Principle of High-Density Electrical Resistivity Survey and Case Analysis

1.Principle of High-Density Electrical Resistivity Survey

High-density apparent resistivity method is an array-based exploration technique, also known as an automatic apparent resistivity system. It evolved from direct current (DC) resistivity methods and combines the functionalities of four-electrode sounding and electrical profiling. By injecting current into the ground through electrodes to create an artificial electric field, the distribution of this field is closely related to the resistivity distribution of underground rock and soil media. By measuring the artificial electric field at different surface locations, the apparent resistivity distribution of subsurface media can be determined. Geological structures are then inferred based on the resistivity variations of rock and soil media.

2.Advantages of High-Density Electrical Resistivity Method

The principle of the high-density resistivity method is the same as traditional resistivity methods. It integrates multiple configurations and electrode spacings, combining electrical sounding and profiling to obtain two-dimensional conductive characteristics of subsurface media. Key advantages include:

• Efficiency: A single electrode setup allows multi-configuration data collection, significantly increasing data volume and improving workflow accuracy.
• ​Sensitivity: Highly sensitive to water content in surrounding rocks. Fractured or water-bearing zones exhibit significantly lower apparent resistivity, while intact or hard rocks show higher resistivity.
• ​Resolution: Provides intuitive imaging with high resolution. Modern computer-aided data acquisition enhances exploration efficiency, coverage, and depth while ensuring reliability in noisy environments.

3. Common Application Fields

1. Detection of coal mine goafs and mining voids
2. Non-destructive detection of termite nests in dam foundations
3. Karst and foundation surveys for railways, highways, and tunnels
4. Delineation of lithological boundaries
5. Investigation of leachate spread in landfills
6. Ancient tomb exploration
7. Sewer pipeline detection
8. Non-destructive leakage detection in reservoirs and river embankments
9. Fractured bedrock zone detection
10. Soil salinity and water quality surveys
11. Detection of ancient tunnels, air-raid shelters, and buried metal objects

4. Application Characteristics

1. Suitable for subsurface exploration where significant conductivity contrasts exist.
2. Limited exploration depth (generally within 100 meters) due to electrode spacing constraints.
3. Less effective in urban areas or near high-voltage power lines due to interference.
4. Challenging electrode deployment on concrete surfaces or exposed bedrock.

5.Field Methodology

1. Configuration Selection
   Choose electrode configurations (e.g., dipole-dipole, Wenner) based on geological objectives.
2. Electrode Spacing Design
   Balance exploration depth (controlled by current electrode spacing) and lateral resolution (controlled by measurement electrode spacing).
3. Data Acquisition
   Ensure reliable measurements by optimizing current injection and minimizing interference.
4. Interpretation
   High-resistivity zones indicate intact rocks, while low-resistivity zones suggest water-bearing or fractured media. Karst cavities may show high-resistivity upper sections and low-resistivity filled lower sections.

6.Engineering Case Study: Karst Cave Detection

6.1 Equipment

The technical tests were conducted using the WGMD-4 centralized high-density electrical method system, which features:
● Transmitting unit and receiving unit are of all-in-one design, portable and lightweight.
● Super high-power, high anti-interference performance and precision: up to 6600w (1100v×6A) power, 48Vp-p voltage range ensures to get a good result in a area with high resistance.
● Automatically achieving compensation of self-potential, drift and electrode polarization, up to ±10V.
● Receiving unit support transient over-voltage protection, transmitting unit supports over-voltage protection, over-current protection and AB open-circuit protection.
● For general resistivity and IP survey function, 9 types of electrode arrays are available. Electrode distance (namely “Program”) can be inputted or called. Geometric factor can also be generated automatically or calculated manually.
● For multi-electrode 2D resistivity imaging, up to 18 electrode arrays are available. It could scanning measure according to fixed section (including AMNB, ABMN, AMBN,AMN, MNB, A-MN-B, Sp M, Sp MN, charge M, charge MN) or continuous rolling scanning measure according to variable section (including A-M, A-MN, AB-M, AB-MN, MN-B, A-MN rectangle).
● Program: the program can memorize 100 groups of electrode distances, avoiding repetitive input. Or you just input a program Id and step to call the corresponding electrode distances.
● Earth resistance inspection: ground condition can be inspected at any time.
● Mass data storage: Up to 1GB capacity (extended) enable store 5,000,000 groups measuring data (including current, voltage, SP, Ro, Ms, metal factor, TH, R, J).
● All parameter and data are under power-failure protection; and thus data will not be lost even thought the system shuts down accidentally.
● USB port enables it to transfer data to computer.

6.2 Project Overview

● Location: Guilin, Guangxi (karst landscape).
● Parameters: 135m electrode array, 5m spacing, Wenner configuration.

Project site: The site is located in Guilin, Guangxi. Guilin landscape belongs to the world-famous karst landform (karst landform). Guilin landscape is a tropical karst landform represented by peak forest-depression. There are many underground caves, mainly soluble arch caves.

my country’s modern karst developed on the basis of the quasi-plain after the Yanshan Movement. The Guilin area is widely distributed as the material basis for the development of karst landforms. Especially since the Quaternary period, the crust has risen rapidly, and the karst landform has developed rapidly, with complex and diverse types.

6.3Data analysis: High-density electrical data inversion analysis

(1) Import the collected data into Swedish analysis software. Use the algorithm in the software to perform inversion analysis on the collected data.

(2) Finally, when we achieve the best inversion effect, we can determine the division of strata based on relevant geological data to find out the distribution of karst caves in this area.
The interpretation and determination of karst in resistivity images follow the following three principles:
① The upper part of the karst zone is hollow and the lower part is filled, showing a string of beads of abnormalities with a continuous combination of high resistivity and low resistivity;
② Karst develops in limestone, and the resistivity value of the background has high resistivity characteristics;
③ Karst develops along the structural fracture zone, and the trend and occurrence of the string of beads are consistent with the regional structure.

The resistivity anomaly images that meet the above three principles are interpreted as karst. Some karsts are completely filled without upper cavities, especially the karsts hidden below the groundwater level, which are completely filled with loose materials and water, showing strong low-resistance characteristics.

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