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Antennas made for use with ground penetrating radar (GPR) are available in a variety of shapes and sizes. The biggest antennas generally radiate the lower frequencies needed to detect the deepest targets. The smallest antennas radiate the highest frequencies that generate the highest resolution necessary for the detection of small, shallow targets.

An antenna with the highest frequency that can still detect objects at the required depth makes it optimal for a job. Mark DeSchepper, President of Kansas-based Echo GPR Services, has over 12 years of GPR experience and provides some excellent examples of how to select the right antenna.

Antenna selection is critical when taking into account the depth of a particular target. If the antenna frequency is too great, the maximum detection depth may be insufficient for finding the desired target. GPR image quality degrades with increasing depth to the point where no reflections can be seen.

On the contrary, if the antenna frequency is too low, subtle details are lost. For instance, if the wrong antenna has been selected, individual tightly-packed shallow targets cannot be detected because they may show up as a continuous reflection band.

When evaluating antenna selection for a particular job, it is crucial to bear the surface cover and soil present in mind. One key parameter is the conductivity of the material – How well does the asphalt, soil or concrete allow an electrical current to pass through it?

The greater the conductivity of a material, the greater the material absorbs the radar waves as they pass through it. That is to say, the radar waves’ depth penetration gradually decreases as the conductivity of a material increases.

For instance, asphalt generally has low conductivity, while the degree of concrete’s conductivity varies depending on its curing state, wetness and composition. Soil conductivity can wildly differ, from radar-friendly sandy soils to radar-foe clay soils.

Therefore, antenna depth specifications are predicated on the material and application. Soil information is used in antenna specifications for soil-based applications, while concrete-based applications use concrete parameters.

Other factors to take into account when evaluating depth penetration include the presence of reinforcing metal between the surface and the target. The closer the rebar or mesh is spaced (for example, in a concrete slab), the more complicated it is to achieve the desired depth penetration.

GPR antennas transmit a range of radio frequencies and typically have an optimal frequency at which they broadcast most efficiently. This value is predicated on the size of the antenna’s transmitter and receiver elements and is generally referred to as the antenna’s “center-frequency.”

Another oft-used term is the antenna’s bandwidth, the frequency range that the antenna broadcasts at power levels that are within half the power level emitted at the antenna’s center frequency. A standard GPR antenna’s bandwidth is nearly the same as its center frequency.

For instance, a 400 megahertz (MHz) GPR antenna should have a center-frequency of around 400 MHz and a range of frequencies radiating at power levels within half the power level at 400 MHz – from 200 MHz to 600 MHz.

Therefore, the bandwidth would be 600 – 200 = 400 MHz. Similarly, a 900 MHz GPR antenna may possess a bandwidth that potentially extends from 450 MHz to 1350 MHz. This bandwidth range means that slight changes in center frequency offer minimal improvement in penetration depth.

That is why GPR antennas are not commonly available in small frequency increments. More ordinarily, antennas can be acquired in center-frequencies that are doubled, such as 200 MHz, 400 MHz, 800 MHz, 1.6 MHz and 2.6 MHz.

The size and depth of the target often determine what the right antenna frequency for a GPR job will be. Choosing a lower frequency antenna will offer deeper penetration, but the compromise is that the targets must be larger to be detected.

Range of available antenna frequencies, their depth of penetration and applications for which each is most suited. Image Credit: Geophysical Survey Systems Inc.

When locating 1-2 feet beneath the surface, for instance, when imaging in concrete and looking for small reinforcing and conduits, a higher frequency GPR antenna which offers greater resolution should be used.

For deeper targets or characteristics, the fundamental choice is lower frequency antennas. These antennas enable users to see much deeper while trading off the ability to resolve smaller or very shallow features and targets.

When scanning utilities, the mid-range antennas offer an adequate resolution to detect 4-5 inch pipes at four to five feet deep. The general guideline for utility locating is a 1:1 ratio; for each foot in-depth, one inch in diameter is needed to return a strong hyperbolic reflection of a potential target.

To scan deeper, for instance, mapping geological features 20-50 feet deep, low frequency antennas would be appropriate as an extremely high resolution is not required.

According to DeSchepper, antenna selection decisions should be made after conducting a pre-job conversation that determines an understanding of the depth of the investigation and what the customer wishes to accomplish. Safety, comfort and convenience are other considerations to take into account when working on a ladder or overhead.

Echo GPR works largely with electricians and plumbers who are remodeling existing buildings.

We do everything from a local grocery store that wants to install new plumbing in the floor and needs to locate existing utilities and underground conduits – all the way up to nuclear power plants.

Mark DeSchepper, President, Kansas-based Echo GPR Services

Additionally, the Echo GPR team works in commercial remodels of strip malls, office spaces and office retail spaces and has conducted GPR scans at all of Kansas City’s hospitals. They also work for geotechnical and environmental firms for the location of underground storage tanks.

Local supermarket hired Echo GPR to conduct a GPR scan to avoid losing power to cash registers or cutting refrigeration pipes in recent renovation project. Image Credit: Geophysical Survey Systems Inc.

DeSchepper explains that grocery stores cannot afford accidents such as cutting the power to their cash registers. Also, underground refrigeration pipes may run to their coolers, and the last thing they want to do is cut into these refrigeration pipes, which may cause catastrophic downtime, potential environmental issues and even store evacuation.

