Ground Penetrating Radar (GPR) utility scanning services from Rush Locates are used to find non-metallic pipes and search for underground objects. For locations in our main service zone we charge $150/hr with a 2-hour minimum for our GPR services. We offer traditional private utility locating services at the same time if pre-arranged. Call us at 503-939-9585 to discuss your project or schedule online.
GPR utility scanning is a way to find non-metallic lines such as sewer, water, and gas lines which don’t have a tracer wire and can’t be otherwise located. Scanning concrete for rebar, post-tension cables, and voids can also be done. Scanning in the area of planned work for unknown and non-traceable objects requires the use of GPR.
GPR has some limitations and requirements that aren’t present in normal private utility locating. GPR is limited by the soil type and objects underground. Soils with lots of clay lower the penetration depth. Lots of rocks increases interference and spot objects. The pathway must be clear for some feet in every direction. The target area must be flat enough to easily move the low-to-the-ground GPR machine across it. Even then, GPR scanning is time consuming and we need multiple passes to eliminate false positives.
Aside from looking for underground utilities with GPR other common uses include: searching for cesspools, locating drywells or excavation pits, and scanning for heating oil tanks under rebar reinforced concrete.
Rush Locates, LLC offers utility scanning GPR services at the very competitive price of $150 per hour with a 2-hour minimum for locations in our main service area. Rush Locates is your one stop shop for all your locating needs! Call or schedule online today – 503-939-9585 – [email protected]
Using Ground Penetrating Radar (GPR) to scan for underground objects can aid in finding objects that traditional radio detection methods cannot locate. Common uses for GPR surveys are: scanning for buried utilities, locating Underground Storage Tanks (USTs), identifying potential grave site or artifact location, scanning for rebar or objects under a rebar reinforced concrete slab, and to look for disturbances in native soil such as the location of a hole or trench. While GPR is valuable in ways other tools cannot be is cannot do everything and has its own limitations.
The nature of how GPR works and how the data is collected causes many limitations in both application and interpretation. If ideal site conditions are present the underground data may still be inconclusive by itself. By performing multiple passes more data is gained which results in a more accurate determination.
How GPR Works to Locate Utilities.
The GPR machines are different sizes depending on the peak frequency of the antenna used which is determined by what you are looking for. The GPR units used in utility locating are roughly the size of a lawnmower. GPR units used for searching for rebar or post tension cables in concrete are smaller and can be handheld. Other types of GPR exist for more specialized uses. Higher frequencies can see more detail but cannot penetrate as deeply and vise versa.
These units are set very close to the ground surface and need to move smoothly over the target area and the area around it. Because of this the area to be surveyed needs to be free of obstructions. Having more room on either side enables the objects in question to more clearly stand out from the background noise.
While the machine is moving it is sending out cone shaped bursts of radio waves which are strongest at the top and fade away at the bottom. These radio waves penetrate into the earth’s surface but they spread apart the further from the source they get. Like a flashlight has a small lens but can create a large circle of light on a wall. The radio waves emitted by the GPR pass through the ground and are reflected when they encounter something with different electromagnetic impedance properties.
The stronger the contrast is then the greater the reflection will be and hence the better signal the GPR machine will display. A shallower object will reflect stronger than a deeper object. There comes a depth that the radar loses coherence and reflections won’t make it back to the GPR to be measured.
For every pulse that is sent out the GPR times how long it takes to get a reflection and plots that on the screen in a single vertical section. Because the radar energy emitted is cone shaped this means that objects which are actually ahead or behind the GPR (as seen from above the surface and looking down) can reflect energy back at the GPR, but because these objects are at an angle the time is takes to strike them and come back is more than if they were directly underneath. An object is located under the peak of this curved hyperbola shape.
The stronger the reflection the higher the displayed signal, and the stronger the hyperbola. Metal objects reflect the most signal of anything, objects which are similar to the material they are in are faint and harder to interpret in a single pass. Objects under metal sheeting or mesh wire cannot be seen as no signal passes under the metal. Rocks, trash, debris, abandoned pipes, and even changes in the mineral or water content of the soil can cause reflections to the GPR machine.
Interpreting GPR Data
Soils which are high in water or fine particles (e.g. clay) can severely limit the depth at which the GPR can penetrate. Rocks can look very similar to pipes. Buried metal trash can give a strong signature. Deep or heavily rusted USTs may not show up at all on a GPR scan.The ambiguity of the data can cause confusion if there is no other information to correlate it with.
Interpretation of the GPR data can only be verified by excavation. Neither the GPR machine nor the operator can determine anything exactly except for where a reflection of the radio waves is being located. GPR determinations can never be guaranteed and objects detected cannot be identified. The information obtained from a GPR survey is best used in combination with other data such as site maps, historical trends, on-site indicators, and records.
While it may be possible to locate things that cannot be otherwise located such as cesspools, drywells, USTs, HOTs, excavation trenches, backfilled pits, non-metallic pipes, and other underground objects the GPR cannot make any determination as to the state of the item. There is no way to tell if a cesspool has been filled in for example.