Journal of Geophysics and Engineering

Coalbed methane (CBM) is produced through desorption, diffusion, and seepage processes. However, pressure during CBM production, especially the seepage process, is a very complicated problem. Methods for solving the pressure models for CBM reservoirs are multidisciplinary and rather sophisticated. In this paper, an elementary and simple method for solving the CBM seepage model is proposed. First, a classical seepage model was built to study CBM seepage at a variable flow rate. After the transformation of pressure into pseudo-pressure and dimensionless treatments, a definite problem for the CBM seepage model was obtained. By Laplace transformation, the definite problem can be solved as a boundary value problem in Laplace space. Then, this boundary value problem was solved by the proposed method which is elementary and simple and its solutions have a structure similar to that of a continued fraction. This explains why this method is called the similar constructive method. With this method, solutions for the model of the pressure response of a CBM reservoir can be obtained. A corresponding program for the method was written, which can help engineers working on CBM reservoirs solve similar problems even without pre-training. In addition to the typical analysis curves for well testing such as the conventional log–log graph of the pressure and pressure differential and semi-log graph, another type of analysis curve, i.e., a log–log graph of the first-order pressure derivative was plotted. On such basis, the diagnosis of the pressure response of a CBM well is more accurate. Our results might be of great significance to the theoretical study of pressure responses of CBM reservoirs. The results of this research could provide great simplicity for software developers to create well testing analysis software and well testing interpreters to interpret CBM well test data.

The new millennium has seen a fresh wave of world economic development especially in the Asian-Pacific region. This has contributed to further rapid urban expansion, creating shortages of energy and resources, degradation of the environment, and changes to climatic patterns. Large-scale, new urbanization is mostly seen in developing countries but urban sprawl is also a major social problem for developed nations. Urbanization has been accelerating at a tremendous rate. According to data collected by the United Nations [1], 50 years ago less than 30% of the world population lived in cities. Now, more than 50% are living in urban settings which occupy only about 1% of the Earth's surface. During the period from 1950 to 1995, the number of cities with a population higher than one million increased from 83 to 325. By 2025 it is estimated that more than 60% of 8.3 billion people (the projected world population [1]) will be city dwellers.

Urbanization and urban sprawl can affect our living quality both positively and negatively. In recent years geophysics has found significant and new applications in highly urbanized settings. Such applications are conducive to the understanding of the changes and impacts on the physical environment and play a role in developing sustainable urban infrastructure systems. We would like to refer to this field of study as 'urban geophysics'.

Urban geophysics is not simply the application of geophysical exploration in the cities. Urbanization has brought about major changes to the geophysical fields of cities, including those associated with electricity, magnetism, electromagnetism and heat. An example is the increased use of electromagnetic waves in wireless communication, transportation, office automation, and computer equipment. How such an increased intensity of electromagnetic radiation affects the behaviour of charged particles in the atmosphere, the equilibrium of ecological systems, or human health, are new research frontiers to be investigated [2]. The first objective of urban geophysics is to study systematically the geophysical fields in cities, searching for principles and processes governing the intensity and patterns of variation of the geophysical properties, as well as the potential consequences on the biosphere.

Secondly, geophysics has already been found to be a useful tool for subsurface detection and investigation, hazard mitigation, and assessment of environmental contamination. Geophysicists have documented numerous cases of successful applications of geophysical techniques to solve problems related to hazard mitigation, safeguarding of lifeline infrastructure and urban gateways (air- and sea-ports, railway and highway terminals), archaeological and heritage surveys, homeland security, urban noise control, water supplies, sanitation and solid waste management etc. In contrast to conventional geophysical exploration, the undertaking of geophysical surveys in an urban setting faces many new challenges and difficulties. First of all, the ambient cultural noise in cities caused by traffic, electromagnetic radiation and electrical currents often produce undesirably strong interference with geophysical measurements. Secondly, subsurface surveys in an urban area are often targeted at the uppermost several metres of the ground, which are the most heterogeneous layers with many man-made objects.

Thirdly, unlike conventional geophysical exploration which requires resolution in the order of metres, many urban geophysical surveys demand a resolution and precision in the order of centimetres or even millimetres. Finally restricted site access and limited time for conducting geophysical surveys, regulatory constraints, requirements for traffic management and special logistical arrangements impose additional difficulties. All of these factors point to the need for developing innovative research methods and geophysical instruments suitable for use in urban settings.

