scale effect on AVO responses drawn from previous numerical modelling studies. For a heterogeneous rectangular reservoir, the effect of reservoir scales on AVO responses could potentially be used to quantitatively estimate reservoir scale.

SUMMARY The seismoelectric method is a modification of conventional seismic measurements which involves the conversion of an incident poroelastic wave to an electromagnetic signal that can be measured at the surface or down a borehole. This technique has the potential to probe the physical properties of the rocks at depth. The problem is that we currently know very little about the parameters which control seismoelectric conversion and their dependence on frequency and permeability, which limits the development of the seismoelectric method. The seismoelectric coupling coefficient indicates the strength of seismoelectric conversion. In our study, we focus on the effects of the reservoir permeability, porosity and frequency on the seismoelectric coupling coefficient through both experimental and numerical modellings. An experimental apparatus was designed to record the seismoelectric signals induced in water-saturated samples in the frequency range from 1 to 500 kHz. The apparatus was used to measure seismoelectric coupling coefficient as a function of porosity and permeability. The results were interpreted using a microcapillary model for the porous medium to describe the seismoelectric coupling. The relationship between seismoelectric coupling coefficients and the permeability and porosity of samples were also examined theoretically. The combined experimental measurements and theoretical analysis of the seismoelectric conversion has allowed us to ascertain the effect of increasing porosity and permeability on the seismoelectric coefficient. We found a general agreement between the theoretical curves and the test data, indicating that seismoelectric conversion is enhanced by increases in porosity over a range of different frequencies. However, seismoelectric conversion has a complex relationship with rock permeability, which changes with frequency. For the low-permeability rock samples (0–100 × 10−15 m2), seismoelectric coupling strengthens with the increase of permeability logarithmically in the low-frequency range (0–10 kHz); in the high-frequency range (10–500 kHz), the seismoelectric coupling is at first enhanced, with small increases of permeability leading to small increases in size in electric coupling. However, continued increases of permeability then lead to a slight decrease in size and image conversion again. For the high-permeability rock samples (300 × 10−15–2200 × 10−15 m2), the seismoelectric conversion shows the same variation trend with low-permeability samples in low-frequency range; but it monotonically decreases with permeability in the high-frequency range. The experimental and theoretical results also indicate that seismoelectric conversion seems to be more sensitive to the changes of low-permeability samples. This observation suggests that seismic conversion may have advantages in characterizing low permeability reservoirs such as tight gas and tight oil and shale gas reservoirs.

Electric fields applying on the fluid-filled porous materials can induce small relative pore-fluid motions due to electroseismic conversions. In order to characterize the electroseismic propagation phenomena, we have designed an experimental apparatus to acquire the electroseismic (ES) signals. The electroseismic measurements on different samples have been conducted to confirm the origin of the recorded signals. We find that a strong acoustic signal generates around the electrode and affects the identification of ES signals. To further confirm and distinguish the ES signal as well as the acoustic signal around the electrode, we have analyzed records obtained with regular movements of the receiver, the sample and the source. Analysis has been made on the characteristics of the traveltime, polarity and frequency of ES signals. Our results show that the traveltime of ES signal relates to the distance between the rock sample and the receiver, the location of the exciting electrode has little impact on the traveltime. The applied electric field influences the polarity of ES signal, the polarity of ES signal reverses along with the changes of the electric field direction. While it has no polarity effects on the acoustic signal generated around the electrode. The frequency spectrum of ES signal is absolutely different with that of the acoustic signal generated around the electrode. The acoustic signal around the electrode has multiple dominant frequencies which are mainly in low-frequency range without being affected by the frequency of the electric field. The ES signal has only one dominant frequency which closely relates to the frequency of the electric field. The understanding of the signal characteristics on electroseismic conversion can contribute to a better application and interpretation of ES exploration.

Phosphorylation affects ubiquitination, stability, and activity of transcriptional factors, thus regulating various cellular functions. E2F transcriptional factor 1 (E2F1) regulates paternally expressed imprinted gene 10 (Peg10) expression, thereby promoting cell proliferation. However, the effect of E2F1 stability on Peg10 expression and the molecular regulation of E2F1 stability by its phosphorylation have not been well demonstrated. Here, we describe a new pathway in which phosphorylation of E2F1 by GSK3β increases E2F1 association with the deubiquitinating enzyme, ubiquitin-specific protease 11 (USP11), which removes K63-linked ubiquitin chains thereby preventing E2F1 degradation in the nuclei. Downregulation of USP11 increases E2F1 ubiquitination and reduces E2F1 stability and protein levels, thereby decreasing Peg10 mRNA levels. Physiologically, USP11 depletion suppresses cell proliferation and wound healing in lung epithelial cells, and these effects are reversed by E2F1 and PEG10 overexpression. Thus, our study reveals a new molecular model that phosphorylation promotes substrate stability through increasing its association with a deubiquitinating enzyme. The data suggest that GSK3β and USP11 act in concert to modulate E2F1 abundance and PEG10 expression in lung epithelial cells to affect cell wound healing. This study provides new therapeutic targets to lessen lung injury by improving lung epithelial cell repair and remodeling after injury.

