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Continuous Resistivity Profiling (CRP)
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Use
CRP for fast and cost effective results!
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The Continuous Resistivity Profile (CRP) surveys use the same
concept
as the Multi-electrode Electrical Resistivity
(MER) surveys. The
system computer operates a dipole-dipole
array as the cable is towed behind a boat. A sonar and GPS units continuously
record depth and position. These data are compiled to produce a
cross section of the bathymetry
of the ocean or lake floor and the electrical resistivity of the
underlying sediments. The electrical
resistivity correlates to the sediment type and allows mapping of the
thickness of recent sediment deposits
over aged sediments. Because CRP surveys are rapid, they are very cost
effective. By returning to GPS
mapped pathways, CRP surveys may be used to map changes in sediment
accumulation over time. CRP
surveys are a superior method for exploring for offshore sand deposits for
beach enrichment projects. The
surveys may be conducted in fresh water environments as well as in salty
or even in hyper-saline waters.
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The graph below
is a typical example of the bathymetric surface and geologic cross section as recorded by our
Marine Super Sting unit.
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CRP in Anguilla, BWI
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The
CRP mapping of the coastal areas of Anguilla were used to evaluate sand
thickness over the limestone to determine the feasibility of:
1)
constructing horizontal beach wells for salt water supply intake
2)
map
the depth to limestone and the physical characteristics of the limestone to
aid in the design of storm water drainage wells
3)
to provide the bathymetry and sea bottom sediment distribution as part of
our analysis of beach erosion.
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CRP in Tierra Verde, FL
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NSN was contracted to
map the geology and bathymetry of the current boat channel providing access
to residents located in
Tierra Verde
,
Florida
. The geologic and bathymetry
mapping were accomplished using our Continuous Resistivity Profiling (CRP),
and Multiple Electrode Electrical Resistivity survey (MER) geophysical
equipment.
NSN performed a series
of CRP marine surveys along the inner boat channel boundary (east shoreline)
and outer boat channel boundary (west shoreline) as well as the channel
leading to the cut at the east end of Shell Key (Figure 2).
NSN used an electrode
spacing of four feet along the CRP transects, allowing for a shallow (10-12
feet) and detailed data set. The
survey data has been broken down into subsections, which allows for ease of
interpretation. In addition to
collecting resistivity data along the CRP transects, NSN has also collected
and displayed a bathymetric or bottom profile and recorded any changes in
water temperature. The
bathymetric profile is displayed over the resistivity profiles and is shown
as a white line.
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The east channel CRP
transects were performed in the north to south direction along the boat
channel located closest to the shoreline.
These profiles generally illustrate two to three feet of water
depth, which is represented on the profiles by blue coloration and low
resistivity. Approximately one
to 1.5 feet of loose sand exists overlying a dense sand layer.
This loose sand layer is likely the result of infilling, and is
represented on the profiles by resistivity values ranging from 0.3 to 0.6
ohm meters (green coloration). The remaining high resistivity values
ranging from 0.6 ohm meters and above (yellow to red coloration) represent
the underlying dense sand layer. As
we travel south the loose sand surface layer exhibits more variability as
the underlying dense sand has a very uneven surface.
The small depressions in the dense sand layer increase the
in-filled sediment thickness to as much as three feet within the
depressions.
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