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| WINTER 1998 |
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| Inside: |
• MOULD — A Growing Health Concern in Buildings
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USING GEOPHYSICS FOR SITE CHARACTERIZATION
by Lori Larsen, P.Ag.
PHH was asked to determine whether contamination was present on a large site that was on the edge of a former inlet in Vancouver. The subsurface geology maps for the area indicated the site was situated on a fill area. To complicate things, the historical aerial photo review had identified a ravine that had been present in the 1940’s and 1950’s but was now filled in. Faced with such a complex subsurface geology, PHH decided on using a geophysical investigation to help characterize the site prior to conducting the borehole-drilling program.
A geophysical investigation is a non-intrusive surveying technique. Geophysical investigations look at changes in the near surface (1-100m). Changes in physical properties are what geophysics uses to detect objects or features of interest. Those features can be underground storage tanks, utility lines, groundwater tables, contamination plumes, layers of soil or rock and others.
How do geophysical investigations work? Simply put, geophysics simply measures the amount of contrast between a target property and the background and that contrast determines how easy it is to detect. Geophysics can use a number of physical properties to measure contrast: electrical conductivity, magnetic fields, gravity (increases or decreases in density), radiation and seismic waves. Most environmental applications of geophysical techniques use electrical and magnetic measurements. You will hear terms like electromagnetic (EM) survey and ground penetrating radar (GPR) being used for geophysical surveys.
Performing a GPR scan on a site. --
For example on our site, we knew that we had a "missing" underground storage tank (UST). The geophysical investigation could take electrical measurements directly, by sticking probes into the ground (like a giant volt meter) and looking for more conductive areas of ground that could be our UST. Electromagnetic (EM) measurements don’t require contact with the ground and are more indirect. They are often called induction techniques because they use one EM field to "induce" a second field in the target conductor. They are very closely related to radio and radar technologies.
Microwave ovens are an example of EM induction. The oven doesn’t actually touch the food; it induces electrical currents inside the food. If you had a small enough sensor you could actually measure the extra current flowing in the food. The fact that the food heats up is a result of the fact that food isn’t a very good conductor; mostly it just heats up rather than conducting electricity. If you ever put a really good conductor in the microwave by mistake (like a fork or tin foil) you’ll see some really good induction currents! Our UST is a really good conductor, but we don’t get close enough to make it spark. If you use a high enough frequency, the energy bounces and scatters off of the UST instead of "inducing". We call this ground penetrating radar or GPR.
How did it work for us? Besides wanting to know more about the "lay of the land" beneath the site we also wanted to know how deep the groundwater table was and if there was any contamination on our site. We had a little bit of advantage before we began by knowing about a previously removed UST in one corner and the approximate area of our "missing" UST on the site.
We used two geophysical measuring techniques on our site, an EM conductivity metre and a SIR-10 ground penetrating radar. The GPR scan was done with two antennas allowing us to have a relatively high detail scan and a deeper scan. Computer algorithms later processed the data collected. The two photos show the GPR scan being done.
How easily the GPR was able to pick up our "missing" UST is clearly shown in Figure 1. The survey determined we were previously wrong about the location of the UST and it mapped its exact location. It even told us how deep the UST was below the surface.
The site had a lot of immovable metal racks on the surface and this interfered with both the EM-31 and GPR scans but despite the hurdles, the survey was able to locate our groundwater table at 0.8m below ground and found some piping that we were unaware of.
Most importantly, the GPR was able to map our ravine on the site. To be able to know the contours of the relatively impermeable silty clay layer under the fill was a great benefit to planning our borehole investigation. It was able to determine the deepest portion of the ravine, information that is shown on Figure 2.
Anomaly map produced by GPR shows depth of confining layers in the ground. --
Of greatest interest to PHH was how well the geophysical scan could map contamination on the site. The EM scan actually records two values, a quadrature phase value and an in-phase value. Soil contaminants are interpreted on the resulting maps where the quadrature phase values rise or fall without a similar response in the in-phase values. You can see the maps of phases in Figure 3 – Quadrature Phase contours and Figure 4 – In-Phase contours.
You can probably tell from a quick look at these figures that they are not that easy to interpret. To obtain good geophysical survey results you must not only collect good data but must have the background to interpret it. Our results did detect a possible change in soil conductivity toward the corner where the old UST was removed. The results were not conclusive but they when we did complete our borehole program; contamination was shown to be greatest in this area.
The cost for the geophysical survey was about one-third to half the cost of the borehole program for the site. Was it worth it? Definitely!! The site had so complex a subsurface geology that a borehole investigation alone would have had to drill about three times the number of boreholes to obtain the knowledge that we obtained by this preliminary survey. A number of the results, like the exact location and depth of the ravine, would have certainly not been able to be as accurately provided in any borehole program. In the long run, the client saved the most by expending a little more money at this stage of the site investigation process. Having the knowledge provided by this survey will very accurately target future remediation techniques.
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