Below are some of the most common questions about GPR and what we do:
Frequently Asked Questions
What can be mapped with GPR?
Almost any buried object or feature can be imaged and mapped in 3D. But, there are some critical variables, including the depth of burial, the size of the target object, and the type of material (soils/sediments) that surrounds and buries it. For example, very small objects can be located and imaged (up to the size of a coin), but only if they are buried at shallow depths (less than 30 cm in the ground). At deeper depths (about 5-10 meters), only fairly large objects can be imaged (such as the size of a scooter). In either case, successful imaging depends on using the correct radar antenna.
Why use GPR over other geophysical methods?
GPR is ideal for mapping specific areas of interest in a variety of environmental settings. Unlike other techniques like magnetometry, GPR will map features of all different compositions, including wood, stone, and metal. It is also the only near-surface geophysical technique that provides actual depth in the ground, making it an ideal tool for the studying of archaeological, environmental, and construction sites.
Do I need to know about the soil types and environmental conditions of the site?
Yes! Soils types and conditions are very important. The difference between sand and clay at a site can be the determining factor of a successful survey. How much moisture is in the ground (how recently it’s rained) is also important to know, as water often impacts GPR readings. While GPR surveys have proven successful at a variety of sites in numerous different soils and sediments, generally those without salts (such as calcrete) or metal (such as magnesium) are best.
What size area can be covered?
Because GPR is a fairly high resolution, near-surface mapping technique, it is best at targeting specific areas of interest for mapping. A typical GPR survey grid is about 20m by 20m, though an area as large as 40m by 40m can usually be done in one day. Often significant expanses of ground can be covered over the course of several days, but this requires more lab time for processing and analyzing the data. Very large areas (many acres or hectares) are simply not feasible for GPR mapping, and other geophysical methods should be used first (such as EM or magnetometry) to delineate specific areas for more detailed mapping with GPR.
How much time does it take to do a survey?
A 40 by 40 meter grid can usually be collected in one day. If the project warrants immediate results, the data can be downloaded to a computer and preliminary analysis and interpretations can be made the same evening. More definitive interpretations, however, typically require a full day of computer analysis per day collecting data in the field.
What types of surface conditions are important?
An abundance of trees, brush, or large rocks can significantly affect the quality of data collected, and should be avoided if possible. A large amount of buried metal (not associated with the site) or modern trash can also create problems. Otherwise, good data can be collected on many surfaces.
Can you collect data in areas with modern disturbance or buried modern materials?
Yes. GPR data can be collected anywhere, but it should be remembered that the more impacted a site is by modern disturbances (such as buried pipes, cut and fill material, or other construction), the more difficult data interpretation will be.
What is your success rate in finding buried materials?
That primarily depends on the site conditions. In ideal soil conditions, most if not all target features that are not buried too deeply can be found and mapped. In poorer environmental conditions, the success rate is lower. Some surveys can be a total failure if the targets are too small, if they are not different enough from the surrounding material, or if they are buried too deeply. Remember, GPR is not a “magic” tool, but a geophysical instrument that when applied correctly to qualifying sites, can be extremely beneficial in mapping features of interest.
What is your final product?
All GPR studies begin with a detailed analysis of each individual radar profile in the survey. These are used to determine the velocity of radar travel in the ground, and in visual interpretations of what was recorded. The data are then processed to remove background noise, and the appropriate filters are applied for optimal resolution of features in the ground. All processed data are then imported into a computer program for amplitude analysis and to create amplitude slice maps of the site. These maps provide a 3D visual image of the subsurface, as the location of features are indicated spatially and with depth in the ground. The final report to the client includes an interpretive report with selected maps and images, as well as a CD with all of the maps, all of the raw field data, and all of the processed data.
What information do you need from a prospective client to do a survey?
We always ask the following:
- What is your target? What are the dimensions and possible burial depth?
- What types of soils do you have? Are the wet? Sandy? Full of clay?
- What is the surface like? Are there any obstructions? Vegetation? Ground conditions in general?
- How much of an area is prospective? Can all the data be gathered in one grid?
- Will you be testing the area after the GPR data are interpreted? If so, can these data be incorporated into the analysis?