Anomaly Effects of Arrays for 3d Geoelectrical Resistivity Imaging using Orthogonal or Parallel 2d Profiles

The effectiveness of using a net of orthogonal or parallel sets of two-dimensional (2D) profiles for threedimensional (3D) geoelectrical resistivity imaging has been evaluated. A series of 2D apparent resistivity data were generated over two synthetic models which represent geological or environmental conditions for a typical weathered profile and waste dump site, respectively, commonly associated with geophysical applications for hydrogeological, environmental and engineering investigations. Several minimum electrode separations and inter-line spacing were used to generate the apparent resistivity data for each electrode array with a view to determining the optimum inter-line spacing relative to the minimum electrode separation. The 2D apparent resistivity data for each array were collated to 3D data sets. The effectiveness and efficiency of the arrays in 3D geoelectrical resistivity imaging were evaluated by determining the mean absolute anomaly effects of the electrodes arrays on the synthetic models. The anomaly effects observed in dipole-dipole (DDP), pole-dipole (PDP) and Wenner- Schlumberger (WSC) arrays were generally larger than that observed in other arrays considered. The least anomaly effect on the synthetic models was observed in pole-pole (PP) array. This indicates that DDP, PDP and WSC arrays are more sensitive to 3D features. In all the arrays, the anomaly effects observed in the data set generated using the conventional square grids were slightly larger than those from parallel or orthogonal 2D profiles. This slight increase is attributed to the increased data density and is insignificant when compared with those of parallel and orthogonal 2D profiles. Hence, the use of parallel or orthogonal 2D profiles for 3D geoelectrical resistivity survey is effective

Effectiveness of 3D Geoelectrical Resistivity Imaging using Parallel 2D Profiles

Acquisition geometry for 3D geoelectrical resistivity imaging in which apparent resistivity data of a set of parallel 2D profiles are collated to 3D dataset was evaluated. A set of parallel 2D apparent resistivity data was generated over two model structures. The models, horst and trough, simulate the geological environment of a weathered profile and refuse dump site in a crystalline basement complex respectively. The apparent resistivity data were generated for Wenner–alpha, Wenner–beta, Wenner–Schlumberger, dipole–dipole, pole–dipole and pole–pole arrays with minimum electrode separation, a (a = 2, 4, 5 and 10 m) and inter-line spacing, L (L = a, 2a, 2.5a, 4a, 5a and 10a). The 2D apparent resistivity data for each of the arrays were collated to 3D dataset and inverted using a full 3D inversion code. The 3D imaging capability and resolution of the arrays for the set of parallel 2D profiles are presented. Grid orientation effects are observed in the inversion images produced. Inter-line spacing of not greater than four times the minimum electrode separation gives reasonable inverse models. The resolution of the inverse models can be greatly improved if the 3D dataset is built by collating sets of orthogonal 2D profile

Structural and Stratigraphic Mapping of Emi Field, Offshore Niger Delta

The Niger Delta, where oil and gas are predominantly trapped in sandstones and unconsolidated sands in the Agbada formation, ranked among the world’s major hydrocarbon provinces. The traps, structure and stratigraphic, could be very subtle and complex and are therefore, difficult to map accurately. The degree of reliability and precision of the mapping can be greatly enhanced by integrating seismic data with well logs commonly used independently in hydrocarbon exploration and exploitation studies. In this paper, seismic data were integrated with well logs to define the subsurface geometry, stratigraphy and hydrocarbon trapping potential of Emi-field, off shore Niger Delta. Lithologic units were identified on the logs and correlated across the wells. The stratigraphic cross-sections produced show a general lateral continuity of the lithologic units across the field. Seismic-to-well ties revealed that, high amplitude reflection events correspond to sand units, whereas, low amplitude reflection events correspond to shale units. Four horizons, H1, H2, H3 and H4 were mapped and structure contour maps produced for each of the horizons. Closures considered as good hydrocarbon prospects were identified and delineated. Stratigraphic plays such as pinch-outs, unconformities, sand lenses and channels are also suspected. The integration of seismic data with well logs proved to be a useful tool in structural and stratigraphic mapping and in predicting lateral and vertical variations in the lithologic units

Anomaly Effects of Orthogonal Paired-arrays for 3D Geoelectrical Resistivity Imaging

A series of 2D apparent resistivity data were generated over two synthetic models representing different geological or environmental conditions commonly associated with geophysical applications for hydrogeological, environmental and engineering investigations. The apparent resistivity data were generated for the following arrays: Wenner-alpha (WA), Wenner-beta (WB), Wenner– Schlumberger (WSC), dipole–dipole (DDP), pole–dipole (PDP) and pole–pole (PP) arrays, which were paired such that apparent resistivity data for 2D profiles in a parallel direction are obtained with a particular array type and those in a perpendicular direction are observed with a different array type. The 2D apparent resistivity data for the orthogonal paired-arrays were then collated to 3D data sets. The effectiveness and efficiency of the orthogonal pairedarrays in 3D geoelectrical resistivity imaging were evaluated by determining the mean absolute anomaly effects of the electrode configurations on the synthetic models. The results show that DDP–PDP, DDP–PP, DDP–WSC, PDP– PP, DDP–WB, PDP–WB and WB–WSC orthogonal paired-arrays produced higher anomaly effects on the synthetic models. This indicates that DDP–PDP, DDP–PP, DDP–WSC, PDP–PP, DDP–WB, PDP–WB and WB–WSC orthogonal paired-arrays are more sensitive to 3D features of the geologic models than the other orthogonal pairedarrays investigate

Application of 2D and 3D Geoelectrical Resistivity Imaging for Engineering Site Investigation in a Crystalline Basement Terrain, Southwestern Nigeria

