Geoscience Reference
In-Depth Information
a)
a)
W
E
DH1
DH2
Transmitter loop (Tx)
U
A
A
U
V
V
(out of page)
U
(into page)
A
Drillhole
V
Z
(into page)
X
Y
(into page)
Section
b)
V
V
b)
DH1
A
A
U
(out o
f pa
g
e)
Transmitter loop (Tx)
A
U
(out of page)
U
(out of page)
DH2
V
Drillhole
Drillhole
Drillhole
Y
X
c)
Map view
Z
(out of page)
Transmitter loop (Tx)
Figure 5.92
The drillhole coordinate system (A, U, V) reference
frame for the vector components measured in DHEM. A is the axial
component, and U and V the perpendicular cross-hole components.
(a) Drill section showing the reference frames speci
Drillhole
Drillhole
Drillhole
c to each
drillhole and how the absolute direction of the components varies
when the drillhole trajectory is curved (DH1) or changes direction
(DH2). (b) Map view of the reference frames for the drillholes shown
in (a). X, Y and Z are coordinates of the absolute (geographic)
reference frame. See text for definition of the orientations of the
components.
Figure 5.91
Typical layout of transmitter loops for DHEM surveying.
(a) Multiple loops for surveying a single surface drillhole; (b) and (c)
single loop with surveys conducted in multiple drillholes.
information critical for determining the target
s location
and orientation. This method also assists in laterally dis-
criminating multiple close-spaced conductors.
Multiple loops are an integral part of surveys using axial
component probes (see
Section 5.8.1.2
), but three-
component probes (see
Section 5.8.1.3
)
can reduce the
need for multiple loops and multi-hole surveys.
'
rotational ambiguity, meaning that the distance to a con-
ductor and its orientation can be determined, but not its
azimuth relative to the axis of the drillhole. This requires
the survey to be repeated with the transmitter loop in
different locations so as to vary the coupling with the
conductors (see
Section 5.8.1.1
)
. Few, if any, DHEM
surveys are conducted now with just a single axial com-
ponent probe.
5.8.1.2
Axial-component measurement
Historically, the probe used in TD-DHEM was a single
multi-turn coil oriented along the drillhole axis, i.e. an
along-hole or axial component sensor. It measures the
strength of the
field in the direction of the drillhole at
The orientation of the probe (the drillhole) at each meas-
urement point is required for analysis of the data, and is
obtained by other means. The measured response does
not provide unique directional information about any
conductors that might be detected. The results have a
5.8.1.3
Three-component measurements
A DHEM probe containing three mutually perpendicular
coils or magnetic (B-
eld) sensors to measure two perpen-
dicular cross-hole or radial components in addition to the
axial component is known as a three-component probe.
Three-component measurements provide a complete
determination of the orientation of the primary and the
secondary magnetic fields at the measurement location. It
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