Geology Reference
In-Depth Information
3000
2000
1000
0
0
100
200
300
400
Distance (m)
500
600
700
800
900
Figure 12.1.2. Magnetic field readings recorded over clinker in Theodore Roosevelt National Park, North
Dakota, USA. The red-dashed lines designate the extent of two bodies of clinker mapped by Gonzalez and Biek
2003.
A third magnetic profile over clinker in New Zealand is shown in Figure 12.1.3 (Lindqvist et al., 1985). The strong
anomaly at mid-profile was recorded over areas of exposed clinker. The anomaly at the western edge of the profile
was thought to correspond to an unexposed lignite bed which had been logged elsewhere on the site.
The examples above all highlight surveys over paleoclinkers. Sternberg and Lippincott (2004) and Sternberg et al.
(2008) also report magnetic survey results over areas of recent coal-seam fires in western North Dakota. About 30
coal-seam-outcrop fires over an area of 30 000 hectares were ignited in 1999 as a result of the so-called Gap
grassland wildfires, which were set by weed burning on the Montana border. In some cases, these fires have
continued to burn underground, and grass fires occasionally are reignited (Beechie, 2004). Four profiles over the
Cummings A burn showed anomalies up to 1000 nT (Figure 12.1.4; Sternberg and Lippincott, 2004). Anomalies to
the south end of the profiles are attributed to paleoclinkers. The extent of the zone marked as
was
based on observations of slumped and discolored soil material. The four profiles show anomalies within this zone
and not beyond, although without much correlation from profile to profile. This suggests the possibility of
anomalies due to mineralogical changes in the burned soil.
“
surface burn
”
62 000
61 500
61 000
60 500
West
East
Clinker exposed
Clinker inferred
Lignite W7
Lignite W6
W8
0
100
200
Distance (m)
300
400
Figure 12.1.3. Magnetic field readings at the southern end of the Waimumu deposit, the New Vale Coal Company,
eastern Southland, New Zealand. From Lindqvist et al. 1985 with permission of the Royal Society of
New Zealand
.
