Frederick Owusu-Nimo

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...

Nitrate Contamination in Groundwater at Farmlands in Nsawam, Ghana: The Role of Fractures from Azimuthal Resistivity Surveys

The role of fractures in nitrate contamination of fractured groundwater aquifers at farmlands within the Nsawam district (Ghana) is investigated using azimuthal square-array resistivity measurements. The study covers seven farming communities where azimuthal resistivity surveys (ARS) were conducted at sites with exposed rocks (outcrops) to measure real and imaginary parts of the electrical conductivity of the rocks, which are then used to provide estimates of anisotropic coefficients and fracture porosities. At each outcrop, fracture orientations, apertures and lengths were measured and used directly to obtain the porosities and specific surface areas of the fractures. Linear regression models were then developed between the specific surface area of the fractures and the electrical parameters. At selected well locations, azimuthal resistivity surveys were performed and the developed regression models were used to estimate porosities and specific surface areas of the subsurface fractures. Groundwater samples were collected from the selected wells to determine the concentration levels of nitrates. The estimated porosities and specific surface areas of the subsurface fractures correlate with concentration levels of nitrate in the groundwater. The investigations suggest that nitrate concentration levels in groundwater in the study area tend to be higher in wells where the specific surface area of the fracture system are lower. Also, wells with lower fracture porosities tend to have lower concentration levels of nitrates. The establishment of linkages between subsurface fracture parameters obtained non-invasively from surface geophysics and nitrate concentration levels is a useful result that can be utilized in environmental impact assessment in complex fractured terrain, such as that of the study area.

Exploring the Linkages between Geotechnical Properties and Electrical Responses of Sand-Clay Mixtures under Varying Effective Stress Levels

The strength and stability conditions of unconsolidated geo-materials, for example soils, are influenced by modifications of their micro-structure, texture, mineralogy and effective stress levels. These modifications in the internal structures of the soils often result in geohazards (e.g., landslides, liquefaction, debris flow), which often claim so many lives, destroy the environment, and cause considerable amounts of property damage. Characterization and monitoring of such geohazards demand knowledge about the geotechnical (physical and mechanical) properties of unconsolidated near-surface geo-materials of interest. The fundamental relations between geo-electrical parameters and the geotechnical properties that influence the mechanical behavior of soils are investigated by performing controlled laboratory experiments on sand-clay mixtures under stress. Spectral electrical response (SER) measurements are performed on these mixtures over a range of frequencies (0.001 Hz–1 kHz) with concurrent measurements...

Artificial neural network and statistical models for predicting the basic geotechnical properties of soils from electrical measurements

The ability to predict the geotechnical properties of subsurface soils using non-invasive geophysical measurements can be undeniably useful to the geotechnical engineer. Using laboratory data, we assess the potential of artificial neural networks to investigate the relations between geotechnical and electrical parameters characterizing a variety of soils. The geotechnical parameters are: fines content, mean grain size, mean pore size and the specific surface area. The electrical parameters obtained from low-frequency electrical measurements (4 Hz) include the resistivity amplitude, phase shift and the loss tangent. Relations that can be used to predict the geotechnical parameters of a soil given its electrical parameters are developed. The predictive capabilities of the neural networks are compared with traditional multivariate regression models. The performances of the neural network and regression models in predicting (a) the geotechnical parameter given the same electrical parameters as inputs and (b) the electrical parameters given the same geotechnical parameters as inputs are compared. In both cases, the neural network outperforms the multivariate regression as the neural network is able to capture and model the non-linear and complex relationships among the variables. The relative importance of the geotechnical parameters on the overall electrical conduction was examined using the neural networks. The results indicate that mean grain size and fines content are the two geotechnical parameters that influence phase-shift values the most; fines content and mean pore size influence the resistivity amplitude the most, whilst fines content and mean grain size influence the loss tangent the most. It was observed that of the four geotechnical parameters, the mean grain size influences the measured resistivity values the least.

On the use of a fractal equivalent circuit model to predict the geotechnical properties of soils

Frequency dependent electrical measurements of soils contains useful information about their texture and structure that can be linked to their geotechnical and transport properties. We perform frequency dependent electrical measurements on twenty-nine natural soils with wide variability in physical and textural properties in a laboratory environment at a constant stress level in the frequency range 0.01Hz -10 kHz. The engineering and hydraulic properties of these soils, that is, the hydraulic conductivity, void ratio, fines content, intergranular void ratio and the dry density are also measured. The electrical behavior of the soils were modeled with an equivalent circuit model described by six circuit parameters. Relationships between the circuit parameters and the soil properties