This special issue on 'Sustainable urban development and geophysics' in Journal of Geophysics and Engineering is a response to the call for the development of novel geophysical techniques especially applicable to city settings. It consists of 11 papers which are selected and expanded from a collection of papers presented to the special sessions on 'Sustainable Urban Development and Geophysics' (U14A, U15A, and U41B) in the Union section of the Western Pacific Geophysics Meeting held in Beijing, China, on 22–27 July 2006 [3]. This indicates that new and innovative geophysical applications in urban settings have emerged, and these innovations may be potentially useful for the planning, implementation, and maintenance of urban infrastructure systems. These 11 research papers can be divided into three groups: (1) geophysics and urban infrastructure; (2) geophysics and urban environment; and (3) geophysical investigations associated with geological hazards.

The first group of papers focuses on urban infrastructure. Fred Stumm et al reported a geohydrologic assessment of fractured crystalline bedrock with borehole radar in Manhattan, New York in preparation for the construction of a new water tunnel. Using GPR, Xie et al conducted a quality control study of the walls of the river-crossing highway tunnel in Shanghai. For the same purpose, S Liu et al investigated the effect of concrete cracks on GPR signatures using a numerical simulation technique. Sun et al, using seismic surface waves, investigated road beds and the degree of weathering of the marble fence in the Forbidden City, Beijing.

In the second group of papers, using a numerical simulation technique, L Liu et al studied the effect of a building coordinate error on sound wave propagation with the aim of locating sound sources in urban settings. Chan et al studied the abundance of radio elements in weathered igneous bedrock in Hong Kong for the purpose of the promotion of public health in the urban environment.

The third group includes five papers on geo-hazards. The three papers by B Zhao et al and Z Zhao et al address the problem of earthquake strong ground motion in urban regions using case studies from Osaka, Japan and the city of Yinchuan, China. The other two papers study the geological hazard of surface subsidence using geophysical tools: G Leucci reported a comprehensive study in Nardo, Italy, while Kim et al reported a similar case study for a small city in South Korea.

One striking feature of all the papers in this special issue is that multiple authors with at least 3 co-authors wrote the majority of the papers, which is an indication of strong team work and interdisciplinary collaboration. This is essential for the successful application of geophysical science and technology in tackling a variety of engineering and environmental problems for the urban setting. The only sole author, Dr Leucci, expressed deep gratitude in his acknowledgements to his team members who carried out substantial parts of the data acquisition.

We are pleased to present this special issue to the engineering and environmental geophysics community and hope that it can serve as a snapshot of the current state-of-the-art studies in urban geophysics.

References

[1] United Nations 1990 World Demographic Estimates and Projections (1950–2025) (New York: Press of United Nations)

[2] Chen Y, L-S Chan and S Yu 2003 J. Geodesy & Geodynamics 23 1–4 (in Chinese)

[3] American Geophysics Union 2006 Eos Trans. AGU 87 (36)

Fractal dimension can effectively describe the microstructure of self-similar fractal porous media, and it is of great significance to predict their macroscopic mechanical properties by different fractal dimensions. Based on fractal theory and the Mori–Tanaka method, homogenization equations of fractal porous media were deduced by tensors in this paper, and the parameters affecting the prediction results and the macro mechanical properties were discussed. The numerical results show that the homogenization equations are reliable, and the value of λ_f,min/λ_f,max has an influence on prediction results and the macro mechanical properties. When the pore fractal dimension is close to 1.4, the prediction results by fractal homogenization equations in this paper are more accurate. Meanwhile, the macro mechanical properties are enhanced with the increase of the solid parameters (Poisson's ratio, elastic modulus), whereas they decrease with the increasing of the pore fractal dimension, and the varying curve can be obtained from the homogenization equations in this paper. The analytical solution in this paper can be utilized to predict the macroscopic mechanical properties of fractal porous media and to provide theoretical support for parameters selection of numerical simulation and scale upgrading.

Journal of Geophysics and Engineering (JGE) aims to publicize and promote research and developments in geophysics and in related areas of engineering.

As stated in the journal scope, JGE is positioned to bridge the gap between earth physics and geo-engineering, where it reflects a growing trend in both industry and academia. JGE covers those aspects of engineering that bear closely on geophysics or on the targets and problems that geophysics addresses. Typically this will be engineering focused on the subsurface, particularly petroleum engineering, rock mechanics, geophysical software engineering, drilling technology, remote sensing, instrumentation and sensor design.

There is a trend, visible throughout academia, for rapid expansion in cross-disciplinary, multi-disciplinary and inter-disciplinary working. Many of the most important and exciting problems and advances are being made at the boundaries between traditional subject areas and, increasingly, techniques from one discipline are finding applications in others. There is a corresponding increasing requirement for researchers to be aware of developments in adjacent areas and for papers published in one area to be readily accessible, both in terms of location and language, to those in others.

