Geological anomalies using Geophysical Method

1.0 Introduction:
SandVik Mining & Construction Ghana Limited client, has proposed the construction facilities including workshops, training centres offices etc. at their proposed site around Pakyi No1 in the Ashanti Region. Atelier Ghana Limited, the project architect acting on behalf of the client, engaged Hydrolog Geoinformatics limited to conduct geotechnical investigations at the site. The geotechnical investigations include conducting surface geophysical survey to delineate any geological anomalies such as fault zones. Electrical resistivity ground geophysical survey which is commonly used for geotechnical investigations and environmental surveys, due to its efficiency and effectiveness in producing images of the subsurface was carried out at the site. The overall goal of the electrical resistivity survey at the proposed site was to obtain the geological structure of the upper most 30m of the subsurface in the lateral and vertical extent. This report discusses the survey procedures and interpretation of electrical resistivity survey carried out at the proposed site for SandVik Mining & Construction Ghana.

2.0 Survey Procedures:

2.1 Equipment:
1. WGMD-3 Multi electrode resistivity measuring system (Figure1). This consists of:
a) WDJD-3 Resistivity/IP Meter (Main frame)
b) WDZJ-3 Multiplex Switcher
c) Intelligent multicore cable
d) Steel Electrodes

Figure1. Resistivity instrument used for field work

2.2 Field Measurements:
Two main field measurements were carried out at the site, which are profiling and vertical electrical sounding techniques. Electrical resistivity profiling survey was conducted along three (3) selected traverses (Line1, Line2 & Line3) (Figure 2) in a regular grid at 30m interval to determine the lateral variation of geological anomalies. The maximum length of the measured profile was 300m with unit electrode spacing of 5m. The Schlumberger electrode array sounding technique was also conducted at four borehole locations (BH1, BH3, BH5 &BH7) to also determine vertical variation in resistivity anomalies, such as groundwater that may affect foundation excavations.

3.0 Discussion of Results:
Processing and analysis of geophysical data and interpretations were carried out using EarthImager2D inversion and IX1D software for profiling and sounding resistivity data respectively. The sequence of processing of each profile data using EarthImager2D inversion software shows the inversion results after 3 iterations with RMS errors (11.2-23.9 %), and L2-norm of (1-10), which are considered the optimum model for interpretation and convergence of inversion. Again, the processing of each vertical electrical sounding data with IX1D software also shows the inversion results after multiple iterations inversion with a fitting error less than 10% are also regarded as the optimum model for interpretation. WDJD-3 Multifunction digital DC Resistivity/IP meter was used to conduct Vertical Electrical Sounding (VES) to determine vertical resistivity anomaly for groundwater beyond the depth of geotechnical exploratory borehole. This instrument enables measuring the potential current ration which multiplies with the geometric factor, k, to give the apparent resistivity values in Ohm-m. The Schlumberger 4-electrode array configuration was used in conducting the sounding measurements with current electrode spacing (AB) =60m at each sounding station. The VES models are represented in the form of resistivity sounding curve, relating the apparent resistivity (ρa) to half of the current electrode spacing AB/2 (depth). Four (4) VES soundings were carried out at four borehole locations (BH1, BH3, BH5 &BH7) at the site.

Figure2: Location of resistivity profile lines and VES sounding points on site

3.1 Interpretation of 2D Inversion Data:
Pseudosection resistivity results (figures 3, 4 & 5) give a pictorial representation of geological anomaly in the subsurface. These profile images (pseoudosections) of the subsurface were obtained through the inversion of measured apparent resistivity profile data. The results from the profiling pseudosection did not detect any geological anomaly such faulting within the site. However, it did provide a clear indication of moisture content variation laterally. The inverted pseudosections show three resistivity zones within the first 10m from the top of the image along the traverse.
These are:
1. Low resistivity zones with resistivity range between 24.6 to 100 Ohm-m (deep blue to light blue to cyan shading). These low resistivity zones are interpreted as soil material with relatively high moisture content.
2. Moderate resistivity zones with resistivity range between 100 to 750 Ohm-m (green to light orange shading). This moderate resistivity zones are interpreted as soil materials with moderate moisture content.
3. High resistivity zones with resistivity range between 750 to 2217 Ohm-m (dark orange to red shading). These are interpreted as slightly weathered material or air field voids with less moisture content.

Figure3. Inverse model resistivity pseuodosection for Line1

Figure4. Inverse model resistivity pseuodosection for Line2

Figure5. Inverse model resistivity pseuodosection for Line 3

3.2 Interpretation of 1D Vertical Electrical Sounding (VES) Resistivity Data:
The results from vertical electrical soundings for BH1, BH3, BH5 and BH7 are presented in (figures 6a, 6b, 7a, 7b, 8a, 8b, 9a & 9b). The layering parameters (resistivity ρ, thickness h and depth) can be obtained from the apparent resistivity data. The interpretation of each VES curve was carried out through the use of automatic interpretation computer software “Interpex 1-D Sounding Inversion Version 3.45” software written by Interpex Limited (2008). The accuracy of the inverse model was improved by using standard methods of data collection followed by moving abhorrent data points from the field curves before computing the inverse model. Finally, a careful consideration of available geologic information was taken into account for each inverse model. The interpreted layer parameters of the subsurface layers for each VES station are illustrated in the figures below. The correlation of the geoelectrical parameters (resistivity, thickness and depth) of the resulting layers with the geological succession from boreholes led to the identification of three layers. The succession can be established as follows: 1. The first layer has soil materials resistivity values ranging between 7.36Ohmm to 188.26Ohm-m; and with varying thickness values from 0.52m to 0.96m.
2. These second layer also has material with resistivity values ranging from 401.87Ohm-m to 7002.9Ohm-m.; and with varying thickness values from 1.98m to 3.6m.
3. The third layer has resistivity values ranging from 279.28Ohm-m to 402.69Ohm-m; and with varying thickness values from 5.3m to 30m. VES soundings did not detect any ground regime within 30m meter depth from the surface, and hence groundwater is not expected within foundation excavations within this depth.

Figure 6a. Resistivity model for Borehole 1

Figure 6b. Three(3) layered resistivity inverse model for Borehole 1

Figure 7a. Resistivity model for Borehole

Figure 7b. Four layered resistivity inverse model for Borehole 3

Figure 8a. Resistivity model for Borehole 5

Figure 8b. Two layered resistivity inverse model for Borehole 5

Figure 9a. Resistivity model for Borehole 7

Figure 9b. Two layered resistivity inverse model for Borehole 7

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