Electrical Resistivity Method, 4th Lecture, 6 Oct 2024 PDF

Electrical Resistivity Method / 4th Lecture/ 6 Oct,2024 Objective: 1- Electrode spacing with depth of investigations 2- Planning for conducting a Resistivity survey 3- Field effects upon measurements Learning Outcomes: 1- To understand electrode spacing and depth of investigat...

Electrical Resistivity Method / 4th Lecture/ 6 Oct,2024 Objective: 1- Electrode spacing with depth of investigations 2- Planning for conducting a Resistivity survey 3- Field effects upon measurements Learning Outcomes: 1- To understand electrode spacing and depth of investigation. 2- To understand the field procedure by Electrical Resistivity method. 3- To understand the challenges that might affect resistivity measurements. Electrode spacing with depth of investigation The depth of investigation (DOI): indicates how deep into the ground the data can provide reliable information about subsurface resistivity and generally varies with electrode array geometry. In a uniform medium, the Wenner, dipole-dipole, and Schlumberger arrays have DOIs that are approximately 30%, 25%, and 20% of their current electrode separation, respectively. Thus, electrode spacing can be manipulated to achieve sufficient depth (U.S. Environmental Protection Agency, n.d.) Parameters Controlling the Depth of Investigation in Resistivity Surveys The depth of investigation in resistivity surveys is primarily influenced by the length of the current electrodes (AB line) and the distance between the current electrodes (AB) and the potential electrodes (MN). Different electrode configurations, such as the Schlumberger, Wenner, dipole- dipole, pole-pole, and gradient arrays, can be used depending on the specific survey objectives. Each of these configurations has its advantages and limitations in terms of depth penetration, lateral resolution, and ease of field setup. However, they all follow the same general principles: The greater the length of the current electrode line (AB), the deeper the electrical current penetrates into the subsurface. The farther the potential electrodes (MN) are placed from the current electrodes (AB), the more the potential difference measured at the surface reflects the resistivity of deeper subsurface layers. Factors Affecting the Measurement: 1. Resistivity of the Ground (ρ): Resistivity refers to how easily or difficult electricity flows through the ground. If the ground has high resistivity (like hard rock), it creates a stronger signal (VMN or potential resultant). If the resistivity is low (like in clayey soils), the signal is weaker and harder to measure. 1 Electrical Resistivity Method / 4th Lecture/ 6 Oct,2024 Example: Hard rocks (e.g., 1000 ohm.m) give a signal 10 times stronger than sedimentary rocks (100 ohm.m) and 100 times stronger than clay (10 ohm.m). Key Point: You can't control this, as it depends on the natural materials underground. 2. Current Intensity (IAB): refers to the amount of electrical current sent into the ground. The higher the current, the easier it is to measure deeper layers. 3. Sensitive Measuring Instruments: the equipment used to measure the signal (VMN) needs to be very sensitive, especially when measuring at greater depths. At these depths, the signal can become very weak due to factors like noise from power lines or natural electromagnetic interference. Modern equipment helps by using filtering techniques, which clean up the signal by filtering out the noise and even averaging out small variations. This allows us to get clear, reliable measurements in a short time. To measure resistivity at greater depths, several factors need to be managed: Ground resistivity is naturally higher in hard rock, producing stronger signals. Increasing current intensity (IAB) by lowering ground resistance (e.g., using saltwater) and using higher voltage equipment helps measure deeper layers. Sensitive equipment that can filter out noise ensures accurate measurements, even when the signal is weak. Understanding and managing these factors allow us to effectively conduct resistivity surveys and investigate the deeper layers of the subsurface. The depth of investigation in electrical methods depends on two main factors: 1-the geometry of the cables (type of array, number of electrodes, spacing between electrodes, number of segments). 2-the measurability of the signal by the equipment, namely the amplitudes of the signal and of the existing noise, the power specifications of the equipment and its ability of filtering the noise through the stacking process. Electrode Array Procedures: 1. Sounding Procedure: In the sounding method, the depth of investigation is gradually increased at a fixed point by expanding the distance between the electrodes for each measurement. This allows for a deeper understanding of the subsurface at a single location. An example of this is using the Schlumberger array, where the current electrodes are moved farther apart while keeping the potential electrodes relatively stationary, providing deeper resistivity readings for each successive measurement. 