One such area that is expanding rapidly is that at the interface between geophysics and engineering. There are three principal developments.

• Geophysics, and especially applied geophysics, is increasingly constrained by the limits of technology, particularly computing technology. Consequently, major advances in geophysics are often predicated upon major developments in engineering and many research geophysicists are working in multi-disciplinary teams with engineers.

• Engineering problems relevant to the sub-surface are increasingly looking to advances in geophysics to provide part of the solution. Engineering systems, for example, for tunnel boring or petroleum reservoir management, are using high-resolution geophysical imaging to reduce uncertainty and associated risk.

• In the economically dominant area of petroleum exploration and production, the focus has moved dramatically from exploration to production. This shift is leading increasingly to integration between petroleum geoscience and petrophysics on the one hand, and petroleum engineering and rock mechanics on the other. This integration means that petroleum engineers need to be aware of developments in geophysics, and geophysicists need to be aware of the problems and requirements of the reservoir engineer.

Journal of Geophysics and Engineering has been established firmly in that context, and we expect this trend to strengthen and extend far into the future.

The Editors welcome your submissions, and comments on this first issue of JGE.

Digital rock physics (DRP) builds a bridge between pore-scale physical processes and the macroscopic physical properties of rock. Its key paradigm is to image and digitize the pore space and mineral matrix of rock, then to simulate the response of various physical fields and calculate the equivalent elastic parameters of rocks through digital rocks and mathematical methods. In this paper, a new approach is proposed to estimate the rock moduli of two-phase media with the staggered-grid high-order finite difference method (FDM) of the Biot theory based on DRP. This new method not only takes into consideration the impact of fluid on elastic wave propagation in two-phase media, but it is also easy to understand and implement, improving the calculation accuracy, requiring less memory and improving on the weaknesses of conventional rock physics experiments which are time consuming and expensive. In order to estimate the rock moduli, we establish two models, and the digital rock sample is embedded in one of those. Using this method, it is possible to model the dynamic wave propagation and measure the time delay of the peak amplitude caused by the inhomogeneous structure of the digital rock sample, with the receivers set at the bottom of the two models. The time delay allows us to estimate the effective velocity of both compressional and shear waves, and therefore calculate the rock moduli. Additionally, comparison between the numerical simulated results obtained through this method and experimental results indicates that they agree well. Comparison with the numerical simulated results obtained via another method tests and verifies the accuracy and feasibility of the new method. Also, the equivalence conditions between this new method and the various rock physics models are inferred.

The decrease in the amplitude and resolution of seismic waves with depth, the so-called earth Q effects, can be conveniently defined in terms of frequency-dependent amplitude attenuation and velocity dispersion. In this paper, we modify Kolsky's (1953 Stress Waves in Solids (Oxford: Clarendon)) attenuation–dispersion model so that it has an accurate representation of the velocity dispersion within the seismic band. Such a modification may lead to at least two advantages: (1) an accurate phase correction in inverse Q filtering that follows, and (2) a good match to other earth Q models. The latter suggests that, when applying them to design an inverse Q filter, filtered seismic sections should in principle be comparable with each other.

X-ray CT scanning is one of the main methods to study the pore structure of the rocks. However, low resolution is currently one of the major challenges of this method, which does not allow for the identification of microscopic pores. Even though adopting images of higher resolutions could reflect the microscopic pores of the rocks, due to the small size of the scanned rock sample in the x-ray CT scanning method, the rock parameters cannot be effectively obtained, especially for the rocks with strong heterogeneity. This would cause the loss of important pore information, such as fractures' information. Focusing on this problem, the current research tries to propose a method for constructing high-precision digital rocks of complexly porous rocks. This method is based on the simultaneous application of x-ray CT scanning images, nuclear magnetic resonance measurements, as well as mercury injection experimental data and fractal discrete fracture network. These methods effectively compensate for the shortcomings of the scanning method where it cannot capture pores smaller than the scanning resolution and for the lack of fracture information due to rock sampling. The research results showed that compared with the digital rock models constructed by single-resolution scanning, the high-precision complex pore digital rock constructed by this method can effectively improve the accuracy of the results of porosity and permeability calculations. These results were closer to the results of rock physics experiments. This approach provides a solid foundation for the numerical simulation of rock parameters in unconventional reservoirs with complex pore structures, such as carbonate, tight sandstone and shale reservoirs.

