# Preventing Overturning of Mobile Cranes Using an Electrical Resistivity Measurement System
## 3 Methodology  
### Geophysical Exploration Methods  
Geophysical exploration methods are primarily used to assess and analyze subsurface ground conditions, as illustrated in Figure 1. These methods include seismic refraction, ground-penetrating radar, electrical resistivity measurement, borehole drilling, gravity surveys, magnetic surveys, and electromagnetic methods, all of which are described in detail in reference [33]. To put it simply, Seismic refraction measures the travel time of seismic waves through subsurface materials, offering insights into the layering and material properties below the surface. Ground-penetrating radar uses high-frequency electromagnetic waves to generate images of the subsurface and provides the detection of voids, pipes, and material composition changes. Electrical resistivity measurement involves injecting electrical currents into the ground and measuring resistivity, which helps infer the soil's composition and water content. Borehole drilling provides direct access to subsurface layers, enabling accurate geological and hydrological analysis. Gravity and magnetic surveys detect variations in gravitational and magnetic fields to map subsurface features, such as voids or mineral deposits. Finally, electromagnetic methods measure the ground's response to induced electromagnetic fields to map conductivity variations, helping to identify different subsurface materials.  
According to [PERSON], the most advantageous geophysical exploration methods for locating groundwater and detecting cavities are electrical resistivity (ER) measurement, electromagnetic (EM), and ground-penetrating radar (GPR) [34]. These methods are favored for their simple measurement processes and portability. EM and GPR methods typically use a frequency range between 10 MHz and 2.6 GHz. Therefore, these methods provide higher resolution than the electrical resistivity method, but the penetration depth of waves to the underground is limited to less than about 3-5 m due to the very high attenuation of electromagnetic waves [38; 39; 40]. Moreover, these methods can be challenging to operate on rough or curved ground surfaces, which are typical of construction sites. In contrast, the ER method, which uses electrode burial, is suitable for use on uneven ground surfaces at construction sites and consumes less power than the EM and GPR methods, allowing for more compact equipment design. The ER method can probe hundreds of meters underground, and the acquired data can be visualized to provide an intuitive understanding of subsurface conditions [41]. However, the ER method is susceptible to noise from buried pipes, wires, buildings, and natural potentials, which can compromise data reliability; therefore, caution is required when interpreting the results.  
The principle of the ER method involves placing four electrodes in a straight line on the ground surface and measuring the electrical resistivity using two current and two potential electrodes. The resistance values vary based on the electrical characteristics of the underground medium, and the measurement depth and resolution depend on the electrode configuration [42]. Common electrode array configurations include Wenner, Schlumberger, and dipole-dipole arrays [43], which are used for one-dimensional (1D) vertical, two-dimensional (2D), three-dimensional (3D), and four-dimensional (4D) explorations with time intervals [44; 45]. The basic ER system comprises a data acquisition device, electrodes, and a controller, with additional software required for data inversion and imaging.  
Figure 1: Geophysical exploration methods and equipment.  
In this study, the ER system was developed with considerations of equipment miniaturization, the ground environment of construction site, and the exploration depth required for integration with mobile cranes. The Wenner array configuration, which allows for equal spacing of electrodes, was selected to facilitate ease of installation by workers.