For seismic data interpretation, horizon picking based on seismic events in stacked or migrated seismic sections is essential for obtaining information on subsurface structures. This conventional work is time-consuming via manual implementation. In this paper, we develop an automatic seismic event tracking method of horizon interpretation using the dynamic time warping (DTW) algorithm. The proposed method consists of two steps: calculating local time shifts between adjacent traces through a pilot trace and then event tracking. In the method, the DTW algorithm is applied to calculate time shifts between two adjacent traces, and an improved multitrace DTW strategy is proposed to improve the robustness. One synthetic seismic trace is used to demonstrate the DTW algorithm, and a synthetic seismic section is used to verify the feasibility of the proposed method handling contaminated seismic data with noise. Finally, we apply the method to a 3D physical model dataset. The result indicates that the proposed method is quantitatively feasible for seismic event automatic tracking and is reasonably stable for noisy seismic section flattening, which also has the potential to extract seismic horizon slices effectively.

Experimental physical inversion of rock from the diagonal group velocities is an effective method for the determination of Thomsen's δ anisotropy parameter in organic-rich shale. We further enhance the inversion accuracy of δ through conducting more reliable experimental measurements and through theoretical expression. First, we assembled two sets of group velocity acquisition methods, a rotational ultrasonic transducer system and a laser ultrasonic system, and then we assessed which of them was more applicable and accurate by comparing the waveforms and observations on the same cylindrical organicrich shale. Second, we combined the δ-based phase velocity approximation and stricter physical constraints of δ, which are deduced on a standard VTI medium, to improve the theoretical part of the inversion. As a result of better observations by the optimal test methods and the proposed δ inversion methods, the least errors between the best fitted curve to the observations are 3.24% for the traditional method and 2.1% for the proposed method, which verifies the superiority of the proposed method. Based on experimental tests on two cylindrical shale specimens, we find that rotational ultrasonic transducer measurement is more applicable for quick velocity anisotropy measurements, while for observations obtained by the laser technique, system relative error and the necessary scattering effect processing should be conducted. The procedure of the inversion is more robust and accurate when conducted on the proposed δ-based inversion.

Sparsity constraint inverse spectral decomposition (SCISD) is a time-frequency analysis method based on the convolution model, in which minimizing the l1 norm of the time-frequency spectrum of the seismic signal is adopted as a sparsity constraint term. The SCISD method has higher time-frequency resolution and more concentrated time-frequency distribution than the conventional spectral decomposition methods, such as short-time Fourier transformation (STFT), continuous-wavelet transform (CWT) and S-transform. Due to these good features, the SCISD method has gradually been used in low-frequency anomaly detection, horizon identification and random noise reduction for sandstone and shale reservoirs. However, it has not yet been used in carbonate reservoir prediction. The carbonate fractured-vuggy reservoir is the major hydrocarbon reservoir in the Halahatang area of the Tarim Basin, north-west China. If reasonable predictions for the type of multi-cave combinations are not made, it may lead to an incorrect explanation for seismic responses of the multi-cave combinations. Furthermore, it will result in large errors in reserves estimation of the carbonate reservoir. In this paper, the energy and phase spectra of the SCISD are applied to identify the multi-cave combinations in carbonate reservoirs. The examples of physical model data and real seismic data illustrate that the SCISD method can detect the combination types and the number of caves of multi-cave combinations and can provide a favourable basis for the subsequent reservoir prediction and quantitative estimation of the cave-type carbonate reservoir volume.

This paper designs a total angle ultrasonic test method to measure the P-wave velocities (vp), vertically and horizontally polarized shear wave velocities (vsv and vsh) of all angles to the bedding plane on different kinds of strong anisotropic shale. Analysis has been made of the comparisons among the observations and corresponding calculated theoretical curves based on the varied vertical transversely isotropic (TI) medium theories, for which discussing the real similarity with the characterizations of the TI medium on the scope of dynamic behaviors, and further conclude a more accurate and precise theory from the varied theoretical formulas as well as its suitable range to characterize the strong anisotropy of shale. At a low phase angle (theta 0.25, the Berrryman curve will be the best fit for the vp, vsv on shale.