2. Profiling Procedure: In the profiling method, the spacing between the electrodes remains constant for all measurements, and the array is moved along a line to collect data across a 2 Electrical Resistivity Method / 4th Lecture/ 6 Oct,2024 larger area. The depth of investigation in this method is determined by the fixed spacing between the electrodes. For example, if the Wenner array is used, all electrodes (ABMN) are moved together along the profile line at regular intervals, allowing for the detection of lateral variations in the subsurface resistivity but with a fixed depth of investigation based on the electrode spacing. Planning and Preparation for Resistivity Surveys 1. Defining Objectives: Before conducting a resistivity survey, it is essential to define the goals and objectives of the investigation. Objectives might include mapping subsurface geological structures, locating groundwater resources, or assessing environmental conditions such as contamination zones or soil properties. Clearly outlining the objectives provides a focused framework for planning the survey, ensuring that the selected methods, equipment, and survey design will yield relevant and useful data. 2. Site Selection: The selection of the survey site is a critical step that directly impacts the success of the resistivity survey. The chosen area must align with the survey’s objectives and consider geological factors such as stratigraphy and rock types. In addition, accessibility is a major factor, especially in rugged or remote terrains, where physical barriers might hinder equipment setup or data acquisition. Topographical features and potential interference from nearby structures, such as buildings or power lines, should also be evaluated to minimize noise and ensure accurate data collection. 3. Geological and Site Information: Acquiring pre-existing geological and site-specific information is invaluable for efficient planning. Resources such as geological maps, borehole logs, and data from previous surveys can provide insights into subsurface conditions, aiding in survey design and instrument calibration. These background materials also help in identifying potential challenges, such as heterogeneous materials or subsurface anomalies that could influence resistivity readings. 4. Instrument Calibration: Proper instrument calibration is a crucial step for ensuring accurate and reliable measurements during the resistivity survey. Calibration involves verifying the settings of the resistivity meter, checking the functionality of cables, and ensuring the electrodes are in good condition. Instruments should be tested under controlled conditions before field deployment, and any necessary adjustments should be made. Regular calibration during the survey can prevent measurement errors and maintain data integrity. 5. Safety Considerations: Field safety must be a priority when conducting resistivity surveys. Before beginning, identify potential hazards at the site, including power lines, unstable terrain, or extreme weather conditions that could pose risks to field personnel. A thorough safety protocol should be developed and implemented to mitigate these risks. This might include proper equipment 3 Electrical Resistivity Method / 4th Lecture/ 6 Oct,2024 handling procedures, personal protective equipment (PPE), and emergency response plans for adverse events. Survey Design and Layout 1. Electrode Configuration Selection The selection of an appropriate electrode configuration is pivotal for obtaining data that meet the survey’s objectives. Various configurations, such as the Wenner, Schlumberger, or dipole-dipole arrays, can be employed depending on factors like the depth of investigation and resolution requirements. For instance, the Schlumberger configuration is suitable for vertical electrical sounding (VES) to assess deeper structures, while the Wenner array is commonly used for shallow investigations or environmental surveys. Consideration of geological conditions is essential in this selection process. 2. Grid or Line Layout The layout of the survey area should reflect the nature and extent of the features being investigated. A grid layout, comprising multiple survey lines in orthogonal directions, is typically employed for large-area mapping, providing comprehensive coverage of the subsurface. Alternatively, a line layout, with survey lines placed along a predetermined path, is more suitable for detailed profiling of specific geological features. The layout should be designed to minimize coverage gaps while maximizing data resolution. 