**12.5 2-D resistivity structure along West-East direction**

The 2D resistivity structure (**Figure 3**) reveals four geologic/geoelectric subsurface layers separated by geologic boundaries; namely topsoil marked by A (generally blue except few points with yellow and green colour), weathered layer marked B (generally blue except few points with yellow and green colour), weathered/fractured

basement marked C (green and yellow colour) and fresh basement marked D (generally red except few points with yellow colour). The topsoil is generally thin but subsume into the weathered layer in many places due to its thickness, the topsoil is characterised mainly by clay with few portions of laterite and sandy clay. The weathered layer is characterised mainly by clay (lowest resistivity represented by blue colour) between stations 5 and 8 (distance at 50 m and 80 m), but the resistivity values increases from the above stations towards the western and the eastern flank signifying the reduction of clay to sand ratio.

A linear feature is noticed between stations 5 and 7 (distance at 50 m and 70 m) which has significant depth extent. The low resistivity zone is between stations 5 and 7 (50 m–70 m) which could be diagnostic of a suspected fault zone with a width of approximately 19 m. The suspected fault zone is flanked on both sides by regions of higher resistivity. The extremely low resistivity value (blue colour) that was noticed at the lower part of the fault zone signifies the effect of high saturation and diagnostic effect of wet clay. The resistive parts are seen at the lower part of the section which is the fresh basement.

**Figure 4.** *VLF profile and inverted 2D model obtained along traverse one (S-N).*

*Geophysical Investigations for Design Parameters Related to Geotechnical Engineering DOI: http://dx.doi.org/10.5772/intechopen.108712*

#### **12.6 VLF profile along South-North and West-East direction**

On the traverse one which is trending South-North, the VLF-EM profile (**Figure 4**) identified peak positive filtered real values at distances 2 m, 40 m, 83 m, 110 m, 138 m, 168 m and 200 m. The amplitude of the peak positive filtered real values are very low, the inverted model shows that most of the peaks manifest as nonanomalous conductive zones, except for the peak positive filtered real noticed at 200 m. The VLF-EM profile and the inverted model are shown on the profile.

On the traverse trending North-South, the VLF-EM profile (**Figure 5)** identified peak positive filtered real values at distances 18 m, 43 m and 58 m. The amplitude of the peak positive filtered real values are very low, the inverted model shows that most of the peaks manifest as non-anomalous conductive zones, except for the peak positive filtered real noticed at 18 m.

On the baseline which is trending West-East direction, the VLF-EM profile and the inverted model are shown in **Figure 6**, the VLF-EM profile identified peak positive filtered real values at distances 18 m, 32 m, 46 m, 60 m, 82 m, 112 m and 138 m, the

**Figure 5.** *VLF profile and inverted 2D model obtained along traverse three (South-North).*

**Figure 6.** *VLF profile and inverted 2D model obtained along baseline (West-East).*

observation agree with conductive zones delineated by the inverted model at distance 20–60 m, 80–100 m and 122–138 m. The conductive zone between distance 122–138 m and 20–60 m are typical of a linear feature (fracture) because of its depth and it is dipping to the west.

On the traverse which is trending West-East, the VLF-EM profile and the inverted model are shown in **Figure 7**. The VLF-EM profile identified peak positive filtered real values at distance 78 m, 102 m, 118 m and 138 m, the observations agree with the conductive zones delineated at distances 3–20 m, 110–140 m, the conductive zones between 110 m and 140 m is typical of a linear feature (fracture) and it is dipping to the east.

### **13. Conclusion**

In this chapter geophysical investigations for design parameters related to geotechnical engineering was explored.

The strength of the geophysical methods lies in the ability of the method to image the subsurface in a faster and cost effective approach when compared with geotechnical investigations. Geophysical methods provides adequate guidance to civil engineers before embarking in boring and drilling operations, this will avoid excessive

*Geophysical Investigations for Design Parameters Related to Geotechnical Engineering DOI: http://dx.doi.org/10.5772/intechopen.108712*

**Figure 7.** *VLF profile and inverted 2D model obtained along traverse two (West-East).*

damage of subsurface materials which result in environmental risks such as flooding activities.

Seismic refraction, electrical methods, electromagnetic method and magnetic method have been proven to be useful for evaluating design parameters/geologic subsurface deductions needed before the construction of engineering structures such as buildings, rail ways, dams, bridges etc. Case studies involving the application of electrical method and Very Low Frequency Electromagnetic method at Study area within Igarra, South-western Nigeria was explained to reveal the importance of geophysical methods in geotechnical engineering. Geologic features such as faults and fractures were delineated from the Electrical Resistivity Image of the subsurface, VLF profile and the Inverted 2D Model, the compositions (sandy clay, clay, clayey sand and laterite) of the subsurface/geoelectric layers were also classified based on their competence.

In conclusion, the importance of geophysical investigations for evaluation of the design parameters related to geotechnical engineering cannot be neglected in the pre-construction phase.
