Fred Kofi Boadu

Fractured rock mass characterization parameters and seismic properties: Analytical studies

Abstract Analytical studies have been carried out based on a modified displacement discontinuity fracture model to characterize the relations between seismic properties and fractured rock mass parameters. Models of a fractured medium are developed to represent vertically aligned fractures embedded in an intact rock material. Seismic properties, including velocities and quality factors (Q), are computed from seismic waves transmitted through the medium. The geometrical properties of the fractures, that is, the fracture length, spacing, and aperture, are assumed to exhibit fractal or Weibull behavior. The discontinuity index Id, fracture density parameter C, and the RQD, which provide a descriptive measure of either the hydraulic properties or the strength of the fractured medium, are computed for simulated distributions of the geometric fracture properties. These parameters are analyzed and then related to the seismic properties. The investigations suggest that consistent low seismic velocity and Q values associated with a fracture zone are indicative of a permeable or hydraulically transmissive zone. Lower rock strengths are also associated with lower seismic velocity and Q values. The results show that reasonable inferences characterizing the hydraulic and strength properties of fractured rock mass may be derived from measured seismic properties.

Predicting the transport properties of fractured rocks from seismic information: numerical experiments

Abstract The hydraulic properties including porosity and permeability of fractured rock masses are estimated from seismic velocities derived from controlled numerical experiments. Models of fractured media are developed to represent fractures embedded in an otherwise intact rock. Fracture porosity and permeability are computed using a hydraulic model that accounts for fracture length, aperture and orientation. Seismic attributes are used as a guide to detect the onset of reflections from the fractured medium. Seismic velocities of the fractured layers are computed from the transit times of seismic waves propagating through the layer. The study shows that the velocity ratio between the fractured and the intact rock correlates with the hydraulic properties. Low velocity ratios are associated with high fracture porosity and permeability. Empirical least-squares regression relationships are developed to describe the correlations for practical use.

Determining subsurface fracture characteristics from azimuthal resistivity surveys: A case study at Nsawam, Ghana

We conducted azimuthal resistivity surveys (ARS) using the square-array configuration to characterize the subsurface fractured rock mass at selected farmland sites in Nsawam District, Ghana. This study is the first of its kind in Ghana, and it provides useful information for future hydrological studies of the area, where groundwater is suspected to be contaminated as a result of the indiscriminate use of pesticides and fertilizers by local farmers. We estimate the fracture orientation, fracture porosity, and coefficient of anisotropy of the fractured rock mass at the selected sites from the azimuthal resistivity measurements; the specific surface area is estimated from field geological mapping of outcrops. High correlations exist between the specific surface area and the real and imaginary parts of the measured resistivity of the fractured rock mass. Fractures at localities with relatively high values of coefficient of anisotropy possess relatively high fracture porosity and relatively low specific surface area and are thus more likely to be intensely fractured and permeable. Results from this integrated geological and geophysical study indicate two dominant fracture directions in the study area, with other minor orientations that may influence groundwater and contaminant transport. The dominant orientations of the fracture systems at Kitase are northwest-southeast in the northern part and northeast-southwest in the southern part. At Amanfrom, the fractures are oriented northwest-southeast, and at Nsakye they are northeast-southwest. These sources of information from a noninvasive geophysical method are useful in assessing the transport properties of the fractured rock mass in the study area.

The fractal character of fracture spacing and RQD

Abstract Fracturing and fragmentation processes occur on all scales and, hence, may be described in terms of fractals. We examined the fractal dimensions of discontinuity spacing measurements in a fractured rock mass and observed a relation between the fractal dimension and the quality of the rock mass in terms of RQD. RQD tends to increase with a decrease infractal dimension for a given scanline length. Thus a relatively more-competent fractured rock mass will tend to have a lower fractal dimension and vice versa.

Inversion of fracture density from field seismic velocities using artificial neural networks

The inversion of fracture density from field measured P- and S-wave seismic velocities is performed using a neural network trained with an output from the modified displacement discontinuity fracture model. The basic idea is to use input-output pairs generated by the fracture model to train the neural network. Once the neural network is trained, inversion of fracture density from field-measured seismic velocities is performed very quickly. The overall performance of the neural network in the inversion process is assessed by means of a loss function. The results indicate that both sources of field information (P- and S-wave velocities) predict the field fracture density with reasonable accuracy. The performance of the neural network was compared to the prediction from least-squares fitting. It is shown that the neural network out performs the least-squares fitting in predicting the field-fracture density values.