3. Electrode Spacing and Array Geometry Electrode spacing and array geometry must be carefully planned to ensure that the survey meets its depth and resolution requirements. Wider electrode spacing increases the depth of investigation, allowing for the detection of deeper features. However, larger spacing may reduce resolution for near-surface structures. Array geometry, influenced by the chosen electrode configuration, also determines the sensitivity of the survey to subsurface heterogeneities. Adjustments to spacing may be necessary depending on geological conditions encountered in the field. 4. Survey Line Orientation Survey line orientation plays a crucial role in optimizing the detection of geological features. Lines should generally be oriented perpendicular to the expected strike of subsurface structures to maximize contrast in resistivity readings. In some cases, multiple orientations may be required to accurately characterize the subsurface. Line orientation should also consider practical limitations, such as terrain and accessibility. 5. Measurement Stations 4 Electrical Resistivity Method / 4th Lecture/ 6 Oct,2024 Measurement stations should be distributed systematically across the survey area, ensuring full coverage and consistency in data acquisition. Stations can be marked using stakes, flags, or GPS coordinates, allowing for precise placement of electrodes during the survey. Consistency in station spacing is essential for obtaining comparable data between different survey lines. Data Acquisition Techniques 1. Current and Potential Electrode Placement: the proper placement of current and potential electrodes is fundamental to the success of resistivity surveys. Current electrodes should be installed at the ends of the survey lines, ensuring that the electrical current is injected uniformly into the ground. Potential electrodes are placed along the survey lines according to the chosen electrode configuration, facilitating the measurement of voltage differences across the subsurface. 2. Current Injection: during data acquisition, a controlled electrical current is injected into the ground through the current electrodes. The magnitude of the current should remain stable throughout the survey to ensure consistency in measurements. Any fluctuations in current levels may lead to erroneous resistivity calculations and misinterpretations of subsurface features. 3. Potential Measurement: the voltage potential is measured between pairs of potential electrodes, with readings recorded at each measurement station. This data, when combined with the known injected current, is used to calculate the apparent resistivity of the subsurface. Careful attention to electrode contacts and environmental conditions ensures that the voltage measurements accurately reflect subsurface conditions. 4. Data Recording: data during the resistivity survey is recorded using either a data logger or a digital recording system. It is important to document relevant parameters such as electrode spacing, instrument settings, and environmental conditions during each measurement. Accurate data recording is essential for the subsequent analysis and interpretation of the survey results. 5. Survey Progression: this survey systematically across the layout, following the predetermined grid or line arrangement. Maintaining consistent electrode spacing and following the designed configuration is essential for obtaining reliable data. Regular checks during the survey can prevent mistakes in electrode placement or data collection, which might otherwise compromise the results. 6. Quality Control: throughout the data acquisition process, quality control measures should be implemented to identify and correct any potential issues. Regular checks on electrode contact resistance, instrument performance, and data consistency help maintain the quality and reliability of the collected data. Interference from environmental factors or equipment malfunction should be addressed promptly. 5 Electrical Resistivity Method / 4th Lecture/ 6 Oct,2024 7. Repeat Measurements: in some cases, repeating measurements or using multiple electrode configurations may enhance the accuracy and reliability of the data. This approach can help cross- verify results and provide a more detailed characterization of subsurface conditions. 8. Post-Processing: following field data collection, post-processing involves data inversion and interpretation to create a subsurface resistivity model. This process translates the raw data into a meaningful representation of the subsurface, allowing for the identification of geological features or anomalies. Field effects upon measurements: 1- Effect of a cliff or road cut: When conducting resistivity surveys, the presence of a cliff or road cut can significantly affect the flow of electrical current in the subsurface. Normally, when electrodes are placed in the ground, the current flows uniformly through the subsurface materials, and the resistivity of those materials can be measured accurately. However, when a cliff or steep surface is present, the current lines are disrupted or "deflected" away from the edge of the cliff. To avoid these issues, it is recommended to place the electrodes at least one electrode spacing away from the cliff or road cut. This distance helps minimize the deflection effects and ensures that the current can flow more uniformly, leading to more accurate resistivity measurements. This adjustment allows for better data collection by reducing the influence of the cliff on the resistivity survey, preventing skewed interpretations of subsurface properties. 