Table of contents

This paper discusses the concepts of multicomponent seismology and how it can be applied to characterize hydrocarbon reservoirs, illustrated using a 3D three-component real-data example from southwest China. Hydrocarbon reservoirs formed from subtle lithological changes, such as stratigraphic traps, may be delineated from changes in P- and S-wave velocities and impedances, whilst hydrocarbon reservoirs containing aligned fractures are anisotropic. Examination of the resultant split shear waves can give us a better definition of their internal structures. Furthermore, frequency-dependent variations in seismic attributes derived from multicomponent data can provide us with vital information about fluid type and distribution. Current practice and various examples have demonstrated the undoubted potential of multicomponent seismic in reservoir characterization. Despite all this, there are still substantial challenges ahead. In particular, the improvement and interpretation of converted-wave imaging are major hurdles that need to be overcome before multicomponent seismic becomes a mainstream technology.

Ore prospecting using gravimetric and magnetic data has become one of the traditional approaches used in past decades, often complemented with electric and electromagnetic methods. However, due to the problem of non-uniqueness inherent to potential-field modelling, constraints provided by structural methods such as seismic reflection are often used. During the exploration of polymetallic massive sulfide minerals in the Iberian Pyrite Belt, Figueira de Cavaleiros sector, located in the Sado Tertiary Basin, several gravimetric and magnetic anomalies were considered to be interesting targets. In order to reduce any ambiguity in the gravimetric modelling and to confirm the geological model of the area, two seismic reflection profiles were acquired. The interpretation of these profiles was assisted by three mechanical boreholes, two of which were located in the research area to make a seismostratigraphic interpretation. Unfortunately, the gravimetric modelling suggests that the anomaly has a lithological and structural origin and is not related to massive sulfides. Nevertheless, a good agreement between the seismic and potential-field data was achieved and new insights into the geological model for the region were obtained from this work, with accurate data about the Tertiary cover and Palaeozoic basement.

Path-integral imaging forms an image with no knowledge of the velocity model by summing over the migrated images obtained for a set of migration velocity models. Double path-integral imaging migration extracts the stationary velocities, i.e. those velocities at which common-image gathers align horizontally, as a byproduct. An application of the technique to a real data set demonstrates that quantitative information about the time migration velocity model can be determined by double path-integral migration velocity analysis. Migrated images using interpolations with different regularizations of the extracted velocities prove the high quality of the resulting time-migration velocity information. The so-obtained velocity model can then be used as a starting model for subsequent velocity analysis tools like migration tomography or other tomographic methods.

Inversion is a critical step in all geophysical techniques, and is generally fraught with ill-posedness. In the case of seismic surface wave studies, the inverse problem can lead to different equivalent subsoil models and consequently to different local seismic response analyses. This can have a large impact on an earthquake engineering design. In this paper, we discuss the consequences of non-uniqueness of surface wave inversion on seismic responses, with both numerical and experimental data. Our goal is to evaluate the consequences on common seismic response analysis in the case of different impedance contrast conditions. We verify the implications of inversion uncertainty, and consequently of data information content, on realistic local site responses. A stochastic process is used to generate a set of 1D shear wave velocity profiles from several specific subsurface models. All these profiles are characterized as being equivalent, i.e. their responses, in terms of a dispersion curve, are compatible with the uncertainty in the same surface wave data. The generated 1D shear velocity models are then subjected to a conventional one-dimensional seismic ground response analysis using a realistic input motion. While recent analyses claim that the consequences of surface wave inversion uncertainties are very limited, our test points out that a relationship exists between inversion confidence and seismic responses in different subsoils. In the case of regular and relatively smooth increase of shear wave velocities with depth, as is usual in sedimentary plains, our results show that the choice of a specific model among equivalent solutions strongly influences the seismic response. On the other hand, when the shallow subsoil is characterized by a strong impedance contrast (thus revealing a characteristic soil resonance period), as is common in the presence of a shallow bedrock, equivalent solutions provide practically the same seismic amplification, especially in the frequency range of engineering interest.

Conventional common-reflection-surface (CRS) parameter estimation is based on coherence analysis using the second-order semblance function. Extensions of the usual semblance can be defined by replacing second-order by higher-order quantities. Here we tested such extensions in an application to CRS parameter estimation. We found encouraging results by a natural extension of semblance to the fourth order. Numerical examples show that for low noise levels, results for all tested orders of the semblance were equivalent, whereas for high noise levels, the new fourth-order semblance function worked better in the linear search. This measure is less dependent on the aperture and noise level, thus resulting in reliable estimates of the local slope more often than when using conventional semblance.