The relationship between seismic velocity and porosity is one of the most important research topics in exploration geophysics. A strong correlation, if it exists, can be very useful for quantitative seismic interpretation of direct hydrocarbon indicators. The Voigt and Reuss bounds are often plotted alongside the data for quality control and interpretation. The relative position of the data within the bounds may supply important information regarding the pore shape, stress condition, lithology, and diagenesis of porous rocks. It is found that the Voigt bound and Reuss bound are actually special cases of the weighted power mean, with the power parameters being 1 and −1, respectively. The geometric mean is a special case of the power mean with the power parameter being zero. The power mean is a monotonic function of the power parameter. For the construction of a rock physics template, the region between the Voigt and Reuss bounds can be evenly divided by varying the power parameter between −1 and 1. The power mean can be a very useful tool for modeling the elastic properties of rocks. We have demonstrated the potential applications of the power mean in the studies of velocity-porosity relation, effective media, and pore fluids effect on seismic velocities.

The recently developed cross-rhombus stencil-based time–space domain finite-difference (FD) method for modeling two-dimensional acoustic equations, controls the temporal and spatial dispersions synchronously and outperforms cross-stencil-based FD (CS-FD). However, it is only applicable to modeling on equally spaced grids and extensive application is hindered. In this work, we extend it further and develop novel cross-rhombus stencil-based FD (CRS-FD) for modeling on arbitrarily rectangular grids. Two kinds of rhombus stencils involving grid points both on and off the axis are developed first to achieve fourth order and sixth order FD accuracies, respectively, for solving the second order temporal derivative on rectangular grids. The plane wave theory is then used to derive the time–space-domain dispersion relations, when the temporal and spatial derivatives are solved by the new rhombus-stencil-based temporal high order FD and the cross-stencil-based spatial high order FD, respectively. The extrapolation stencil is a mixture of rhombus and cross stencils, and the involved FD coefficients are determined by applying the Tayler expansion on the time–space domain dispersion relation. Our new CRS-FD is high-order accurate in both time and space, and applicable to modeling on arbitrarily rectangular grids. Dispersion analysis, stability analysis and modeling examples on rectangular grids show that the CRS-FD is more accurate and stable than the CS-FD. Meanwhile, we develop the variable-length schemes for CRS-FD to further increase efficiency and apply them to extrapolate wavefields in reverse time migration. The results validate that the CRS-FD is more efficient than the CS-FD because much larger time steps can be used while reaching a similar accuracy. The variable-length scheme further reduces the computational time in comparison with the fixed-length scheme.

Two perpendicular high-resolution shallow seismic and resistivity profiles were acquired to assist in imaging the near-subsurface sedimentary architecture for hydrogeological prospecting in the Nylsvley Nature Reserve along the Nyl River floodplain in South Africa. We deployed 48 channels of 14 Hz geophones with 1–2 m dense source-receiver spacing providing fold coverage of 24 (line 2) and 48 (line 1). The resistivity profiles were acquired with a Schlumberger electrode configuration with 2–3 m electrode spacing and a total of 40 steel electrodes. The raw seismic shot records were characterized by low-frequency, high-amplitude source-generated noise (surface and guided waves). Reflections from bedrock were not obvious. To enhance the reflected seismic signal, we employed an extensive seismic processing workflow which enhanced the seismic reflectivity on the stacked sections. The seismic reflection interpretation was constrained and integrated with seismic refraction and resistivity tomography, and the one-dimensional multichannel analysis of surface waves to generate the model that best represents the real subsurface geological model. The integrated results show the bedrock-overburden contact at 8–12 m depth, which correlates well with boreholes drilled in the area. In addition, the reflection seismic and refraction tomography show the bedrock undulation and velocity changes associated with erosional surfaces or weathered/fracture systems. We further interpret these characteristics to be associated with groundwater movement and storage related to the fractured/weathered zone within the bedrock. The integrated data also delineate the interface between the unsaturated sand and saturated sand-gravel that represents the groundwater water table. This study demonstrates the potential of combining several surface geophysical techniques for near-surface investigations, especially for hydrogeological prospecting.

The fault fracture zone has is characterized by low strength, easy deformation and easy activation under the influence of high-strength mining, resulting in instability of the roadway coal rock mass in the affected area and thus severe deformation of the surrounding rock. Aiming to prevent serious deformation of a large section roadway along a fault in response to drivage, combined with specific geological conditions and supporting engineering practice of the panel of a coal mine, UDEC polygonal methods are used to compare and analyze the fracture development, stress distribution and deformation of roadway surrounding rocks and the bearing force of the supporting body of the original support scheme and the new support scheme. In the numerical simulation scheme, the study area is divided into polygonal blocks and the mechanical parameters of the blocks and contacts in the Voronoi program are determined by fitting the rock mass properties obtained through laboratory tests. A time-dependent strength degradation process is used to simulate the weakening effect of the fault on the surrounding rock mass. Then a new scheme for combined support involving key area strengthening and high-strength yielding bolts is proposed, which can ensure that the stress distribution of surrounding rock tends to be reasonable and that the deformation of roadway surrounding rock is in a reasonable range. In addition, the research results are applied successfully in the field.