2- Effect of a Wrong A or B Moving Electrodes: A- Case i (Shortened Distance): If one electrode is not moved while the other is, the distance between A and B becomes too short. This leads to abnormally high resistivity values because the current is concentrated in a smaller area. Solution: Ensure both electrodes are moved correctly, maintaining the intended distance based on the geometric factor K. B- Case ii (Overshooting the Distance): If an electrode is moved too far, the distance between A and B becomes too long, leading to abnormally low resistivity values due to dispersed current flow. Solution: Mark positions carefully and verify the distance to prevent surpassing the correct AB/2 mark. 3-Effect of Leakage in Cables: The current will run into the ground not only at the electrodes but additionally at the point of leakage. The displayed point will drop out upward. 6 Electrical Resistivity Method / 4th Lecture/ 6 Oct,2024 4-Effect of Wire-Mesh Fence: When an electrode passes a wire-mesh fence, the current flows into the ground at the start of the fence, not at the new electrode position. It’s as if the electrode hasn’t moved, even though it has. This keeps the current density higher than it should be at the new spot, causing the resistivity to drop unexpectedly. The fence disrupts the measurements, leading to inaccurate results. 5-Effect of a ditch: A ditch is a narrow, typically shallow trench or channel dug into the ground. It is often used for drainage purposes, to direct water away from roads, fields, or other areas, preventing flooding or erosion. In geophysical or geological contexts, ditches can alter the current flow during resistivity surveys, affecting the readings and causing variations in the apparent resistivity values due to their different conductive properties compared to the surrounding ground. A- Empty Ditch (Air-Filled): Current Flow: The presence of air in an empty ditch result in high resistance, which restricts the flow of electrical current. Resistivity: This leads to an increase in resistivity values due to the poor conductivity of air. Resistivity Curve: The resistivity curve shifts upward, indicating a bad conductor in the subsurface. B-Water-Filled Ditch: Current Flow: Water, especially if it is saline or contains dissolved minerals, enhances conductivity, allowing electric current to flow more freely. Resistivity: This causes a decrease in resistivity values because water is a good conductor. Resistivity Curve: The resistivity curve shifts downward, indicating a good conductor. C- Ditch with Soil Saturation: Current Flow: If a ditch is filled with saturated soil, the resistivity can vary depending on the soil type and moisture content. Resistivity: The resistivity may be lower than that of dry soil but higher than water, resulting in variable readings. Resistivity Curve: The resistivity curve may display fluctuations based on the soil's conductive properties. 6-Effect of Water Pipe on Resistivity Readings In resistivity surveys, a water pipe can significantly affect the readings: Current Flow: When the electrode is positioned before the pipe (position B), current flows through the earth. However, at position B', current preferentially flows through the pipe instead of the surrounding soil due to its better conductivity. 7 Electrical Resistivity Method / 4th Lecture/ 6 Oct,2024 Resistivity Measurement: This leads to a drop in apparent resistivity, causing an upward shift in the resistivity curve, which may misrepresent the presence of high-resistivity bedrock at greater depths. Interpretation: The presence of the pipe can complicate the interpretation of the data, potentially leading to inaccurate conclusions about subsurface conditions. 7-Effects of topography: Current Flow Distortion: Irregular landforms, such as hills and valleys, can alter the path of electrical current. This distortion can result in inaccurate readings that do not reflect true subsurface conditions. Data Interpretation Challenges: Misleading readings due to topography can lead geophysicists to incorrectly identify subsurface features, impacting resource exploration and environmental assessments. Mitigation Strategies: To reduce topographic effects, geophysicists can carefully plan survey designs, use advanced data processing, and combine measurements with other geophysical methods. 8- Effects of dipping layer: To get the most accurate resistivity readings, it’s crucial to conduct surveys parallel to the strike direction of the geological layers. This alignment helps ensure that the measurements reflect the true characteristics of the layers without the distortions. 8