Electromagnetic Offset Log (EOL)
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The EOL survey utilizes a very large surface transmitter coil at low frequency to induce a magnetic field in the subsurface. A receiver coil is placed in a nearby well and measures the signal which is proportional to the subsurface resistivity. This signal is transmitted up hole by way of an electrical cable and recorded at the surface. The measurements are taken and recorded at 0.1 foot intervals. Figure 1 shows the layout of the acquisition system.

Once the recordings are made at a location, the transmitter coil is moved to a new data point on the surface and another recording is made.

Figure 2 shows the transmitter being moved to successive locations around the receiver well. This process us followed until the required transmitter point records are completed for the specific site. Under normal conditions, clear subsurface definitions are obtained horizontally out to 200 to 300 feet from the receiver well. Within this lateral distance, clear vertical resolution is obtained from near the surface to two feet above the total well depth; which can be in excess of 300 feet. In figure 2’s animation, the high resistivity anomalies represent how the EOL survey “sees” the hydrocarbon plume.

Figure 3 shows a transmitter layout pattern that was used at an automotive repair center. The engineer recording the data can see the variation in resistivity as recorded shown in figure 4, and can choose supplemental transmitter points as deemed necessary.

Once the data acquisition is complete, the data is ready for editing, processing, and modeling. Editing, done first, eliminates extraneous noise. The data is then normalized to eliminate differences caused by data acquired from separate receiver wells.

Once this is done, apparent resistivity and second order resistivity logs are generated versus depth. Figure 4 shows examples of the subsurface resistivity changes that occur with depth.

The next step is to generate two-dimensional models. The three-dimensional plots are generated using advanced modeling software. Figure 5 shows the three-dimensional figures generated for the automotive test center presented in figures 3 and 4.

This center experienced a leaking underground storage tank (LUST) and the owners were required by the state to perform a site characterization of the facility. The EOL resistivity anomalies in figure 4 clearly indicate the leaking fuel migrated under the building and downward to the water table located just above 25 feet. The FID values taken from samples collected with a hydro-punch device clearly confirm the presence of the plume and jointly they give a clear picture of the entire site and the subsurface contamination from the LUST.

Confirmation of the hydrocarbon plumes related to resistivity anomalies are an integral part of the overall EOL operation. Being able to see the contamination clearly prior to sampling is a strong economic factor of the EOL process. With the EOL survey, only a few samples are required for confirmation. Many more are required in finding and defining the plume by drilling and sampling alone.

There are several key ingredients in obtaining the best survey possible. First, an in-depth review of all subsurface geology and hydrogeologic data is performed. Next, an onsite review is made to locate all electrical noise locations which can interfere with the survey. From this, the transmitter pattern is laid out and receiver-well placements are selected. It is important that the receiver wells not be located in a contaminated area. Like viewing a cloud, it is much easier to see the high resistivity features from the outside looking in, than from the inside looking out.

The EOL survey offers several advantages in addition to the economic ones. Much of the survey in figures 3 through 5 was conducted inside the automotive repair center building. This was possible because of the low frequency at which the system operates. EOL surveys can be used successfully inside metal buildings, and over steel-reinforced concrete or asphalt paving found in floors, highways, and runways. When the transmitter is properly supported, it can be floated and used over water. EOL surveys have been performed on refineries, bulk-petroleum terminals, railroad terminals, airport aprons, taxi ways, and runways, bus terminals, over water, and at numerous service stations. It has been used to monitor remediation type operations and to define geologic strata as well.

The EOL survey has been used to map hydrocarbon migration paths through fractures in carbonate geology. Figure 6 provides an example. The surface layout covers two service stations to the north and an apartment house below and separated by a retaining wall to the south.

Gasoline was discovered at point A in the basement of the apartment house and the problem was to define which station(s) was the source of the gasoline release. The picture clearly suggests station two released the product near its UST and perhaps some product from the french drain to the north. The product migrates under the retaining wall through the limestone fracture about 8 feet below the backyard’s ground level and into the basement of the apartment house. The migration was confirmed by a back hoe digging at the apex of the retaining wall near point B and was also confirmed with color dye tracers injected at the service stations.

While the EOL survey has some very strong incentives in the site characterization, remediation, and monitoring market, it also has the strong backing of clients. The EOL survey has been successfully utilized by the branches of the armed services, on small service stations, large aircraft fueling areas spills, and for remediation monitoring of hydrocarbon contamination. It has been successfully used by state regulatory agencies for tracking leaking underground storage tank contamination and in determining responsible parties. And, the technique has been used by large and small corporations in the private sector for the same applications.