Geology Reference
In-Depth Information
2
1
2
2
1/2
(
)
1
(
2
2
)
1/2
2
1
2
2
1/2
(
)
2
(a)
(b)
Figure 8.12
The Biot-Savart Law. (a) Basic geometry for the Biot-Savart
equation. Current
I
flows in the wire, length
l
, and the field
(F)
is mea-
sured at a point offset a perpendicular distance d from one end of the wire.
(b) Application of the Biot-Savart equation when the line from the observa-
tion point to the end of the wire is not perpendicular to the wire. The effect is
identical to that of two wires positioned so that the equation can be applied,
but in one of which (in this case) there is a negative current
(-I)
.
currents in the two coils were also measured, any non-zero value being
anomalous. There was no receiver-transmitter reference cable, but absolute
phases and ratios relative to a single base could be calculated if each suc-
cessive reading was taken with the trailing coil placed in the position just
vacated by the leading coil. The method is now seldom used for CWEM
surveys, but large fixed sources are common in TEM work (Section 8.4).
B
S
A
P is the point of observation
ABCD is the transmitter loop
A
1
is the area of PQBS
A
2
is the area of PRAS
A
3
is the area of PRCT
A
4
is the area of PQDT
I is the current in the loop
r
2
r
1
P
R
Q
[
]
r
1
r
2
r
3
r
4
A
1
A
2
A
3
A
4
F = kI
+
+
+
r
4
r
3
In this example r
1
and r
4
lie wholly
outside the loop and must therefore
be given negative signs
C
D
T
Figure 8.13
Application of the Biot-Savart Law to a rectangular loop.