Today, coal and oil are the main energy sources used in the world. However, these sources will last for only a few decades. Hence, the investigation of possible energy sources to meet this crisis has become a crucial task. Coal bed methane (CBM) is a potential energy source which can be used to fulfil the energy demand. Since the amount of carbon dioxide (CO₂) emitted to the atmosphere from the use of CBM is comparatively very low compared to conventional energy sources, it is also a potential mitigation option for global warming.

This paper reviews CBM recovery techniques with particular emphasis on CO₂-enhanced coal bed methane (CO₂-ECBM) recovery. The paper reviews (1) conventional CBM recovery techniques and problems associated with them, (2) CBM production-enhancement methods, including hydro-fracturing and enhanced CBM recovery techniques, such as N₂-ECBM and CO₂-ECBM, (3) the importance of the CO₂-ECBM technique compared to other methods and problems with it, (4) the effect of CO₂ injection during the CO₂-ECBM process on coal seam permeability and strength and (5) current CO₂-ECBM field projects and their progress.

Although conventional CBM recovery methods are simple (basically related to the drawdown of the reservoir pressure to release methane from it), they are inefficient for the recovery of a commercially viable amount of methane from coal seams. Therefore, to enhance methane production, several methods are used, such as hydro-fracturing and ECBM (N₂-ECBM and CO₂-ECBM). The CO₂-ECBM process has a number of advantages compared to other methane recovery techniques, as it contributes to the mitigation of the atmospheric CO₂ level, is safer and more economical. However, as a result of CO₂ injection into the coal seam during the CO₂-ECBM process, coal mass permeability and strength may be crucially changed, due to the coal matrix swelling associated with CO₂ adsorption into the coal matrix. Both injecting CO₂ properties (gas type, CO₂ phase and pressure) and coal seam properties (coal rank and temperature) affect this swelling. Although there are many related studies, a number of gaps exist, especially in the area of coal rank and how the effect of other factors varies with the rank of the coal seam. To date, there have been few CO₂-ECBM field projects in the world. However, the reduction of CO₂ injectability after some time of CO₂ injection, due to coal matrix swelling near the well bore, is a common problem in the field. Therefore, various permeability-enhancing techniques, such as hydro-fracturing and injection of an inert gas such as N₂ or a mixture of inert gases (N₂ + CO₂) into the seam to recover the swelled areas are under test in the field.

Geophysical methodologies have been implemented, tested and validated as diagnostic and /or monitoring tools in artworks or historical monuments. They are non-destructive and can give an image of internal structure of investigated medium. This paper is a review about the main geophysical techniques applied to the study of cultural built heritage (excluding the archaeology field). A brief description of the used methodologies is presented, the main investigations done in this field are showed, the method or methods most appropriate to answer each problem (moisture detection, characterization of the materials, study of the structural continuity of the material, assessment of intervention's effectiveness) are indicated and the main advances and gaps and future developments are also pointed out.

In a vertical borehole, free heat convection arises when the temperature gradient equals or exceeds the so-called critical gradient. The critical temperature gradient is expressed through the critical Rayleigh number and depends on two parameters: (a) the ratio of formation (casings) to fluid (gas) conductivities (λ_f/λ) and (b) the convective parameter of the fluid. Both these parameters depend on the temperature (depth). An empirical equation for the critical Rayleigh number as a function of the ratio λ_f/λ is suggested. For the 0–100 °C range, empirical equations for convective parameters of water and air are proposed. The analysis of the published results of field investigations in deep boreholes and modelling shows that the temperature disturbances caused by thermal convection do not exceed 0.01–0.05 °C. Thus, in deep wells the temperature deviations due to thermal convection are usually within the accuracy of the temperature surveys. However, due to convection cells the geothermal gradient cannot be determined with sufficient accuracy for short well sections. In shallow boreholes the effect of thermal convection is more essential (up to 3–5 °C). To reduce the effect of convection on the temperature regime in shallow observational wells, it is necessary to reduce the diameter of the wellbores and use well fillers (fluids and gases) with low values of the convective parameters. The field observations and numerical calculations indicate that the distorting effect due to casing pipes is small and its influence is localized to the ends of the pipes, and this effect is independent of time.