Electromagnetic conductivity surveys

Inducing an electromagnetic field eliminates the need for an electrode array and, therefore, the electromagnetic field method is a more rapid tool for surveying. The elimination of electrodes to measure electrical properties of subsurface materials also enables electromagnetic surveys to be conducted in many areas where resistivity surveys cannot be considered (i.e. pavement areas, very dry sandy soils, frozen ground, railroad tracks, etc.). Electromagnetic conductivity surveys may also be used to produce rapid continuous profiles up to depths of 15 meters. 

In addition, electromagnetic methods are subject to interference from many cultural features and the presence of nearby electrical fields. Finally, the ability of electromagnetic conductivity surveys to identify groundwater contaminant plumes requires that a significant electrical conductivity contrast exists between contaminated and natural groundwater (2). 

Figure 5, Cross-sectional view of transmitted electromagnetic field and generated secondary electromagnetic field measured by a receiver for completing electromagnetic conductivity surveys.

Electrical resistivity measurements were also taken at 28 locations selected across the site to verify the data collected by electromagnetic methods. The data obtained was in agreement with the results of the electromagnetic conductivity survey. 

Figure 14. Conductivity contour map resulting from electromagnetic conductivity survey completed at an effective measurement depth of 50 feet.

In order to map the extent of the minor solution cavities found in other areas of the site, detailed electromagnetic conductivity surveys were completed on a grid spacing of twenty feet. 

Figure 17. Detailed geologic map of proposed field-application site based upon results of seismic refraction and electromagnetic conductivity surveys and test boring data.

Ground Penetrating Radar (GPR), Electromagnetic Conductivity (EM), Electrical Resistivity Surveys, or Seismic Surveys. 

Electromagnetic (EM) surveys are most widely used to create profiles or maps of subsurface conductivity. Lateral variations in conductivity, at a given depth, may be interpreted as contaminant plumes, sand and gravel deposits, clay deposits, karst features, salt water intrusion or buried objects (Figure 6). Because field surveys may be completed rapidly, EM techniques are often implemented to map large sites and to provide detailed maps of variable site subsurface features such as isolated karst features, and buried tanks or 55 gallon drums. 

Electromagnetic (EM) Conductivity Measures the electrical conductivity of materials in microohms over a range of depths determined by the spacing and orientation of the transmitter and receiver coils, and the nature of the earth materials. Delineates areas of soil and groundwater contamination and the depth to bedrock or buried objects. Surveys to depths of SO to 100 ft are possible. Power lines, underground cables, transformers and other electrical sources severely distort the measurements. Low resistivities of surficial materials makes interpretation difficult. The top layers act as a shunt to the introduction of energy info lower layers. Capabilities for defining the variation of resistivity with depth are limited. In cases where the desired result is to map a contaminated plume in a sand layer beneath a surficial clayey soil in an area of cultural interference, or where chemicals have been spilled on the surface, or where clay soils are present it is probably not worth the effort to conduct the survey. 

A comparison of resistivity and electromagnetic techniques (resistance versus specific conductance) shows that resistivity surveys are slower because the technique is necessarily limited to measurements at the fixed electrode points, which must be moved for each new survey. Electromagnetic surveys are not limited by fixed points. Also, certain applications of resistivity methods are best suited for sites where the... 

Field equipment required for a survey consists of a low frequency transmitter coil and a receiver coil. The receiver coil intercepts a portion of the secondary electromagnetic field and produces an output voltage which is linearly related to subsurface conductivity (11). 

Electromagnetic surveys may not be conducted in areas characterized by unusually high or low values of subsurface conductivity. 

Forester Electromagnetic Detector A test conducted at the Comhusker Army Ammunition Plant (CAAP) required unexploded ordnance (UXO) support in order to investigate the subsurface in areas where ordnance disposal activities may have occurred. An UXO geophysical team used the Forester Electromagnetic Detector (MK 26 Ordnance Locator), Eagle 2, White 6000 for both surface sweeps and subsurface surveys. 

Similarly, electromagnetic disturbances that affect users have become a significant problem, and a number of surveys have been conducted by utility companies and public organizations related to home appliances [4, 5, 6, 7, 8-9]. For example, it was reported that there were more than 1000 cases of damage to home appliances over the course of 1 year for one Japanese utility company. 

---