Geoscience Reference
In-Depth Information
This value is twice the Nyquist sampling frequency and should be adequate for most situations.
Technically, “oversampling” is never a real problem, but measuring more stations than necessary
costs extra money and should be avoided for economic reasons. Economics and other practical
considerations (e.g., spatial survey accuracy) are the only limitations on using very small distances
between stations.
6.4.3 d
e P t h
o f
P
e n e t R a t i of n
Surveys should be designed so that the instrument depth of penetration is twice that required to see
the deepest object anticipated in the survey area. The depth of penetration varies with the sepa-
ration of transmit and receive coils (Geonics Limited [Date Unknown]—Technical Note TN-31).
However, from a practical point of view, the only way to know the depth of penetration for a par-
ticular survey site is to make an initial guess based on estimations from the skin depth calculations
(Equation (6.2)). The skin depth can be calculated using the following relationship:
1
1
one skin depth ==
m
(6.2)
12
/
α
12
/
2
ω
µε
σ
εω
1
+
−
2
22
2
where α is the attenuation constant in the wave equation (Balanis, 1989), σ is the subsurface elec-
trical conductivity, μ is the subsurface magnetic permeability, ω is angular frequency, and ε is
the subsurface electrical permittivity. A plot of the skin depth versus frequency is provided in
Figure 6.10.
6.4.4 i
n t e R f e R e n c e
e
f f e c t s
The physical surroundings of a survey and the proximity of interfering features should be a consid-
eration for any survey. Some of these features are shown in Figure 6.11. Overhead or underground
power lines can give off a signal that interferes with commercial systems, although most systems
have 60 Hz filters that eliminate most of the power line related noise. There are no foolproof rules
of thumb for eliminating these noise sources. However, they should be avoided whenever possible.
10
6
σ
= 10
-5
10
4
ε = 1
ε = 20
ε = 50
10
2
σ
= 10
2
10
0
10
-2
σ = 10
-5
σ = 10
0
σ = 10
2
10
-4
10
2
10
4
10
6
Frequency, Hz
10
8
10
10
10
12
0
fIGURe 6.10
Skin depth plotted as a function of frequency. One skin depth is the depth at which the signal
is attenuated by the amount 1/
e
. The area in gray is the electromagnetic induction region, where attenuation is
primarily influenced by the conductivity (σ) and is minimally affected by the permittivity (ε).