Below, we detail a selection of real-world examples of antenna selection. These are taken from the nearly 8,000 GPR scans Echo has conducted across the United States.

For simple concrete cutting or core drilling, Echo GPR usually selects higher frequency antennas, such as the 1600 MHz or 2600 MHz antenna.

I usually reach for the 2600 MHz antenna first. This is the highest frequency and gives the highest resolution. But higher resolution gives less depth of investigation. If the project is in a thicker concrete environment, we will switch out to a 1600 MHz antenna, which is a very sharp resolution antenna but can penetrate a bit deeper into the ground.

Mark DeSchepper, President, Kansas-based Echo GPR Services

Presently, Echo GPR is tasked with locating rebar prior to anchor-drilling steel plates for support mounts on a basement remodeling job with a steel erection company.

“The engineer elected to hire us because they know that once we have marked out all the steel they can establish where the premanufactured supports are and will be able to drill into the ceiling without hitting steel. This speeds up their process.”

Using a 2 GHz Palm antenna, the contractor located the plastic-capped rebar thread couplers in the Z axis. Image Credit: Geophysical Survey Systems Inc.

For this purpose, DeSchepper opted for a 2 GHz Palm antenna, which offers high resolution data with a decent depth of penetration while being very lightweight.

I am working upside down and off a ladder, with both hands above my head as I am scanning, so my antenna choice in this case is based on comfort and convenience.

Mark DeSchepper, President, Kansas-based Echo GPR Services

During a project at a local children’s hospital, the Palm antenna was crucial to the success of the task at hand. The hospital was erecting nine stories of research labs on top of an existing parking deck.

Contractors had to thread new vertical rebar into form savers that were embedded in concrete. The profile of the form savers was ¾ to 1 inch in diameter, capped with a piece of plastic.

Contractors found it difficult to locate the form savers and contacted Echo GPR to assist. Employing the 2 GHz Palm antenna, Echo was able to locate the plastic-capped rebar thread couplers in the Z axis:

“It was so accurate that contractors could hammer drill and find 16-30 rebar couplers in each column. We had scanned 800-900 of these rebar couplers and the contractor later told us we had never missed a piece of post tension cable and only missed 2 form savers in the entire project. This was gratifying because the vertical and plastic capped profile is such a hard thing to image,” explains DeSchepper.

Top image: Echo GPR scanned the area and identified 16 pieces of rebar reinforcement. Bottom image: Successful drill locations as identified in GPR scan. Image Credit: Geophysical Survey Systems Inc.

According to DeSchepper, a 400 MHz antenna or the 350 MHz digital antenna with HyperStacking (HS) technology are among the premium antenna choices for customers who wish to install new drain lines but have little knowledge as to where the existing sewer is located.

While the resolution may be less than other antenna options, these antennas allow users to see deeper targets – 6 to 8 feet below ground.

Another challenge Echo GPR is often faced with is when a request comes in to look for a clay tile pipe drain line buried in the clay soil that is so common in Kansas City.

As DeSchepper explains, “In this instance we reach for the newer 350 HS antenna because the digital antenna offers a clearer image than a traditional GPR antenna and a 50 percent improvement in-depth penetration. It also works with the newer control unit we use (the GSSI SIR 4000), which has more power and flexibility to image these more difficult to image targets.”

Along with the location of sewer lines and drainpipes, Echo GPR also utilizes the 350 HS antenna for detecting underground storage tanks.

Echo GPR uses a 100 MHz bistatic antenna when working for a geotechnical company at a property where sinkholes or underground caves may be present; one of the few GPR firms to do so. This antenna allows the company to scan up to and in excess of 20 feet underground while mapping out soil layers or deeper anomalies.

DeSchepper does state that it is not always easy when selecting the correct antenna for achieving the requisite depth penetration.

He offers this example: “Recently, a contractor asked us to scan a 20 foot by 200 foot area because they needed to cut out a hole and install a new piece of equipment. I grabbed the 2600 MHz antenna, my usual go to for concrete cutting. Now, when I teach GPR for those getting the industry certificate through the Concrete Sawing & Drilling Association (CSDA), the first thing I tell students is that the most important thing when scanning concrete is to find the bottom of the concrete – for both slab on grade and suspended slab applications. This lets you know you are seeing what you need to assist the cutting/coring contractor.”

He explains that challenges and difficulties are frequent occurrences – primarily because actual conditions in the field turn out to be different than those initially provided by a contractor.

When I scanned with the 2600 MHz antenna I could not see the bottom of the concrete. This was a definite red flag, since the contractor said the concrete was 8 inches thick and the 2600 MHz was set up to look up to 12 inches deep. I quickly realized this was not the right antenna. I switched out to the 1600 MHz – and lo and behold, the concrete they thought was 8 inches thick was actually 18 inches thick!

Mark DeSchepper, President, Kansas-based Echo GPR Services

Luckily, DeSchepper was able to contact the concrete cutting company before they left the shop; they came out with the correct saw and appropriate size blade to cut through the concrete.

Good antenna selection should be made by acquiring a deep understanding of what the customer needs to accomplish, as well as safety, comfort and convenience when working on a ladder or overhead.

This information has been sourced, reviewed and adapted from materials provided by Geophysical Survey Systems Inc.

For more information on this source, please visit Geophysical Survey Systems Inc.

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