DETECTION OF WATER LEAKS USING GROUND PENETRATING RADAR

Laboratory experiments were used to investigate the potential of using ground penetrating radar (GPR) to detect water leaks in the underground distribution system. Leaks not only waste precious natural resources, they create substantial damage to the transportation system and structure within urban and suburban environments. Surface geophysical methods are noninvasive, trenchless tools used to characterize the physical properties of the subsurface material. This characterization is then used to interpret the geologic and hydrogeologic conditions of the subsurface. Many geophysical techniques have been suggested as candidates for detecting water leakage, including GPR, acoustic devices, gas sampling devices and pressure wave detectors. GPR is a reflection technique which uses high frequency electromagnetic waves to acquire subsurface information. GPR responds to changes in electrical properties, which are a function of soil and rock material and moisture content. A series of laboratory experiments were conducted to determine the validity and effectiveness of GPR technology in detecting water leakage in metal and plastic PVC pipes. Initially, a prototype laboratory model was designed to simulate a pipe leak. Holes were drilled in the middle of the pipe to allow the water leak into a simulated soil (sand). The metal and PVC pipes were tested separately by burying them in sand to a depth of 18 and 20 cm, respectively. Water was then injected into the pipe from the surface through a plastic hose. A 1.5 GHz antenna was used to collect GPR data. Although the experiment was very well controlled, results obtained so far indicate that GPR is effective in detecting water leaks. An outdoor test bed is currently under construction in collaboration with Central Arkansas Water (CAW) to simulate and detect water leaks in underground water systems using the GPR technique. Pipes that are commonly used for water distribution in the city of Little Rock, AR, will be used for the test. The test bed will be constructed using soil material that is representative of the region. Advanced digital signal processing will be implemented to enhance the anomalies. Also model simulations will be used to select an appropriate equipment configuration (frequency band, type of antenna and real-time imaging software) prior to data acquisition. INTRODUCTION Leaks waste both a precious natural resource and money. A large percentage of water usually is lost from the distribution systems in transit from the treatment plant to the consumer. According to an inquiry made in 1991 by the International Water Supply Association (IWSA), the amount of lost or unaccounted for water is typically 20 to 30 percent of total water production. Some distribution systems, mostly older ones, may lose as much as 50 percent. The primary economic loss comes from the cost of raw water, its treatment, and transportation. Leakage inevitably also results in secondary economic loss in the form of damage to the distribution network itself (e.g. erosion of pipe bedding and major pipe breaks) and to the foundations of roads and other manmade structures. Leaky pipes also create a public health risk, as every leak is a potential entry point for contaminants if pressure should drop in the system. Economic cost and scarcity of public water sources mandate that a systemic leakage control program be developed. In such a program, there are two components: water audits and leak detection surveys. Water audits measure water flow into and out of the distribution system, or parts of it, and to help identify those parts of the distribution system that have excessive leakage. However, water audits do not identify the specific location of a leak. To find the specific location needing repair, a leak detection survey must be performed (Hunaidi et al., 2000). Detection of fluid loss due to leakage from underground distribution pipes represents a major challenge to scientists and engineers. The key to the solution is threefold: selection of the right combination of sensing equipment, proper adaptation of procedure for each field operation, and data analysis. Since each problem is unique considering soil conditions, type of distribution system, groundwater conditions, and intensity of the leak, it is essential that a pre-tested combination be used to effectively devise the appropriate corrective measures in the shortest possible time. Testing and guessing in the field might rush a wrong decision. Bose and Olson (1993), Carlson (1993), and Turner (1991) classified the leak detection methods in three groups which may be applied to monitor the integrity of a pipeline (Zhang, 1996). These are: Biological Methods: Experienced personnel or trained dogs may detect and locate leaks by visual inspection, odor or sound. Hardware-Based Methods: Different hardware devices are used to assist in the detection and location of leaks. Typical devices used include acoustic sensors, gas