Ground-Penetrating Radar (GPR) Concrete Imaging Technology

A Brief Overview of Ground-Penetrating Radar (GPR) Technology

During our concrete imaging and scanning surveys, we harness the same ground-penetrating radar (GPR) technology used to conduct our utility locating surveys, the only difference being that we replace the mid frequency 400 MHz antenna and large survey cart with a higher frequency antenna and small handcart. We currently use the GSSI StructureScan module, which is manufactured by Geophysical Survey Systems, Inc. of Nashua, NH. To understand the theory behind GPR technology, please visit our Ground-Penetrating Radar (GPR) Technology UtilityScan Module page.

GPR Technology With The GSSI StructureScan Module

The StructureScan module is comprised of a SIR-3000 or SIR-4000 control unit, a handcart with a survey wheel, and a high frequency (high resolution) antenna, enabling us to scan a variety of concrete structures in real-time without the need for shutting down the work area or gaining access to the bottom of the slab.

The GSSI StructureScan Module

High Resolution Antenna Frequencies

We use four different antennas, each of which has a specific application purpose and depth penetration capability.

1000 MHz - Deep Antenna

The 1000 MHz Deep Pentrating GPR Antenna

 1600 MHz - All-Purpose  Antenna

The 1600 MHz All-Purpose GPR Antenna

2000 MHz - Palm  Antenna

The 2000 MHz GPR Palm Antenna

2600 MHz - High Resolution  Antenna

The 2600 MHz High Resolution GPR Antenna

GSSI StructureScan Mini System

In addition to the multi-purpose control unit and handcart, we have a self-contained unit that can be used to scan in areas that are difficult to access with the larger equipment.

The 2700 MHz All-Purpose, Deep Penetrating StructureScan Handcart

2700 MHz - All-Purpose Antenna

Determining the Feasibility of Conducting a GPR Concrete Imaging Survey

Prior to performing test scans, a visual inspection is performed to analyze the following three factors:

Running Test Scans & Calibrating the Depth Scale Using the Ground Truth Method

Test scans are performed by making long passes perpendicular to one another across the concrete structure in question. If the results are good, scans are then performed across a target of known depth such as an exposed rebar or conduit to calibrate the depth scale on the y-axis of the data screen profile.

Setting Up an Orthogonal GPR Survey Grid

To set up an orthogonal survey grid, the location of known objects (from an as-built or test scans) are used as a reference point. Specifically, the grid is oriented so that it allows for the greatest number of scans to be performed perpendicular to known objects, the direction they must be crossed to produce the narrowest hyperbolas possible. In most situations the orthogonal grid can be imaginary, but if documentation is required, an alphanumeric grid must be created with chalk. An example of typical GPR survey grid is shown below.

The spacing of grid line intersections is dependent upon the purpose of the survey and the size of the survey area, and this decision can only be made through an analysis of multiple factors and field experience. In most concrete imaging surveys, a 3' x 3' survey grid will be created around the proposed core drilling location in question. A brief rule of thumb to determining the spacing of an imaginary grid is to perform the scan, stop at the end of the grid, pivot the wheels of the handcart, turn 180° until the wheel overlaps the track of the previous scan, and then perform a scan parallel to the first.

Alphanumeric, Orthogonal GPR Survey Grid

Conducting a GPR Survey

A GPR survey is performed using the same principles outlined in the seven-step process shown on our  Ground-Penetrating Radar (GPR) Technology UtilityScan Module page.

Advantages of GPR Technology for Concrete Imaging 

Limitations of GPR Technology for Concrete Imaging