Overview

Planning the location of boreholes is a very common geotechnical problem. This case study illustrates the use of GPR to identify underground obstructions and define clear zones for borehole drilling.

Gasoline station site that required an environmental site assessment as part of a real estate transaction due diligence. Borehole locations were sited in areas clear of buried structure.

Problem

A gas station sale required a environmental site assessment. Known subsurface obstructions included buried utilities, old foundations, buried tanks and other possible buried debris. The goal of the assessment was to determine if hydrocarbons had leaked into the soil by conducting a borehole soil sampling program.

GPR Solution

GPR is commonly used prior to such borehole programs to identify areas clear of subsurface obstructions.  Clearing programs will often use multiple methods.  Metal pipes and cables are often identified using traditional electromagnetic tracking techniques.

Scanning the area with GPR provides the benefit of detecting both the metal pipes and cables as well as non-metallic structures. As a result, areas of disturbed soil, undocumented utilities and storage tanks can be identified.

The example of a GPR cross-section shows the response from two buried tanks located under a concrete pad plus the presence of some of the buried product pipe.

The normal approach is to scan over the area with GPR, much line mowing the grass.  Any localized changes are immediately visible on the real-time display and can be immediately marked out.  While GPR cannot uniquely identify the type of target, reference back to as-built drawings and records plus onsite visual observations usually provides this information.

The sampling investigation sites boreholes in favourable areas which are clear of any identified subsurface features. 

Example of a GPR cross section that shows two underground fuel tanks beneath a concrete slab and associated product piping.

Results & Benefits

Using GPR to clear a site is very common.  The benefits of GPR include:

• Very quick deployment on site
• Immediate visual response
• Detection of all manner of buried obstructions
• Complements other techniques such as EM induction and magnetics
• Inexpensive, fast and simple.