Orthogonal set of 2D geoelectrical resistivity field data, consisting of six parallel and five perpendicular profiles, were collected in an investigation site using the conventional Wenner array. Seven Schlumberger soundings were also conducted on the site to provide ID layering information and supplement the orthogonal 2D profiles. The observed 2D apparent resistivity data were first processed individually and then collated into 3D data set which was processed using a 3D inversion code. The 3D model resistivity images obtained from the inversion are presented as horizontal depth slices. Some distortions observed in the 2D images from the inversion of the 2D profiles are not observed in the 2D images extracted from the 3D inversion. The survey was conducted with the aim of investigating the degree of weathering and fracturing in the weathered profile, and thereby ascertaining the suitability of the site for engineering constructions as well as determining its groundwater potential

Numerical Evaluation of 3D Geoelectrical Resistivity Imaging for Environmental and Engineering Investigations Using Orthogonal 2D Profiles

Field design for 3D data acquisition in geoelectrical resistivity imaging using a net of orthogonal sets of 2D profiles was numerically investigated. A series of 2D apparent resistivity pseudosections were generated over a synthetic horst structure representing the geological environment of a crystalline basement in low latitude areas using RES2DMOD code. Different minimum electrode separations and inter-line spacing were used with a view of determining the optimum inter-line spacing relative to the minimum electrode separation. The 2D apparent resistivity data were collated to 3D data set and then inverted using RES3DINV, a full 3D inversion code. The relative effectiveness and imaging capabilities of Wenner-alpha (WA), Wenner-beta (WB), Wenner-Schlumberger (WSC), dipole-dipole (DDP), pole-dipole (PDP), and pole-pole (PP) arrays to image the structure using a net of orthogonal set of 2D profiles are presented. The normalized average sensitivity of the inversion results show that WSC, DDP, and PDP arrays are more sensitive to the 3D structure investigated. Interline spacing of not greater than four times the minimum electrode separation gives reasonable resolution

Experimental Evaluation of 3D Geoelectrical Resistivity Imaging using Orthogonal 2D Profiles

Numerical evaluation of 3D geoelectrical resistivity imaging was conducted using orthogonal set of 2D pseudo-sections generated over two synthetic models, horst and trough models. The models represent geological environment that simulates a typical weathered profile and refuse dump site in a crystalline basement complex, respectively. Different arrays including Wenner-alpha (WA), Wenner-beta (WB), Wenner-Schlumberger (WSC), dipole-dipole (DDP), pole-dipole (PDP), and pole-pole (PP) arrays were used for the data generation. The 2D apparent resistivity data were collated to 3D data set and then inverted using a full 3D inversion code. The effectiveness of the technique for 3D resistivity imaging as well as the imaging capabilities of the selected arrays is evaluated. The observed anomaly effect and normalized model sensitivities of the arrays indicate that DDP and PDP arrays are more sensitive to the 3D features, while WSC show moderate sensitivity to 3D features. Field example in which the technique was applied in a 3D geoelectrical resistivity imaging for engineering site investigation in the crystalline basement complex of southwestern Nigeria is also presented

Geophysical Imaging of Archaeological Materials at Iyekere, Ile-Ife Southwestern Nigeria.

Non-invasive geophysical methods are increasingly being used in archaeological studies. In this study, magnetic and electrical resistivity tomography geophysical techniques were integrated to locate subsurface archaeological materials. The survey consists of four parallel and three perpendicular profiles with station interval of 0.5 m for both magnetic and electrical resistivity tomography. Wenner array with electrode spacing ranging from 0.5 – 3.0 m was used to collect the electrical resistivity data. The results show that high total magnetic intensity anomalies correspond to high inverse model resistivities. The regions with high magnetic and resistivity anomalies were thought to be locations of archaeological materials; the corresponding depths to these materials were inferred from the resulting geophysical images. Test units conducted at the regions of high total magnetic intensity and inverse model resistivity yield archaeological materials including burnt pipes (Tuyere), iron slag, iron smelting, and pottery fragments at approximate depths inferred from the geophysical images

Applications of magnetic methods and electrical resistivity tomography for imaging archaeological structures at Iyekere Ile-Ife Southwestern Nigeria.

Magnetic and electrical resistivity tomography geophysical techniques were integrated to locate subsurface archaeological materials. The magnetic survey comprises seven profiles in N-S and E-W direction with station interval of 0.5 m. [Orthogonal set of] 2D electrical resistivity tomography data consisting of four parallel and three perpendicular profiles were collected using Wenner array with electrode spacing ranging from 0.5 – 3.0 m. Trial pits carried out at regions of high total magnetic intensity and model resistivity yield burnt pipes “TUYERE”, iron slag, iron smelting, and pottery fragments

Geophysical Imaging of Archaeological Materials at Iyekere, Ile-Ife Southwestern Nigeria

Non-invasive geophysical methods are increasingly being used in archaeological studies. In this study, magnetic and electrical resistivity tomography geophysical techniques were integrated to locate subsurface archaeological materials. The survey consists of four parallel and three perpendicular profiles with station interval of 0.5 m for both magnetic and electrical resistivity tomography. Wenner array with electrode spacing ranging from 0.5 – 3.0 m was used to collect the electrical resistivity data. The results show that high total magnetic intensity anomalies correspond to high inverse model resistivities. The regions with high magnetic and resistivity anomalies were thought to be locations of archaeological materials; the corresponding depths to these materials were inferred from the resulting geophysical images. Test units conducted at the regions of high total magnetic intensity and inverse model resistivity yield archaeological materials including burnt pipes (Tuyere), iron slag, iron smelting, and pottery fragments at approximate depths inferred from the geophysical images. Keywords: Archaeogeophysics, Artefacts, ERT, Magnetic methods, 2D imagin