Since interpretation of seismic data is usually a tedious and repetitive task, the ability to do so automatically or semi-automatically has become an important objective of recent research. We believe that the vagueness and uncertainty in the interpretation process makes fuzzy logic an appropriate tool to deal with seismic data. In this work we developed a semi-automated fuzzy inference system to detect the internal architecture of a mass transport complex (MTC) in seismic images. We propose that the observed characteristics of a MTC can be expressed as fuzzy if-then rules consisting of linguistic values associated with fuzzy membership functions. The constructions of the fuzzy inference system and various image processing techniques are presented. We conclude that this is a well-suited problem for fuzzy logic since the application of the proposed methodology yields a semi-automatically interpreted MTC which closely resembles the MTC from expert manual interpretation.

The pseudo-seismic waves obtained by the transient electromagnetic method (TEM) can be used to explain geological conformations of underground objects. However, the directly transformed pseudo-wave always has a dispersive and broader waveform so that it cannot clearly indicate the geological–electrical boundary of the explored underground objects. This study shows that the reason for the waveform dispersion is because the variance of the kernel function with the Gauss distribution is the real time; the wave distribution range will increase with increasing time. Therefore, this study presents a technique for enhancing the resolving power of pseudo-seismic data processed by a de-convolution method, and detailed theory and algorithms are reported. The pseudo-seismic signals of two geological–electrical models are taken as the examples to verify the present algorithm. Results show that the pseudo-seismic waves after de-convolution processes have sharper waveforms and narrower distribution zones, and this can effectively control the waveform broadening phenomenon and is able to enhance the resolving power using the TEM pseudo-seismic waves to survey the boundary of underground objects.

In this study, the quantification and incorporation of pore geometry, a qualitative parameter, and a source of dynamic information, will be demonstrated in the integrated reservoir studies. To quantify pore geometry, mercury injection capillary pressure (MICP) curves have been exploited. For each MICP curve, 20 parameters were derived and multi-resolution graph-based clustering was applied to classify the curves into nine representative distinct clusters. The number of clusters was determined based on petrography and cluster analysis. The quantified pore geometry in terms of discrete variable has been called pore-facies, and like electro-facies and litho-facies could be used in facies modelling and rock typing phases of an integrated study. The dependence of dynamic reservoir rock properties on pore geometry makes the pore-facies an interesting and powerful approach for incorporation of small-scale dynamic data into a reservoir model. A comparison among various facies definitions proved that neither litho-facies nor electro-facies is capable of characterizing dynamic rock properties, and the best results were achieved by the pore-facies method. Based on this study, it is recommended that for facies analysis in reservoir modelling, methods based on pore characteristics such as pore-facies, introduced in this paper, be used rather than traditional facies that rely on matrix properties. The next generation of the reservoir models which incorporate pore-facies-based rock types will provide a basis for more accurate static and dynamic models, a narrower range of uncertainty in the models, and a better prediction of reservoir performance.

An efficient algorithm is presented in this paper to improve the perturbation efficiency of the adaptive downhill simplex simulated annealing (ASSA) method for magnetotelluric inversion with oblique correlated misfit valleys. The correlated model space is rotated to a less dependent space which is defined by the eigenvectors of the parameter covariance updated progressively by the inversion itself. The downhill simplex step and rotation work together to decorrelate the correlated model space. The application to two synthetic cases and real data indicates that ASSA in the rotated space generally has a better convergence and efficient behaviour than ASSA without rotation. In the rotated space, the high rejection rates, which occurred in the unrotated space, are avoided. At low temperature, the estimated covariance can be used to approximate the global covariance. For all cases, ASSA in the rotated space gives better inversion results.

Approximate and rigorous solutions of seven heat transfer models were statistically examined, for the first time, to estimate stabilized formation temperatures (SFT) of geothermal and petroleum boreholes. Constant linear and cylindrical heat source models were used to describe the heat flow (either conductive or conductive/convective) involved during a borehole drilling. A comprehensive statistical assessment of the major error sources associated with the use of these models was carried out. The mathematical methods (based on approximate and rigorous solutions of heat transfer models) were thoroughly examined by using four statistical analyses: (i) the use of linear and quadratic regression models to infer the SFT; (ii) the application of statistical tests of linearity to evaluate the actual relationship between bottom-hole temperatures and time function data for each selected method; (iii) the comparative analysis of SFT estimates between the approximate and rigorous predictions of each analytical method using a β ratio parameter to evaluate the similarity of both solutions, and (iv) the evaluation of accuracy in each method using statistical tests of significance, and deviation percentages between 'true' formation temperatures and SFT estimates (predicted from approximate and rigorous solutions). The present study also enabled us to determine the sensitivity parameters that should be considered for a reliable calculation of SFT, as well as to define the main physical and mathematical constraints where the approximate and rigorous methods could provide consistent SFT estimates.

