Subsurface utility detection using ground penetrating radar and electromagnetic locator – a comparative study

The process of finding and mapping underground utilities is crucial for managing underground infrastructure effectively, especially to avoid damaging them during construction and digging. This research studies two main methods used for this purpose: Ground Penetrating Radar (GPR) and Electromagnetic Locator (EML). GPR works by using electromagnetic waves to detect and assess things underground, giving an idea of how deep they are. On the other hand, EM L help to find metal utilities by using electromagnetic fields. This shows how important it is to have accurate maps of these utilities. Having precise maps is like having a solid foundation for planning and maintaining infrastructure. It also points out that when combine information from GPR and EML and analyse the results together, it can make the findings even more accurate. In a nutshell, these methods are crucial for keeping us safe, preventing delays in projects, and making sure to use resources wisely across different industries. In this research, Root Mean Square Error (RMSE) is used to determine the correlation in depth accuracy error which is 0.05821 which is calculated based on the depth measured by GPR and EML.


Introduction
Utility location involves the process of identifying and labelling underground public utility mains, which encompass various types such as telecommunication lines, electricity distribution, natural gas pipelines, fibre optics and wastewater pipes.Preserving the accuracy and reliability of underground utility mapping and its database is crucial for planning, upgrading, and maintaining infrastructure, particularly below the ground.This mapping encompasses the positioning, location, and identification of buried infrastructure and objects, including archaeological artefacts.Utility detection and mapping applications extend to areas like metallic and non-metallic utilities, environmental assessment, damage prevention, geological investigation, turf assessment, archaeology, forensics, and road inspections [1].
In today's modern world, there's a growing demand for services delivered through underground pipes and cables, collectively referred to as utilities.These services include telecommunication, internet access, water supply, and sewage removal for both residential and commercial purposes.As a result, a complex network of pipes and cables has emerged underground, varying in depth and direction.Just as space is limited for buildings above ground in cities, the underground space also faces limitations for access and accommodating utilities.GPR is an effective geophysical technique for investigating buried sites, especially when features of interest lie within 2-3 meters beneath the surface.Archaeologists increasingly use GPR for landscape analysis, as it provides maps and images guiding excavation placement and offers a broader context for studying human interaction with ancient landscapes.
Various methods and equipment, such as GPR, EML, seismic waves, excavation, and x-ray scanning, are employed to identify buried utilities, chosen based on project requirements and cost considerations.To prevent accidents and damage during excavation, utility maps displaying as-built infrastructure are essential references.Consequently, utility mapping has become an integral part of the land survey profession, ensuring sustainable development by safeguarding underground utilities from harm [2].
Utility detection and correlation using GPR and EML are essential techniques in industries like construction and utilities management.Both GPR and EML are often used together to cross-verify and correlate findings, providing more accurate data for safe and efficient excavation and construction, ultimately reducing project risks and costs.However, their effectiveness can vary based on factors such as soil conditions and operator expertise, so it's crucial to follow local regulations and consult professionals when employing these technologies [3].

Background
Mapping underground systems, such as pipes and cables, is essential across various industries to support effective construction, upkeep, and repair activities.Two widely employed methods for underground utility mapping are EML and GPR.EML devices employ electromagnetic induction for detecting buried cables and pipes, whereas GPR relies on radar waves to generate images of the subsurface.Researchers Siu and Lai [4] undertook a study to assess the precision of underground utility locations using EML (RD8100) and GPR (Mala ProEx).Their findings indicated that GPR outperformed EML in terms of accuracy.
Furthermore, another investigation by Siu and Lai [4] centred on the precision of depth measurements obtained with GPR (Mala ProEx) and EML (RD8000).The research highlighted the significance of taking into account variables like depth, location, material composition, geological conditions, and proximity to other utilities when applying these methods.Additionally, Siu and Lai conducted a study [4] that compared the precision of GPR (Mala ProEx) and EML (RD8000) in various types of utility lines.
Investigating the relationship between EML and GPR for underground mapping is a significant research area.Grasping the advantages and constraints of each method can assist industry professionals in selecting the most appropriate approach tailored to their specific underground mapping requirements.To sum up, numerous studies have delved into the connection between EML and GPR for mapping underground systems.Although EML devices provide convenience, reliability, and ease of operation, GPR devices excel in achieving greater accuracy when pinpointing underground utilities.Additional research and in-depth analysis are required to gain a comprehensive understanding of the relationship between these two methods and to enhance their synergistic utilization in underground mapping.

Ground penetrating radar
GPR is a geophysical technique that uses electromagnetic waves to image and detect subsurface features and objects.It has become a widely used tool in various fields, including archaeology, geology, civil engineering, and utility detection.This literature review aims to provide an extensive overview of the applications, methodologies, and advancements in GPR technology.