SUMMARY Purely numerical methods for modelling seismic wave propagation are now fairly popular. In this paper, we report on the development of a hybrid finite element-finite difference method. Our method first employs semi-discretization of the finite element method in a part of the spatial domain (in the z-direction for 2-D situation) to obtain a wave equation in weak form, and then uses the finite difference method to solve it. This offers significant advantages in applying non-uniform grids with improved accuracy. To improve the computational efficiency, we introduce the spectral element method to replace the traditional finite element method, which leads to a diagonal mass matrix with sufficient accuracy. After that, we carry out detailed analyses of the dispersion behaviours for both uniform and non-uniform cases; the dispersioncurvesdemonstratehighprecisionofourmethodinnumericalmodelling,especially in dealing with non-uniform grids. Some examples are presented to demonstrate performance of this method and confirm the analytic results.

SUMMARY We have carried out two seismic physical experiments to acquire wide-azimuth P-wave 3D seismic data with a scaled down model (1:10 000) and scaled-up frequencies (10 000:1). Our aims are to verify the physical basis of using P-wave attributes for fracture detection, to understand the usage of these attributes and their merits, and to investigate the effects of acquisition geometry and structural variations on these attributes. The base model consists of a fractured layer sandwiched between two isotropic layers (Epoxylite). Inside the fractured layer there is a dome and a fault block for investigating the effects of structural variations. The two experiments were carried out using different acquisition geometries. The first experiment was conducted to maximize the data quality, with an offset-depth ratio of only 0.68 to the bottom of the fracture layer. For comparison, the second experiment was carried out to maximize the anisotropy effects, with the offset-depth ratio to the bottom of the fracture layer raised to 1.34. For each experiment, about 20 km 2 of wide-azimuth 3-D data were acquired with a P-wave source. The physical modelling confirms that the P-wave attributes (traveltime, amplitude and velocity) exhibit azimuthal variations diagnostic of fracture-induced anisotropy. For the first experiment with noise-free data, the amplitude from the top of the fracture layer yields the best results that agree with the physical model parameters and free of the acquisition footprint. The results from other attributes (traveltime, velocity, AVO gradient) are either contaminated by the structural imprint, or by the acquisition footprint due to the lack of offset coverage. For the second experiment, despite the interferences from multiples and other coherent noise, the traveltime attributes yield the best results; both the acquisition footprint and the structural imprint are reduced due to the increased offset coverage. However, the results from the amplitudes are affected by the noise and are less reliable. Analysis of the two experiments reveals that the offset-depth ratio to the target is a key parameter for the success of the P-wave techniques. Smaller offset-depth coverage may only be applicable to amplitude attributes with high quality data; whilst large offset coverage makes it possible to use traveltime attributes. A reliable estimation from traveltime attributes requires an offset-depth ratio of 1.0 or more.

Using a seismo-physical modelling technique with controlled crack densities introduced in laboratory, the effect of crack density and shape on seismic shear waves is studied. The physical models used in the experiment are made of epoxy resin with silicon rubber slices embedded in them to simulate the cracks. In the experiments, different degrees of shear-wave splitting can be observed in the media containing orientated cracks at various crack densities. The experimental results show that the fast shear-wave velocity is stable, whereas the slow shear-wave velocity increases with increasing crack density. The time difference between the two split shear waves is dependent mainly on the travel time of the slow shear wave. In the case of lower crack densities (

In this study, we analyse the influence of fluid on P- and S-wave anisotropy in a fractured medium. Equivalent medium theories are used to describe the relationship between the fluid properties and the rock physics characteristics in fractured rocks, and P-wave and S-wave velocities and anisotropy are considered to be influenced by fluid saturation. However, these theoretical predictions require experimental measurement results for calibration. A new construction method was used to create synthetic rock samples with controlled fracture parameters. The new construction process provides synthetic rocks that have a more realistic mineral composition, porous structure, cementation and pressure sensitivity than samples used in previous research on fractured media. The synthetic rock samples contain fractures which have a controlled distribution, diameter, thickness and fracture density. In this study, the fracture diameter was about 4 mm, the thickness of fractures was about 0.06 mm, and the fracture density in the two fractured rock samples was about 3.45%. SEM images show well-defined penny-shaped fractures of 4 mm in length and 0.06 mm in width. The rock samples were saturated with air, water and oil, and P- and S-wave velocities were measured in an ultrasonic measurement system. The laboratory measurement results show that the P-wave anisotropy is strongly influenced by saturated fluid, and the P-wave anisotropy parameter, e, has a much larger value in air saturation than in water and oil saturations. The S-wave anisotropy decreases when the samples are saturated with oil, which can be caused by high fluid viscosity. In the direction perpendicular to the fractures (the 0° direction), shear-wave splitting is negligible, and is similar to the blank sample without fractures, as expected. In the direction parallel to the fractures (the 90° direction) shear-wave splitting is significant. The fractured rock samples show significant P- and S-wave anisotropy caused by the fractures and controlled by the saturated fluids.