This paper proposes a method for the parameterization and characterization of fault facies models including a fault core and a fault damage zone containing either fractures or deformation bands, typically associated with carbonate and sandstone reservoirs, respectively. We represent the faulted reservoir models with a relatively small number of parameters and focus on the inverse problem; that is, how to estimate transmissibility of the fault core and the parameters of the fractures or deformation bands that determine the effective stiffness and permeability tensors in the damage zone. Our workflow is based on a consistent stiffness-permeability model for the fractured or composite porous media in the damage zone, and a Bayesian (Monte Carlo Markov chain) method of inversion, which provides information about uncertainties as well as the most likely values of the model parameters. For simplicity, we have assumed that the damage zone consists of a single set of fractures or deformation bands that are parallel with the (vertical) fault core, but the forward modelling part of our workflow can easily be extended to deal with more complex situations involving multiple sets of fractures and/or deformation bands that are characterized by different shapes and orientations. The results of our numerical experiments suggest that one can indeed obtain an improved characterization of fault zones by quantitative integration of seismic AVAZ and production data using the workflow presented in this paper.

The finite-difference (FD) method is a powerful tool in seismic wave field modelling for understanding seismic wave propagation in the Earth's interior and interpreting the real seismic data. The accuracy of FD modelling partly depends on the implementation of the free-surface (i.e. traction-free) condition. In the past 40 years, at least six kinds of free-surface boundary condition approximate schemes (such as one-sided, centred finite-difference, composed, new composed, implicit and boundary-modified approximations) have been developed in FD second-order elastodynamic simulation. Herein we simulate seismic wave fields in homogeneous and lateral heterogeneous models using these free-surface boundary condition approximate schemes and evaluate their stability and applicability by comparing with corresponding analytical solutions, and then quantitatively evaluate the accuracies of different approximate schemes from the misfit of the amplitude and phase between the numerical and analytical results. Our results confirm that the composed scheme becomes unstable for the V_s/V_p ratio less than 0.57, and suggest that (1) the one-sided scheme is only accurate to first order and therefore introduces serious errors for the shorter wavelengths, other schemes are all of second-order precision; (2) the new composed, implicit and boundary-modified schemes are stable even when the V_s/V_p ratio is less than 0.2; (3) the implicit and boundary-modified schemes are able to deal with laterally varying (heterogeneous) free surface; (4) in the corresponding stability range, the one-sided scheme shows remarkable errors in both phase and amplitude compared to analytical solution (which means larger errors in travel-time and reflection strength), the other five approximate schemes show better performance in travel-time (phase) than strength (amplitude).

We present a new 3D correlation imaging approach for magnetic total field anomaly and its vertical gradient for deriving a 3D equivalent magnetic dipole distribution in the subsurface. In this approach, we divide the subsurface space into a 3D regular grid, and then at each grid node calculate cross correlation between the observed magnetic total field anomaly (or its vertical gradient) and the theoretical magnetic total field anomaly (or its vertical gradient) due to a magnetic dipole. The resultant correlation coefficients are used to describe the equivalent magnetic dipoles distribution in a probabilistic sense. The approach was tested both on the synthetic magnetic data of a model of multiple rectangular prisms and on the real aeromagnetic data from an iron-ore deposit area in the middle–lower reaches of the Yangtze River, China. The results show that the equivalent magnetic dipole distribution derived by the approach basically reflects the subsurface magnetic sources and also illustrate that the approach for the vertical gradient produces a higher resolution of the equivalent magnetic source distribution than that for magnetic total field anomaly alone.

