Data
Examples
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.
GEM-2
Data Examples
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).
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