detectors, negative pressure detectors and infrared thermography. Software–Based Methods: Various computer software packages are used to detect leaks in a pipeline. The complexity and reliability of these packages vary significantly. Examples of these methods are flow/pressure change detection, mass/volume balance, the dynamic model-based system, and pressure point analysis. Acoustic equipment (a Hardware-Based method) is the most used among the leak detection methods. In general, it has a fair percentage of success in metal pipes, however, the effectiveness of this traditional method for plastic pipes is limited. The equipment was developed primarily for metal pipes, however the acoustical characteristics of leak signals in plastic pipes are not as pronounced as in metal pipes. This has prompted an extensive investigation of the effectiveness of acoustic methods and the potential of alternative non-acoustic methods for leak detection in plastic pipes (Hunaidi and Giamou, 1998). This, if combined with the large increase of using plastic pipes in the water distribution system, makes the problem more acute and significantly more challenging. This leads to the increased need for development of noninvasive techniques to explore and retrieve information about the subsurface, either to obtain the soil conditions or to locate specific targets. To this end, the trend is toward development of more and more sophisticated systems like the ground penetrating radar (GPR) technique, which is safe for use in urban environments, as well as protecting the geological, environmental and archaeological integrity of subsurface settings (Gamba and Lossani, 2000). The GPR geophysical method is a rapid, high-resolution tool for non-invasive subsurface investigation. GPR produces electromagnetic radiation that propagates through the ground then returns to the surface. The radar waves travel at velocities that are dependent upon the dielectric constant of the subsurface. Reflections are produced by changes in the dielectric constant due to changes in the subsurface material and/or conditions. The travel time of the electromagnetic waves as they leave the transmitting antenna into media and reflect back to the receiving antenna at the surface is a function of the depth of the reflection point and the electric properties of the media. Thus, interpretation of this reflected energy may yield information on subsurface structural variation and condition of the media. As in seismic geophysical techniques, there is a trade-off between frequency and structural resolution. The high-frequency waves produce higher resolution models at shallow depth only, whereas low frequency waves produce lower resolution models that may be located at greater depth. The choice of appropriate antenna is a target dependent on the projects goal. Data are most often collected along a profile, so that plots of the recorded signals with respect to survey position and travel-time can be associated with images of the subsurface structure. GPR signals can be collected fairly rapidly and initial interpretations can be made with minimal data processing, thereby making the use of ground penetrating radar for shallow geophysical investigation cost-effective with the least technical support (Cardimona et al., 2000). GPR could, in principle, identify leaks in buried water pipes either by detecting underground voids created by the leaking water as it erodes the material around the pipe, or by detecting anomalous change in the properties of the material around pipes due to water saturation. Unlike acoustic methods, application of ground penetrating radar for leak detection is independent of the pipe type (e.g., metal or plastic). Therefore, GPR could have a higher potential of avoiding difficulties encountered with commonly used acoustic leak detection methods as it applies to plastic pipes (Hunaidi and Giamou, 1998). GPR could also be used as a supplement to these methods to increase accuracy in high risk areas such as high traffic streets and large structures. THEORETICAL BACKGROUND The speed of an electromagnetic wave in any medium is dependent upon the speed of light in free space (c = 0.3 m/ns), the relative dielectric constant (εr), and the relative magnetic permeability (μr = 1 for non-magnetic materials). The speed of electromagnetic wave (Vm) in a material is given by: Vm = ) 1 ) 1 )(( 2 / ( 2 + +P r r c μ ε , (1) where P is the loss factor, such that P = ωε σ , σ is the conductivity, ω = 2πf (where f is the frequency), and ε = εrεo ( where ε is the permittivity and εo is the permittivity of free space (8.854x10 F/m)). In low-loss materials, P≈0, and the speed of electromagnetic wave is given by: Vm = r c ε = r ε 3 . 0 m/ns (2) The depth of penetration (D) can be determined by, first, calculating the velocity of the medium, Vm, using Equation (1) and (2). Second, the two way travel time (T) can be determined from the graphic representation of the GPR signals. This will allow the use of the follo