Statistical distribution of natural fractures and the possible physical generating mechanism

We have fitted field measurements of fracture spacings (from the vicinity of Lake Strom Thurmond, Georgia, U.S.A.) to the Weibull, Schuhmann and fractal distributions. The fracture spacings follow a fractal and Weibull distribution which implies that they were formed as a result of a repetitive fragmentation process. The limited variation of the fracture density with orientation in the study area suggests that the stress distribution generating these fractures may be uniform.

Effects of Clay Content and Salinity on the Spectral Electrical Response of Soils

The spectral electrical responses (SER) of sand and clay mixtures subjected to an effective stress of 20MPa and saturated with varying concentrations of NaCl are measured in a laboratory environment in the frequency range 10−2to104Hz. Changes in clay content and fluid concentrations result in characteristic changes in amplitude and phase spectra. The equivalent circuit model due to Dias is used to characterize the spectral electrical behavior of the saturated mixtures. Two circuit elements of Dias model, the electrochemical parameter η, and the dimensionless parameter δ, are particularly sensitive to the amount of clay and the concentration of the pore fluid. The electrochemical parameter η is shown to increase significantly with an increase in clay content when the soil sample is saturated with freshwater. The dimensionless parameter η on the other hand is directly related to the concentration of the pore fluid solution (NaCl), increasing with an increase in fluid concentration. Effects of electrode pola...

Predicting seismic velocity and other rock properties from clay content only

It has been shown that accurate predictions of compressional and shear‐wave velocities can be derived from measurements of clay content, porosity, and bulk density. In this paper, we test the efficacy of using a sequence of established petrophysical formulations to go a step further and predict porosity and bulk density as well as velocity from clay content alone. Other variables, like grain and pore‐fluid properties, are assumed to be constant, while empirical relationships are used to model such behaviors as grain packing and compaction.The sequence of formulations, referred to as the “model,” is evaluated using well‐log and core data from the Amazon Fan collected during Ocean Drilling Program Leg 155. Clay contents are estimated from the borehole gamma‐ray logs. These are input to the model, which in turn predicts observed porosities to a rms error of <20%, and bulk densities and compressional velocities to rms errors of ≤10%. The results suggest that with better measurements of clay content and improv...

Predicting oil saturation from velocities using petrophysical models and artificial neural networks

Abstract The degree of oil saturation has been estimated from velocity measurements of unconsolidated sediments at a laboratory scale using a petrophysical model and artificial neural network (ANN) as an inversion tool. Laboratory measurements of velocities, V p , V s and their ratio V p / V s as well as the oil saturation levels of unconsolidated materials from an oil field were performed and the data were analyzed. It was observed that the ratio V p / V s increase with an increase in temperature for all saturation level. Beyond a critical saturation level ( S oil =40%), V p increases with an increase in temperature while V p / V s decreases with an increase in temperature. An ANN is trained with simulated data based on the petrophysical model. The weighting coefficients developed from the training are then used to invert for the unknown oil saturation level given the laboratory measured velocities. Simultaneous use of V p , V s and V p / V s as input variables to the network in training the network give more accurate predictions than when say, V p or V s is used individually as input attribute in the inversion process. The results show a good match between the predicted and the measured degree of oil saturation.

Influence of petrophysical and geotechnical engineering properties on the electrical response of unconsolidated earth materials

We have investigated the influence of petrophysical and geotechnical engineering properties of unconsolidated near-surface geomaterials (soils) on their electrical responses or measurements. Complex resistivity measurements were performed at a constant effective stress on 32 samples of varying textures and compositions. Petrophysical and engineering properties that inherently affect the mechanical and strength behavior of the samples — that is, fines content, pore-size parameter, specific surface area, and fractal dimension of the grain-size distributions — were obtained from geotechnical analysis. The electrical parameters that describe the electrical response of the samples — that is, resistivity amplitude ρ , phase shift Φ , percent frequency effect (PFE), loss tangent tan δ , and the normalized phase φN — are computed from the electrical measurements. Crossplots of the electrical and engineering parameters provide useful infor-mation on how the geotechnical properties of the soil material influence th...