12
results for the EM38-MK2 are shown in Fig. 2A for 1 m coil separation and 2B for 0.5 m coil
separation where the function
gives the fractional response to the meter reading from a thin
horizontal sheet of thickness dz buried at a depth z. We see that in the vertical dipole mode the
relative sensitivity to near surface material is very low (being zero right at the surface), that the
sensitivity increases with depth, becomes a maximum at about 0.4 (0.2 m for 0.5 m coil
separation) meters and decreases slowly thereafter. On the other hand in the horizontal dipole
mode of operation the relative sensitivity is greatest to material at the surface, and decreases
thereafter with depth. The large difference in the response to near surface material in the two
coil configurations is important; the horizontal dipole mode will be relatively sensitive to
variations in the near surface whereas the vertical dipole mode will be insensitive.
This difference in sensitivity allows a quick method for determining whether the near surface
material is more or less conductive than material at depth. One simply places the instrument on
the ground in the vertical dipole mode, notes the reading, rotates the instrument to the horizontal
dipole mode and again notes the reading. If the second reading is greater than the first, the near
surface material is the more conductive and vice versa. If both readings are essentially the same
then the conductivity of the ground is essentially uniform to a depth of about 1.5 meter, the
effective depth of exploration for the EM38-MK2.
5.3
Multi-Layer Calculations
The functions R
v
(z/s) and R
h
(z/s) referred to in TN-6 are illustrated in Fig.3. It will be recalled
that these curves give the fractional response arising from all material below depth z for both the
vertical and horizontal dipole modes (the curves in TN-6 refer to the depth normalized with
respect to the intercoil spacing which for the EM38 is one meter so the curves apply directly).
These curves, when used with the techniques outlined in TN-6, allow simple calculation of the
instrument response in either coil configuration to a multi-layered horizontally stratified earth.
5.3.1
Variation of EM38 Response with Height
The curves of Fig. 3 also give the decrease of the apparent conductivity reading when the
instrument is lifted above a uniformly conductive earth, since in general for a two-layered earth
σ
a
σ
1
[1
R
(
z
1
)] σ
2
R
(
z
1
)
and if the first medium is air, with conductivity
1
=0
σ
a
σ
2
R
(
z
1
)
where z
1
is now the instrument height above the ground.
Examination of the curves shows that in the vertical dipole mode an increase of elevation from 0
to 20 cm results in a reduction of apparent conductivity reading to 92.5% of the surface reading,
whereas in the horizontal dipole mode the reading is reduced to 68%. We thus see that the
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