About GPR

UNDERSTANDING GPR DATA

GPR data can be easily understood with a minimal amount of training and practice. Many of our customers report that they master this process within the span of a day.

It is important to understand that radar signals emitted by a GPR device spread in a fan shape in the direction of travel when scanning. This creates a razor-thin signal with no width to it at all. The signal then broadens as it passes through the subsurface. Because of this, an object will be visible to the radar before and after moving directly over it. This is the reason that a point-shaped object will show up as a hypberola (arc shape) on the imaging data.

Since the GPR signal will always act quickest when the antenna is directly over the target the center line of the target will always be the highest point of the hyperbola in the data.

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An example of unprocessed 2D GPR data of two pipes:

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3D GPR DATA

3D data can be represented in one of 3 ways: either a 3D alignment of 2D traces, one or more depth slices, or isosurfaces.

3D alignments of 2D traces requires almost no post-processing. This results in faster production ahd the least amount of assumptions regarding velocity variation. The images are aligned sequentially and the data analyst can determine findings by examining them.

Depth Slices require an accurate model of the velocity of the medium being investigated. The more consistent the material is (i.e., concrete), the quicker and easier it is to achieve this. In some cases, velocity can be measured and sometimes worked out through a combination of educated guesses and trial and error. Depth slices tend to be good for modeling linear features such as rebar and conduit.

Isosurfaces require the most amount of post-processing and filtering. However, the end result is often most desirable for those that need to construct a full 3D model of their findings. The results can be good for modeling more complex features, but also have a tendency to filter out smaller and fainter features. Depending on project needs, this can be the most desirable data representation.

3D alignments of 2D traces requires almost no post-processing. This results in faster production and the least amount of assumptions regarding velocity variation. The images are aligned sequentially and the data analyst can determine findings by examining them.

3D Alignment of 2D GPR Slices

Depth Slices require an accurate model of the velocity of the medium being investigated. The more consistent the material is (i.e., concrete), the quicker and easier it is to achieve this. In some cases, velocity can be measured and sometimes worked out through a combination of educated guesses and trial and error. Depth slices tend to be good for modeling linear features such as rebar and conduit.

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Isosurfaces require the most amount of post-processing and filtering. However, the end result is often most desirable for those that need to construct a full 3D model of their findings. The results can be good for modeling more complex features, but also have a tendency to filter out smaller and fainter features. Depending on project needs, this can be the most desirable data representation.

isosurface

PLANNING A GPR SURVEY

There are three main approaches to surveying with GPR. The method you choose depends on desired outcome and whether real-time results are required or if post-processing is desired.

If the goal is to identify one or more specific targets, the easiest way to achieve this is to examine the survey site for any clues such as manholes, catch basins, valves, etc. These environmental touch-points can serve as an excellent starting location for the investigation. here, one would move the radar across the medium being investigated until they detect the object on screen. The operator would then determine the precise center of the object and either mark this on the ground or log the GPS coordinates of the point. After this, move over and repeat the process; essentially tracing the target. This works best when real-time results are needed.

A markout is generally accomplished by scanning a site on a grid pattern. The operator marks the target's location on the ground when detected; usually in the form of a flag or spray painted mark. The radar data will reveal only that a target is in the earth with a different composition than the surrounding material. This means that it is best to pass over the object more than once to determine if it is a rock, utility, or other material. The easiest way to differentiate buried objects is to make another pass and see if the target continues or not. One can choose to either place marks at every target and connect the dots later or to immediately move over and make another pass on the target to determine if it is linear or not.

More complex sites with many targets can also benefit from using 3D GPR data. Generating 3D data requires that data be collected on a regular grid in perpendicular directions and also usually entails some degree of post-processing. The amount of post-processing required increases as the uniformity of the medium being investigated decreases.

There is an additional practical correlation between the uniformity of the soil being investigated and the clarity of the images produced through post-processing. Furthermore, usable 3D presentations usually require that data be collected on a much denser grid than is necessary with 2D data. In many cases, the number of survey lines is doubled or quadrupled. For these reasons, 3D data tends to be used more often on smaller scale surveys of concrete floors and walls than it is on large-scale ground surveys.

SYSTEM CONFIGURATIONS

There are two basic model configurations for our GPR systems: Cart and handheld. Handheld units are all-in-one units with either 1000 MHz or 2000 MHz antennas or both. These antennas are ideal for scanning floors and walls, but generally do not offer sufficient depth to locate exterior buried utilities. Cart systems are modular, expandable, and can support a variety of antennas ranging from 100 MHz to 2000 MHz. They can be configured to be handheld for walls and floors or cart-based and to interface with almost any available GPS unit.

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GPS INTEGRATION

The Quantum Imager, Q series and Seeker SPR can be integrated with almost any type of GPS from handheld to RTK. For most applications, GPS positions are logged constantly while surveying. In the field, or at the office, as points of interest are identified, they are logged for future reference and can have numbers and descriptions assigned to them. These points can then be exported for use in spreadsheets, databases, GIS and CAD software. For certain specialized applications such as lakes and terrain with very irregular topography, GPS positioning can be used to trigger each scan made by the system. High accuracy RTK GPS systems are usually the type of GPS system which yields the best results for these applications.

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  • Ground Penetrating Radar Maps

SAFETY INFORMATION

Electromagnetic emissions from ground penetrating radar systems manufactured by US Radar Inc. do not constitute a safety or health hazard under normal operating conditions. The emissions are far below the 10mW/cm² (100W/m²) level specified by United States Occupational Safety and Health Administration (OSHA) regulations and similar regulations in other jurisdictions.

Here is the average power density data at 50mm:

Antenna Model

Average Power Density

(W/m² @ 5cm)

OSHA Spec.

(W/m²)

100

< 0.001

100

250

< 0.001

100

400

< 0.001

100

500

<0.001

100

900

< 0.001

100

1000

< 0.001

100

2000

< 0.001

100