GPR applications.
GPR has revolutionized archaeological investigations by providing nondestructive imaging of buried structures, artefacts, and geological features.Conyers [5] discusses the use of GPR in garden archaeology, where it has been used to map buried features, identify ancient pathways, and locate potential excavation sites.GPR has also been used to study ancient landscapes and human interaction with the environment [5].
IOP Publishing doi:10.1088/1757-899X/1308/1/0120133 GPR plays a crucial role in civil engineering projects, including infrastructure assessment, pavement evaluation, and utility detection.It can provide information about subsurface conditions, such as the thickness of pavement layers, the presence of voids, and the location of buried utilities.Dusan and Aleksandar [6] highlight the use of GPR and GPS technology for the detection of underground utilities, emphasizing its accuracy and efficiency in utility mapping.
GPR is widely used in geology and environmental studies to investigate subsurface structures, soil properties, and groundwater resources.It can help identify geological formations, map bedrock depths, and detect groundwater levels.Jol [7] provides a comprehensive overview of GPR theory and applications in geology, discussing its use in mapping geological structures, characterizing aquifers, and studying soil properties.
2.1.2.GPR methodologies.Data Acquisition: GPR data is acquired by transmitting electromagnetic waves into the ground and recording the reflections from subsurface features.The data is collected using antennas or transducers that emit and receive electromagnetic signals.The choice of antenna frequency depends on the desired depth of investigation and the resolution required.Higher frequencies provide better resolution but have limited penetration depth, while lower frequencies have greater penetration but lower resolution [7].
Data Processing and Interpretation: GPR data processing involves filtering, stacking, and migration techniques to enhance the quality and clarity of the subsurface images.Various software tools, such as RADAN 7, are used for data analysis and interpretation.

Electromagnetic locator
EML are essential tools used in various industries for accurately detecting and mapping underground utilities such as pipes and cables.This literature review aims to provide an extensive overview of the applications, methodologies, and advancements in EML technology.

EML applications.
EMLs are widely used in construction and infrastructure projects to locate and map underground utilities before excavation.This helps prevent accidental damage to utility lines, ensuring worker safety and minimizing costly repairs.EMLs are also used for mapping utility networks during the planning and design stages of construction projects [8].
EMLs play a crucial role in maintenance and repair operations, allowing technicians to quickly and accurately locate faults or breaks in underground utility lines.This enables efficient repairs and reduces downtime for utilities such as water, gas, and electricity [5].
Telecommunications: EMLs are extensively used in the telecommunications industry to locate and trace underground cables.This is essential for installing new cables, repairing damaged lines, and identifying cable routes for network expansion.EMLs help ensure the efficient and reliable operation of telecommunications networks [6].

EML methodologies.
Electromagnetic Induction: EMLs use electromagnetic induction principles to detect and locate underground utilities.They emit electromagnetic signals into the ground and measure the response from metallic objects, such as pipes and cables.The strength and frequency of the signals can be adjusted to optimize detection accuracy and depth penetration [6].
Signal Detection and Interpretation: EMLs employ various signal detection and interpretation techniques to accurately locate underground utilities.These techniques include peak detection, signal strength analysis, and signal filtering to distinguish between different types of utilities and minimize interference from nearby objects [7].

Methodology
The methodology for utility detection and correlation plays a vital role in ensuring the safety and efficiency of construction and excavation projects.Key methods in this process are GPR and EML.These techniques help identify and map subsurface utilities such as water pipes, gas lines, and electrical cables with precision.Using GPR and EML effectively can prevent costly and hazardous utility damage.This methodology emphasizes the importance of accurate utility mapping and the advantages of combining GPR and EML findings for improved results.
The site selection for this study is located at the Evaluation and Verification Facility (EVF), Blok 60, Complex Dengkil, Malaysian Nuclear Agency as shown in Figure 1.In the initial stage of fieldwork data detection, the EML RD8100 and GPR GSSI equipment are installed and operated.In this stage, some of the adjustments and calibration on both the equipment are configured to achieve the required standard.
The antenna used by GPR GSSI is 400 MHz frequency which is capable of detecting both metallic and non-metallic underground objects.The 400 MHz antenna is considered a medium high frequency which can penetrate from 2.0m to 5.0m depth beneath the ground.GPR has been calibrated by measuring the real depth of the targeted MS pipe to the road surface which is visible from the inspection sump shown in Figure 1 and Figure 2. The measurement was taken and calculated based on the distance of the top of the MS pipe to the top curb, minus the distance of the road surface to the top of curb as shown in Equation ( 1).

