The GEM-2 outputs �ppm� data for inphase and quadrature components at each frequency. For most cases where the primary survey purpose is �finding something in the ground,� the raw GEM-2 data are sufficient. In other words, the GEM-2 data require no interpretation for locating buried objects or seeing variations across a site. Here, we show such examples of the raw data for bump-finding surveys.
On the other hand, the multifrequency GEM-2 data provide many other possibilities, depending on survey purposes, budgets, and the interpretation skills of the users. For instance, the GEM-2 data at each frequency can be converted to an �apparent conductivity� map that can be useful for geologic mapping. In addition, GEM-2 data at low frequencies can be converted into an �apparent magnetic susceptibility� map, since the GEM-2 acts like a magnetometer with its own magnetic source. Here, we show examples of conductivity and susceptibility maps derived from the GEM-2 data.
For more data examples, also check out the �GEM-2 Principle of Operation� and �Publications.�
Fig l. GEM-2 Data over buried waste. Example GEM-2 data at 4,050 Hz and 12,150 Hz at a 7-acre site in the desert, containing several debris pits. GEM-2 has been used to locate the pits. The GEM-2's multiple frequency features are important because shallow targets are best identified using high frequencies, while deep targets are best observed when low frequencies are used. As shown below, different features are identified depending on what frequency or component is examined. At low frequency (left), we see mainly the burial pits (red). At high frequency (right), we clearly see the background geology of dry meandering streambeds.
Fig 2. GEM-2 data over a 12-acre (about 5-ha) site, an abandoned military airport in Japan. Other than concrete tarmac, the site is devoid of any surface structures. All features indicated by the GEM-2 data are subsurface features.
Fig 3. Concrete culvert indicated by the GEM-2 data.
Fig 4a. Raw GEM-2 data (in ppm) at three frequencies (1,050Hz; 7,290Hz; and 18,270Hz) from a 4-acre utility plant in New York. Survey line spacing was 5 feet at a horizontal coplanar mode � holding the GEM-2 ski horizontally. Notice the remarkable frequency dependence among the maps. High frequency senses shallow objects and low frequency deep objects.
Fig 4b. Total field magnetic data using a cesium-vapor magnetometer. Notice the dipolar anomalies that are harder to interpret than the GEM-2 data. Also notice relatively poor spatial resolution in comparison with the GEM-2 data.
Fig 4c-1.Apparent Conductivity Map at 18,270Hz. Fig 4c-2.Apparent Conductivity Map at 7,280Hz.
Fig 4c-3.Apparent Conductivity Map at 1,050Hz.
Fig 4c. Apparent conductivity maps derived from the GEM-2 data of Fig 4a. Each frequency, inphase and quadrature generates an apparent conductivity map. Again, the lower the frequency the deeper the conductivity sources.
Fig 4d. Apparent magnetic susceptibility map derived from the 1,050-Hz GEM-2 data, the lowest frequency used in this survey. At low frequencies, the GEM-2 acts like a magnetometer, with a vertical source field in this case since the GEM-2 was held horizontally. This produces a �reduced-to-pole� magnetic map that is easier to interpret since anomalies are monopolar in general. Compare this map with the total field magnetic data of Fig 4b.
Fig 5. Apparent conductivity and susceptibility maps derived from the GEM-2 data at an environmental site with burial trenches and leaking chemical plumes.
Fig 6. GEM-2 data over Geophex Test Site in Raleigh, North Carolina.
Fig 7. GEM-2 data over Metro Subway near Anacostia Station in Washington, DC. The tunnels are about 40feet (or 13m) deep at this section. Multiple frequency data are inverted to determine the conductivity cross-sections (lower figures) using the "electromagnetic induction tomography" method, a generalized geophysical diffraction tomography (from Witten et al., Imaging Underground Structures Using Broadband Electromagnetic Induction, Journal of Environmental and Engineering Geophysics, 1997, p. 105-114).