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Electromagnetic (EM) Surveys – Data Contour Maps or Not?

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Traditionally, results of electromagnetic surveys utilizing EM instruments such as Geonics EM31 or EM61 are usually presented in contour maps. A picture is worth a thousand words -- these data contour maps, especially when drawn in colors, can be very powerful presentation materials showing details of investigation results.

However, when the budget of a project is limited, it’s possible to lower the survey cost, but still achieve the same investigation goal without contour maps as the final output. This can be done with the reconnaissance survey mode without setting up GPS or a manual grid. By doing so, both field time and office time are saved, hence the cost is lower.

So, should you set up an EM survey with or without data contour maps as the final output? It depends on a lot of factors. Here are a few things to consider:

1. Cost v.s. value. Data contour maps could be very good records and they certainly can preserve and present great details of the investigation results. They can be eye-catching and sometimes easier to understand than many words. However, they usually come with a cost. It’s hard to predict their exact values but the responses to them from your clients or the end users might be helpful.
   
   
2. The GPS satellite coverage of the survey area. If the survey area is an open field without nearby tall buildings or tree canopies, etc, the field times for the EM survey are similar either way. Note that even only a small portion of survey area couldn’t be covered by GPS, the cost could be significantly increased due to manual grid setup and the issue of merging non-GPS EM data with GPS-positioned EM data.
   
   
3. The target of the survey. If you are looking for underground metallic objects, which are easily identifiable as you go along with the survey (i.e., in the reconnaissance survey mode), they can be marked on-site and data contour maps might not be necessary. In the other hand, if you are looking for subtle variations of soil electrical conductivity, such as locating the septic field of a septic sewer system, data contour maps are usually necessary since the targets are hard to “see” without the “big picture”.
   
   
4. The size of the survey area. It’s usually a good idea to collect the data with positioning, i.e., making data contour maps eventually for a large survey area. Some targets, potential targets or surprises can be readily recognized only through the “big picture” - data contour maps.  A site can be defined as a large site with the size of tens of acres or even only a few acres. However, if the area is congested with interested targets, it might still be a good idea to map them using the data, even when the area is fairly small. Sometimes it’s actually easier or more accurate to mark the anomalies on-site after the contour map is generated.

Before starting an EM survey, the client requesting it and the contractor conducting it should reach a general understanding whether the data contour maps are needed or required. Usually it’s too late to request data contour maps if the survey is done in reconnaissance mode without positioning information. Please feel free to contact us to discuss the options of conducting EM surveys at your sites.

Detecting screen section of a potable well using acoustic televiewer

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An old potable water well was abandoned and to be closed. In order to properly close the well, the depth of the screen section inside the well needed to be found out. Due to the cloudy well water, regular tools such as well cameras didn't work. Enviroprobe was called upon for a solution and we decided to use our borehole logging equipment to conduct the investigation and the acoustic televiewer was determined as our primary investigation tool.

Two different sections of the well are clearly observed in the acoustic televiewer log. In the suspected screen section, a distinct “spotty pattern” and narrow linear bands are visible in the data images of both travel time and amplitude. Note that the televiewer relies on built-in 3-axis magnetometers to orient the recorded images with respect to magnetic north but the strong variations of the magnetic field along the depth indicates the well was made of steel from top to bottom. As a result, the orientations of the images are not reliable, and straight linear features on the wall could look like zigzag paths in the data images. The transition zone between the solid steel casing section and screen section is also indicated in the images.

These two different sections can also be identified by observing the optical televiewer log, but only vaguely.

Underwater/Sub-bottom Utility Locating

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In the fall of this year, a geophysical crew from Enviroprobe Service Inc. was mobilized to a coastal city in New Jersey to per

form a geophysical investigation. The purpose of the investigation was to locate underwater/sub-bottom telephone utilities within a creek. The investigation utilized a cable locator with a submersible antenna, a Geonics EM31-MK2 system, and an EdgeTech 3100 portable sub-bottom profiler system.

It turned out the cable locator with submersible antenna was the most successful tool for this project. The underwater cables were pinpointed along their routes and their positions were marked on-site with anchored buoys. A high accuracy differential GPS was also used to record their positions. The recorded positions, along with estimated utility depths, were marked on maps for future references.

The EM31 data confirmed some findings of the cable locator survey. Due to the sub-bottom soil content and cable sizes, no signals indicating presences of cables were observable in the sub-bottom data.

Fall of Potential - Electrical Resistance Testing of a Grounding System

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In September, Enviroprobe performed a Fall-of-Potential survey in southern New Jersey. The purpose of the survey was to measure the electrical resistance of the grounding system for a new building. The survey utilized a AGI SuperSting R8 resistivity system with its standard passive cables.

 There were four grounding wells around the building and they were interconnected with each other forming the grounding system. The dimension of this grounding system was in the scale of 100'x100'. Two of the these wells were tested to measure the electrical resistance to the earth. Each test was carried out using the “Fall of Potential” method specified in IEEE Standard 81. The method involves passing an electrical current into the ground well, the earth, and a remote current electrode forming a current loop, and noting the influence of this current in terms of voltage between the ground well under test and a test potential electrode. The results from these two tests were similar, confirming valid testing.