To analyse the resolution and accuracy of cross-borehole ground penetrating radar (XBGPR) methods in monitoring the variation due to fluid infiltration in the heterogeneous vadose zone, we built a hypothetical two-dimensional model by using the Sandia-Tech Vadose Zone (STVZ) model for unsaturated-flow modelling. The spatial variation in water content provided by the flow modelling was converted to a dielectric constant and electrical conductivity. We then used these parameters in finite-difference time-domain (FDTD) electromagnetic (EM) forward modelling to simulate the response of cross borehole radar surveys with antenna configurations identical to those of the STVZ site. We inverted the synthetic GPR data by using a damped least-squares method with straight ray-path assumptions. The results from the resolution analysis suggest that attenuation within a clay layer is overestimated near the boreholes but underestimated between the boreholes in the inverted images synthesized by FDTD modelling. Compared to the attenuation images, inverted water-content images are more representative of the input model. The inverted attenuation also shows that artefacts such as 'bull's eye' structures appear in the attenuation images. The reason for the poorer resolution and artefacts in the attenuation images concerns the straight-ray assumptions that we made in the inversion to approximate propagation of the EM waves with additional energy loss when they crossed the velocity boundaries. In addition, the clay layer serves as a waveguide when both the transmitter and receiver located in the clay layer and our current inversion algorithm do not account for the waveguide physics. Thus, it is necessary to properly incorporate physics into the inversion algorithms in order to correctly invert for the attenuation.

There has been widespread interest in acoustic reflection logging recently due to its higher resolution compared to seismic exploration and larger lateral extension compared to logging techniques. Acoustic reflection logging is particularly useful for delineating near-borehole subtle structures and fractured reservoirs. The primary data processing of acoustic reflection logging is to extract the reflection signals from the full waveform records which are overwhelmingly dominated by the borehole direct modes. We present in this paper a method combining a frequency–wavenumber (f–k) filter with a modified parametric prediction to separate the direct and reflection waves. Afterwards, we apply dip stacking to enhance and to further separate P–P and S–S reflection signals, and suppress the residual direct waves and other noises. Processing examples obtained with synthetic and real data show the capabilities of the proposed workflow and algorithms.

Velocity anisotropy caused by fine layering has a strong effect on the travel paths at large vertical offsets in crosshole seismic data. This effect must be considered in seismic waveform tomography, so that the inversion procedure can match the waveforms in real data. In this paper, we propose to properly take into account this anisotropic effect in two stages. First, we invert for the anisotropy parameter simultaneously with the (horizontal) velocity in travel time tomography. Then we incorporate this estimated anisotropy parameter model in the waveform simulation during waveform tomography, to refine the velocity image. When considering this anisotropic effect, far-offset data are properly used in the inversion, then both travel time and waveform tomography may have a better ray coverage, producing a velocity image with a higher resolution.

Based on the porosity-wave mass-transport and heat-transfer conceptual model in the continental lithosphere, the effects of upward throughflow on the continental lithosphere thermal structure are discussed. Previous theoretical works have proposed three thermal structure modes in the continental lithosphere, namely the heat-conductive-only lithosphere, the heat-conductive-and-advective lithosphere with a pore fluid of constant density, and the heat-conductive-and-advective lithosphere with a pore fluid of variable density (temperature-dependent density). The main purpose of this paper is to use the theoretical model to conduct parametric analysis and compare the numerical results produced with geological observation data. Two points are indicated. (1) The heat-conductive-only lithosphere may represent the thermal structure mode in the shield (or the stable platform), while the heat-conductive-and-advective lithosphere with a pore fluid of variable density often represents the lithospheric thermal structure mode in the continental active region or the orogenic belt. (2) The upward throughflow is the major factor to affect the stable lithospheric thickness of the Qinghai-Xizang plateau, North China, South China and the marginal sea region, demonstrating that the upward throughflow can have significant effects on the thermal structure mode of a continental lithosphere.

The amplitude of the analytic signal of the in-phase component of very low frequency electromagnetic method (VLF-EM) data is used in the delineation of the direction of basement fractures which host uranium. The images of the 2D and 3D amplitude of the analytic signal, particularly 3D images, bring out precisely the basement fractures. The in-phase component with the Hilbert transform yields an approximate depth to the basement fractures. The entire study is exemplified with the in-phase component of VLF-EM data to map the spatial distribution of basement fractures where uranium mineralization is reported in Sambalpur granitoids in the Raigarh district, Chhattisgarh, India.

