There are basically 4 common ways to locate subsurface objects. Some work only on metallic objects, some can locate both metallic and non-metallic objects, such as buried PVC pipe. PVC pipe detection is difficult, and requires a unique process, so not every underground electrical locator is going to be able to do it. The following list briefly describes the different ways for locating underground objects:
The dowsing rod method dates back hundreds of years, its basic principal involves holding branches or rods over the ground, and determining subsurface locations based on their movements. As expected its accuracy has been questioned, but some people swear by it. Many modern "dowsers" use a pair of L-shaped metal rods, holding one rod in each hand, with the short part of the "L" pointed upwards, and the long part pointing forward. When the rods detect an underground object, they cross over each other, theoretically marking an "X" over the object. The rods may point in opposite directions, which is said to indicate objects that are long and straight, such as a buried metal pipe.
Another acoustic method for sewer detection or water main detection involves banging on a fire hydrant or water meter with a hammer, creating a sound on the surrounding metal pipes that can be heard above ground using a leak detector, or other monitoring device. The LD-12 Water Leak Detector from SSI Locators is a great example of this type of tool. The distance at which you can trace the pipe noise above ground depends on the type of metal in the pipe, the amount of soil covering and added noise from the surrounding environment. It usually takes more than one person to do this.
A few pipe locator manufacturers make devices that mimic a second person by placing noise either into the water or against the pipe's wall. There are two types of technology to do this. One of them uses a battery-powered device with adjustable frequency to bounce a tapping sound against the exterior of many buried objects. However, the typical maximum distance attained with a water leak detector is approximately 300 feet. Specialized sound filters may extend the range to about 500 feet. This method of sewer detection is greatly affected by ambient noises, such as traffic.
Another version of gas pipe locator places a pressure wave with a pulsing sound into the underground water via the fire hydrant or water meter. This method is more labor intensive, you need to remove the hydrant's cap, properly place the leak detection device, and then pressurize the hydrant. For residential lines, you'll probably have to replace the meter with the pipe locator or water detection device. As you may have guessed, there are better tools for water main detection available.
Using GPR as a wire locator or buried cable locator can be fairly effective. GPR has the ability to locate buried non-metallic and metallic objects. It's not as effective as the AML PVC Pipe Detector, which falls under the 'ultra-high microwave frequencies' category below. Ground penetrating radar produces a two-dimensional cross-section image of subsurface items, including buried PVC pipe or other object. GPR images can then be converted to 3D using the raw field data and some post-processing software. Engineers will then analyze the 3D images of underground power lines, pipes or water mains. GPR units are somewhat large, and kind of look like push lawnmowers. They also don't work in all surfaces, so they aren't the best PVC pipe detector available, even though they can range up to $50,000 each to purchase.
Using electromagnetic conductivity is another expensive way to go, and it's highly specialized. EM instruments can be used as a wire locator or underground electrical locator, but can't find subsurface PVC pipe. As a metallic pipe detector, it works well if you know how to properly operate it, but it doesn't work as a PVC pipe locator. An EM device can be carried around the work site, and used for various purposes. It's useful as a gas pipe locator, and can detect other underground objects like metal petroleum tanks or clay units. EM has been used to map sinkholes, underground metallic facilities and changes in a subterranean area. Again, it is ineffective as a PVC pipe detector.
This technology belongs exclusively to the AML PVC Pipe Detector by SSI Locators. Using tech originally created for lunar exploration, this PVC pipe locator is a patented scientific instrument. The AML has many advantages over typical GPR technology. This pipe detector functions in wet soil, snow, clay or standing water, while GPR can be inhibited by different surfaces. Pipe detection is achieved with the AML by sweeping the hand-held pipe locator over the ground, and reading different indicators on the unit. The AML uses ultra-high frequencies, or microwaves, to locate underground PVC pipe. Locating subsurface PVC pipe is not an easy process, and it requires a unique process. SSI designed this unit specifically for the water, utility, cable and gas industries. It works as a buried cable locator and PVC pipe detector because it can find almost any subsurface material including concrete, wood, plastic, metal and cable.