An unexpected underground storage tank found during utility markout

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Enviroprobe conducted an underground utility locating survey at a gas station in Morristown, NJ. During the process of searching for underground utilities, an anomaly showed up in the ground penetrating radar data close to the pump island. The experienced Enviroprobe geophysicist immediately realized this could be an orphaned underground storage tank. Further investigation utilizing metal detectors confirmed the finding. The finding was notified to the client, a reputable environmental consulting company, so further appropriate actions could be taken.

Soil resistivity testing with Wenner Array Method

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Enviroprobe performed a soil resistivity testing with Wenner Array 4-point Method in Moorestown, NJ. This method is the most used test method to measure the resistivity of soil for electrical grounding design. The Wenner array consists of a line of four equally spaced electrodes. Current is injected through the outer electrodes and potential is measured between the inner electrodes. The testing utilized an Advanced Geoscience Inc. (AGI) SuperSting R8 resistivity system and was done in a perpendicular pair at the test location using the standard traverse setup with the maximum “a” spacing as 100'.

SOIL RESISTIVITY TESTING FORM

Misconception of “Concrete GPR”

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We’re frequenctly requested by clients to bring “concrete GPR” to sites for searching something underground. However, sometimes these requests were based on the misconception of concrete GPR. Just because the medium is concrete it does not mean that the "concrete GPR” is the best approach.

The main difference between a concrete GPR and a regular GPR, such as those for underground utility locating, is the antenna frequency. The antenna frequency of a concrete GPR system is usually in the range of GHz, while those of regular GPRs are typically between 250MHz and 800MHz. Higher frequency means shallower penetration depth but better resolution. If you look for something small and shallow, such as rebar and small conduits embedded in concrete, it’s usually ideal to use a high frequency antenna as in a concrete GPR system. However, if your targets are deep, it usually means you’d better use a low frequency antenna to reach targeted investigation depth, no matter whether the ground surface is covered by concrete or not.

Based on our experiences, if you look for something right underneath the concrete or even within the concrete, sometimes it helps to use lower frequency antennas. Depending on the mixtures and the age of concrete, some concrete might be electrically more conductive than others, making GPR penetrating depth shallower, thus demanding lower frequency GPR antennas. When requested by our clients for “concrete scanning”, our GPR experts usually carry extra GPR antennas in case they are needed depending on the concrete and sub-concrete soil conditions. Also, for any underground utility locating jobs, we also carry a cable/pipe locator for extra scrutiny.

7 site condition considerations for geophysical services

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Before you sign a contract with a geophyscial service provider, there are some general considerations for the site conditions you need to discuss with them for cost-effective and successful geophysical surveys.

1. Is the site generally accessible through vehicles or ATVs? Geophysical equipment for some tasks could be heavy and vehicle access could speed up the data acquisition process significantly. Sometimes the data acquisition design/logistics could be totally different depending on the easiness of vehicle access.

2. Is the site generally walkable? Is it heavily vegetated? Dense vegetation can require additional costs and/or permissions to clear survey areas.

3. What’s the ground surface material? Asphalt and concrete pavement can require additional work for electrical methods. Reinforced concrete could interfere with electromagnetic and electrical method. Concrete could also interfere with seismic refraction method.

4. Is the ground surface smooth, rough and/or with significant topography? Although most common geophysical techniques can now deal with difficult terrains, a flat and smooth ground surface is generally required for ground penetrating radar surveys.

5. Presence of highly electrical conductive surface materials such as newly constructed concrete or clay soils can significantly attenuate signals from ground penetrating radar resulting in shallow penetration depth.

6. Presence of aboveground and subsurface infrastructure, especially those containing metallic components can interfere with valid data acquisition and interpretation. For example, grounded electrical systems, metallic fences or underground utilities can prevent acquisition of valid electrical resistivity measurements.

7. Any other noise sources nearby? For example, a power plant can generate significant noises for electromagnetic surveys. Also, roadways and railroads can generate significant noises for seismic surveys.

Also, photos of the site could be very helpful. Some potential problems can be immediately identified from them by experienced geophysicists. Finally, a site visit by the service provider might be needed depending on the complexity of site conditions and the nature of geophysical services.

Environmental and Engineering Geophysical Services

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Over the years, the geophysical service provided by Enviroprobe for environmental and engineering communnities has grown tremendously.  Our geophysicists and geophysical technicians employ varieties of geophysical methods including but limited to: ground penetrating radar (GPR), precision locating (for underground utility lines), borehole geophysical logging, DC electrical resistivity, sub bottom profiling, seismic, and EM mapping.

Our services include but not limited to:

• Locating underground metallic and nonmetallic pipes and utility cables

• Delineating underground storage tanks (metallic and nonmetallic)

• Mapping rebar in concrete structure

• Mapping landfill boundaries

• Delineating pits and trenches containing metallic and nonmetallic debris

• Delineating leach fields and industrial cribs

• Delineating previously excavated and backfilled areas

• Mapping groundwater tables

• Mapping bedrock topography

• Mapping subsurface voids, cavities and sinkholes

• Characterization of archaeological sites

• Mapping soil and groundwater contamination

• Bedrock fracture delineation