Technical Bulletin
Is GPR Always Reliable?

INTRODUCTION 

Ground Penetrating Radar (commonly called GPR) is one of the most widely used geophysical instruments today but reliance on just GPR during geophysical surveys can lead to major mistakes.

 

BACKGROUND

GPR is a geophysical method that has been developed over the past thirty years for shallow, high-resolution, subsurface investigations of the earth. GPR uses high frequency pulsed electromagnetic waves (generally 10 MHz to 1,000 MHz) to acquire subsurface information. Energy is propagated downward into the ground and reflected back to the surface from boundaries at which there are electrical property contrasts. GPR is a method that is commonly used for environmental, engineering, archeological, and other shallow subsurface investigations.

 

OVERVIEW

GPR can be a wonderful method when site conditions are optimal but there are many misconceptions about GPR. GPR penetration depths as well as accuracy are directly proportional to the conductivity of the materials below. If you are working in an area with increased soil conductivity then the likelihood of signal attenuation (absorption) is high. In some cases depth of penetration can be

less than two feet. This is a partial list of materials that cause significant signal loss:

  • Clay Soils
  • Highly Saline Soils
  • Saturated Soils
  • Reinforced Concrete
  • Foundry Slag
  • Certain Paving Materials
  • Schists
  • Fly Ash
  • Metal Debris

The signal loss from the GPR in these materials can cause an operator to misinterpret data or overlook an anomaly completely. GPR is still one of the most effective tools when it comes to shallow investigations, but reliance on this method alone can lead to negative results and other instruments may be more effective. For example, investigations for USTs in clay-rich soils should also include an electromagnetic method.

 

AN ALTERNATIVE APPROACH

Following are GPR Transects of Underground Storage Tanks in a Sandy Environment and in a Clay Environment. 

                                   

This Bulletin illustrates the importance of using the right geophysical method for the local conditions and why the success of a survey depends largely upon the experience and expertise of those designing and conducting the survey.  Delta’s team has worked together for over six years and has the experience necessary to get the most from the various geophysical techniques.

If you have questions about how Delta Geophysics conducts geophysical surveys, or the proper techniques that should be used, please feel free to contact us at info@deltageophysics.com . Thank you for taking the time to read our technical bulletin.