𝑇𝑇𝑇𝑇𝑇𝑇 𝑇𝑇𝑜𝑜 𝑀𝑀𝑀𝑀 𝑇𝑇𝑝𝑝𝑇𝑇𝑝𝑝 𝑡𝑡𝑇𝑇 𝑡𝑡𝑇𝑇𝑇𝑇 𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐 − 𝑇𝑇𝑇𝑇𝑇𝑇 𝑇𝑇𝑜𝑜 𝑐𝑐𝑇𝑇𝑟𝑟𝑟𝑟 𝑠𝑠𝑐𝑐𝑐𝑐𝑜𝑜𝑟𝑟𝑐𝑐𝑝𝑝 𝑡𝑡𝑇𝑇 𝑡𝑡𝑇𝑇𝑇𝑇 𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐 = 𝐺𝐺𝑐𝑐𝑇𝑇𝑐𝑐𝐺𝐺𝑟𝑟 𝑇𝑇𝑐𝑐𝑐𝑐𝑡𝑡ℎ 𝐷𝐷𝑝𝑝𝑇𝑇𝑡𝑡ℎ (1)
After the first calibration, a GPR scan was done in the study area to locate the MS pipe and taken as the Ground Truth Depth that will be used in equations ( 2) and (3), followed by an EML scan which collected the Measured Depth used in the same equations.
The research methodology can be simplified by dividing it into three (3) main stages.
Stage 1 : Data collection by using GSSI GPR and EML RD 8100 shown in Figure 3 to determine the depth of the mild steel pipe at the survey Area.
Stage 2 : Data processing in which the data collection by GPR will be post-processed using RADAN software and EML is using Google Earth by recording the GPS data on the site as shown in Figure 4.
Stage 3 : Data interpretation and analysis in terms of accuracy between two (2) types of instruments using statistical error analysis shown in Table 1 to determine the consistency and capability of both utility instruments between GPR and EML.

Result and discussion
This section serves as the critical juncture where the outcomes of our utility detection and correlation methodology are unveiled and comprehensively analysed.Through the utilization of advanced techniques like GPR and EML, the intricate network of subsurface utilities has been meticulously mapped and assessed.This section delves into the empirical findings and their implications, offering insights into the accuracy, effectiveness, and real-world applications of our methodology.Additionally, it provides a platform for scrutinizing the synergistic advantages of combining GPR and EML data, thus advancing our understanding of how these technologies can be harnessed to optimize utility management, mitigate risks, and enhance the efficiency of construction and excavation projects.11 points of data have been recorded as shown in Table 2.For data collected using GPR GSSI, the radargrams (hyperbolic images) have been post-processed and analysed with using RADAN 7 software to determine the depth of utility objects.Statistical analysis with error analysis has been used and applied to identify the accuracy of data.Tables 2 shows the result, while Figure 4 shows a graph analysis of data obtained by GPR GSSI and EML RD 8100.4, GPR shows more consistency with minimum variation in the depth readings.EML survey shows accurate results in locating the pipe horizontally but shows some variations in the depth readings.For GPR equipment used, the antenna was shielded perfectly and was free from noise or other disturbances from metallic and non-metallic underground objects.On the other hand, the data obtained from EML RD8100 uses the electromagnetic induction method is less accurate and inconsistent.This is because the electromagnetic signal induced by the EML can easily be disturbed by any metallic objects near the study area, causing it to jump or fade to the non-targeted metallic objects.

m
Based on the error analysis by comparing GPR and EML depth readings, the average RMSE value is 0.05821.This value shows that the EML reading is acceptable and can complement the GPR reading in terms of utility locating purposes.

Conclusion
The site selection of the EVF facility, Blok 60, Malaysian Nuclear Agency is relevant as a study area since the depth and location of underground utilities are known and can be measured for data verification.The study is carried out to determine the accuracy of depth values obtained from GPR GSSI and EML RD8100 which are based on radar and electromagnetic concepts respectively.During the determination of accuracy using GPR GSSI and EML RD8100, it indicates that not all buried pipes, cables and ducts can be detected and mapped in the certain consideration of depth, location, material type, geology, and proximity to other utilities due to limitation and difficulty in determining the actual depth.Based on the result and analysis, it can be concluded that the accuracy achieved from GPR GSSI is almost equal to EML RD8100.It has been proved by statistical data analysis in which the Root Mean Square Error (RMSE) values from 0.05821.
In terms of the reliability of the dataset for both types of equipment for underground utility mapping, it shows that the range value of depth for all the types of utility between GPR GSSI and EML RD8100 is not significantly diverse.In other words, both types of equipment can complement each other's information and improve the ability to detect depth, it is also able to reduce the risk of false positives or false negatives.This is a good sign as a firm decision can be made for good.

Figure 1 .
Figure 1.The study area and data collection points located in Blok 60, Malaysian Nuclear Agency.

Figure 2 .Figure 3 .
Figure 2. Distance between the top of the MS Pipe to the top of the curb measured from the inspection sump.

Table 1 .
Error analysis formula use in this study.

Table 2 .
Calculated value for ADE, MSE and RMSE based on Point no values.Graph analysis for EML vs GPR based on Depth value.