5th Lecture: Electrical Resistivity Method PDF

5TH LECTURE Electrical Resistivity method 1 10/27/2024 OBJECTIVE ▪ Various field technique to conduct a Resistivity survey. 10/27/2024 2  LEARNING OUTCOMES ▪...

5TH LECTURE Electrical Resistivity method 1 10/27/2024 OBJECTIVE ▪ Various field technique to conduct a Resistivity survey. 10/27/2024 2  LEARNING OUTCOMES ▪ To understand various field technique such as CST and VES ▪ How each technique is conducted ▪ Do they have similar results ? ▪ Concept of inversion in VES technique 10/27/2024 3 INTRODUCTION ▪ Traditionally resistivity surveys were divided into profiling and sounding surveys. ▪ The distances between the electrodes are kept fixed in a profiling survey, such as in the Wenner survey , and the four electrodes are moved along the survey line. ▪ The data interpretation for profiling surveys was mainly qualitative. 10/27/2024 4 ▪ The second type of survey is the vertical sounding method, such as with the Schlumberger array ,where the center point of the electrode array remains fixed but the spacing between the electrodes is increased to obtain deeper subsurface information. ▪ Apparent resistivity plotted as a function of the current electrode spacing gives information on subsurface resistivity variations 10/27/2024 5 ▪ Quantitative data interpretation for sounding surveys uses a one-dimensional (1-D) earth model with a series of horizontal layers (Figure 2b). One commonly used method for 1-D data inversion is the damped least-squares method (Inverse theory, linear) (Inman, 1975). 10/27/2024 6 10/27/2024 7 VERTICAL ELECTRICAL SOUNDING METHOD (VES) ▪ VES is a technique where data are collected in 1D direction. ▪ The surface position ( x & y coordinates) remains fixed and depth of the data points changes. 10/27/2024 8  PROS. AND CONS ▪ Cost-effective equipment can be used for Vertical Electrical Sounding (VES). ▪ The field procedure is simple and allows for quick data collection. ▪ Data is gathered by moving electrodes farther apart to achieve greater depth penetration. ▪ VES provides a 1D subsurface model, requiring assumptions about conditions outside the measurement area. ▪ To achieve accurate results, the method assumes: ▪ Homogeneous resistivity within each layer. ▪ Horizontal, non-dipping layers. In reality, these ideal conditions are uncommon, as geological formations are rarely uniform and horizontal. 10/27/2024 9  APPLICATION ▪ Groundwater Exploration: VES identifies aquifers by detecting water-bearing formations based on resistivity contrasts (low resistivity indicates water-saturated zones)​. ▪ Mineral Exploration: VES helps in locating massive ore bodies highlighting conductive or resistive layers, useful in detecting ores like metals​. But not for mineralization in narrow veins. 10/27/2024 10 VES SURVEY ▪ The field procedure : ▪ what do we do when we take measurements ? 10/27/2024 11 10/27/2024 12 VES SURVEY ▪ Results during measurement ▪ Ves Curve 10/27/2024 13 CONCEPT OF INVERSION 10/27/2024 14 10/27/2024 15 CONSTANT SEPARATION TRAVERSING (CST) 10/27/2024 16  CONSTANT SEPARATION TRAVERSE ▪ Constant Separation Traverse (CST) or what is known as horizontal profiling or mapping : is a method used to assess lateral variations in apparent resistivity. ▪ In this technique, the electrode spacing is kept constant while the entire array is moved along a survey line after each measurement ▪ It is crucial to carefully select the electrode spacing, as it dictates the depth of investigation. 10/27/2024 17 ▪ Commonly, electrode arrays like Dipole-Dipole and Wenner are used in CST. ▪ The dipole-dipole array is known for its high resolution and ability to capture detailed subsurface images, making it ideal for mapping horizontal variations. However, its signal weakens with distance, limiting its effectiveness in deeper investigations. ▪ The Wenner array, on the other hand, offers better depth penetration but provides a more general picture and can struggle with accurately detecting both horizontal and vertical resistivity changes simultaneously​. ▪ To optimize the electrode spacing, it's often recommended to first perform a Vertical Electrical Sounding (VES) in the area. This helps determine the ideal electrode spacing that will ensure appropriate depth penetration for the CST survey. 10/27/2024 18  APPLICATION The Constant Separation Traverse (CST) method is primarily applied in geological and geotechnical surveys for: 1. Mapping lateral resistivity variations: It is useful for identifying horizontal changes in subsurface conditions, which makes it suitable for locating faults, dykes, or other lateral geological features. 2. Environmental and hydrogeological studies: CST can detect subsurface contamination, groundwater flow patterns, and potential aquifer boundaries. 3. Civil engineering: It is used to investigate ground conditions before construction projects, such as tunnels or foundations, where understanding lateral variations in the subsurface is crucial. 4. Archaeology: CST can help detect buried structures by identifying changes in resistivity caused by man-made objects or disturbed soil. 10/27/2024 19 ▪ In the Constant Separation Traversing (CST) technique used in resistivity surveys, longitudinal traverse and transverse traverse refer to the orientation of the survey line relative to a geological structure, such as a fault, boundary, or other feature of interest. 1. Longitudinal Traverse: 1. A longitudinal traverse is performed when the survey line is oriented parallel to a geological feature or structure. In this arrangement, the goal is to map the resistivity variations along the length of the feature. This helps in understanding how the properties of the subsurface vary along the feature's strike or trend, providing insights into the lateral continuity of geological units. 2. Transverse Traverse: 1. A transverse traverse, on the other hand, involves orienting the survey line perpendicular to the geological feature. This method is used to detect resistivity changes across the structure, giving a cross-sectional view of the resistivity contrast. It is especially useful in identifying boundaries, faults, or lateral discontinuities between different rock layers or materials. ▪ Both types of traverses are used to gather complementary information during resistivity surveys. Longitudinal traverses are ideal for understanding the horizontal variation of resistivity along a feature, while transverse traverses are more suited for detecting lateral boundaries and vertical changes in resistivity 10/27/2024 20 ▪ The results of profiling technique is represented as a profile of resistivity variation along the traverse or as iso resistivity contour maps. 10/27/2024 21 CONSTANT SEPARATION TRAVERSING (CST) 10/27/2024 22 10/27/2024 23 10/27/2024 24