Geology Reference
In-Depth Information
Magnetic Anomalies
M
agnetic measurements taken along a profile can be represented as simple
plots. A few examples will suffice
to show the results that typify measurements over clinkers and burning coal seams.
x-y
Figure 12.1.1, taken from Hasbrouck and Hadsell (1978), is a magnetic profile across a clinker area near Decker,
Montana, USA, with a peak-to-peak change of 1500 nT. There was a surface indication of the burn from red-
colored ground to the left on the profile, from 0 to 25 m. Although the ground color beyond 25m was not indicative
of further burning, the magnetic anomaly suggests a clinker edge more than 100m further, past 150m, as indicated.
The dramatic change of about 1200 nT at 160m is an edge effect as the magnetic clinker butts up against the
nonmagnetic coal seam. The location of the edge of the clinker, as interpreted on the profile from the magnetics,
was corroborated by rocks recovered from subsequent drill holes. Furthermore, once the clinker has been passed
and the subsurface coal seam continues underground, the profile is very subdued. Over the clinker, the smaller
magnetic anomalies are probably reflective of the variable magnetization of the clinker. Interestingly, one such
anomaly coincides with the soil color change. This and other examples in Hasbrouck and Hadsell (1976, 1978)
show that the magnetic survey can integrate the subsurface magnetic nature of the clinker and reveal its presence
where otherwise not visibly indicated on the ground surface.
In a profile over clinkers from the area of the Coal Vein Trail, Theodore Roosevelt National Park, North Dakota,
USA, Sternberg and Lippincott (2004) traversed 850 m (Figure 12.1.2). Very quiet zones indicate the normal
stratigraphy of sedimentary rocks, consistent with control profiles run over areas that had no coal fires or clinker.
Two zones where clinker was shown on the geologic map of the Park (Gonzalez and Biek, 2003) are marked on the
profile. The magnetics suggests that the northern border of the southern clinker zone is actually about 50 m south of
where it was mapped. On the other hand, for the northern clinker zone, the southern border of the clinker zone from
the magnetics is about 70m further south than what is shown on the geologic map. The northern and southern
edges of both zones show layer of high magnetization clinker juxtaposed against a layer of low magnetization
unbaked sediments.
59 000
A
E
58 500
Interpreted edge
of clinker
58 000
57 500
Soil-color
boundary
B
Drill holes
0
Unbaked sediments
Clinker
20
Unbaked sediments
Coal
40
60
Unbaked sediments
80
Unbaked sediments
Coal
0
50
100
Traverse distance in meters
150
200
250
Figure 12.1.1. Magnetic field readings over several coal seams and clinker near Decker, Montana, USA. Magnetic
units are in gammas, the cgs (centimeter
second) numerical equivalent of the SI unit of nanoteslas (nT).
From Hasbrouck and Hadsell 1978, with permission of the Colorado Geological Survey
-
gram
-
.