Inversion codes based on a modified genetic algorithm (GA) have been developed to invert multimode Love wave dispersion curves. The multimode Love wave dispersion curves were synthesized from the profile representing shear-wave velocity reversal using a full SH (shear horizontal) waveform. In this study, we used a frequency–slowness transform to extract the dispersion curve from the full SH waveform. Dispersion curves overlain in dispersion images were picked manually. These curves were then inverted using the modified GA. To assess the accuracy of the inversion results, differences between the true and inverted shear-wave velocity profile were quantified in terms of shear-wave velocity and thickness errors, E_S and E_H. Our numerical modeling showed that the inversion of multimode dispersion curves can significantly provide the better assessment of a shear-wave velocity structure, especially with a velocity reversal profile at typical geotechnical site investigations. This approach has been applied on field data acquired at a site in Niigata prefecture, Japan. In these field data, our inversion results show good agreement between the calculated and experimental dispersion curves and accurately detect low velocity layer targets.

Restoration of historical buildings is a delicate operation as they are often built over more ancient and important structures. The Machado de Castro Museum, Coimbra, Central Portugal, has suffered several interventions in historical times and lies over the ancient Roman forum of Coimbra. This building went through a restoration project. These works were preceded by an extensive geophysical survey that aimed at investigating subsurface stratigraphy, including archeological remains, and the internal structure of the actual walls. Owing to the needs of the project, geophysical data interpretation required not only integration but also high resolution. The study consisted of data acquisition over perpendicular planes and different levels that required detailed survey planning and integration of data from different locations that complement images of the surveyed area. Therefore a multi-method, resistivity imaging and a 3D ground probing radar (GPR), high-resolution geophysical survey was done inside the museum. Herein, radargrams are compared with the revealed stratigraphy so that signatures are interpreted, characterized and assigned to archeological structures. Although resistivity and GPR have different resolution capabilities, their data are overlapped and compared, bearing in mind the specific characteristics of this survey. It was also possible to unravel the inner structure of the actual walls, to establish connections between walls, foundations and to find older remains with the combined use and spatial integration of the GPR and resistivity imaging data.

The effects of the wettability and solubility of natural gas in formation water on the electrical properties of natural gas reservoir rocks are studied using the finite element method based on digital cores. The results show that the resistivity index of gas-wet reservoir rocks is significantly higher than that of water-wet reservoir rocks in the entire range of water saturation. The difference between them increases with decreasing water saturation. The resistivity index of natural gas reservoir rocks decreases with increasing additional conduction of water film. The solubility of natural gas in formation water has a dramatic effect on the electrical properties of reservoir rocks. The resistivity index of reservoir rocks increases as the solubility of natural gas increases. The effect of the solubility of natural gas on the resistivity index is very obvious under conditions of low water saturation, and it becomes weaker with increasing water saturation. Therefore, the reservoir wettability and the solubility of natural gas in formation water should be considered in defining the saturation exponent.

We have investigated the accuracy of Rüger's approximation for PP reflection coefficients in HTI media (relative to an exact generalization of Zoeppritz to anisotropy derived by Schoenberg and Protazio) within the context of seismic fracture characterization. We consider the inverse problem of seismic amplitude-versus-angle and azimuth (AVAZ) inversion with respect to fracture density and azimuthal fracture orientation, as well as the forward problem of calculating PP reflection coefficients for different contrasts and anisotropy levels. The T-matrix approach was used to relate the contrast and anisotropy level to the parameters of the fractures (in the case of a single set of vertical fractures). We have found that Rüger's approximation can be used to recover the true fracture density with small uncertainty if, and only if, the fracture density and contrast are significantly smaller than the values that are believed to occur in many practically interesting cases of fractured (carbonate) reservoirs. In one example involving a minimal contrast and a fracture density in the range 0.05–0.1, Rüger's approximation performed satisfactorily for inversion, although the forward modelling results were not very accurate at high incident angles. But for fracture densities larger than 0.1 (which we believe may well occur in real cases), Rüger's approximation did not perform satisfactorily for forward or inverse modelling. However, it appears that Rüger's approximation can always be used to obtain estimates of the azimuthal fracture orientation with small uncertainty, even when the contrast and anisotropy levels are extremely large. In order to illustrate the significance of our findings within the context of seismic fracture characterization, we analysed a set of synthetic seismic AVAZ data associated with a fault facies model where the fracture density decreases exponentially with distance from the fault core, and a set of real seismic AVAZ data involving offset-averaged (root mean square) maps for six azimuthal sectors. Our results suggest that the difference between the maps of inverted fracture densities (which partially determines the effective permeability) obtained using Rüger's approximation can be very different from those obtained using the exact reflection coefficients; but Rüger's approximation can still be used to obtain qualitative information about the trends in the spatial distribution of fractures.