Geology Reference
In-Depth Information
Field Work
T
he surveys described here all use standard procedures for magnetic surveying (Milsom, 2003; Burger et al.,
2006). Studies of the Hatrurim Formation, Israel, show how the airborne and ground results are complementary
(Khesin and Itkis, 2002; Khesin et al., 2007). Aeromagnetic surveys flown at elevations of 1000m showed
anomalies with amplitudes of 20 nanoteslas (nT are an SI unit for measuring the strength of the magnetic field),
whereas ground-based surveys showed anomalies up to 4000 nT. The amplitude decreases because of the greater
height of the airborne sensor above the magnetized rocks causing the anomalies. The anomalies in the ground
surveys also show shorter wavelengths due to the highly variable magnetization of the near-surface rocks.
Schaumann et al. (2008) carried out airborne magnetics over coal fires in China with a cesium vapor
magnetometer (an instrument used to measure the Earth
s magnetic field). An area over 100 km
2
was covered,
with line spacings varying from 50 to 250 m and along-profile sampling intervals of about 2m. This was
supplemented by ground surveys using an optically pumped magnetometer, at measurement spacings of 10m,
over an area of about 1300m by 600 km. Ground surveys, carried out closer to the source, are capable of providing
higher resolution of the anomalies if the data are collected at close spacings.
'
In perhaps the first published reports of ground magnetic surveys over clinkers, Hasbrouck and Hadsell (1976,
1978) reported on ground magnetic profiles at clinker locations in Wyoming and Montana, USA. Station spacings
were on the order of 10m, and profile lengths were 250
-
750m long. Anomalies up to 2500 nT were observed.
Sontag (1984) collected ground magnetic data over clinkers in Wyoming, USA, situated in the Tertiary Fort Union
Formation. Readings were taken along three profiles with lengths of 300
800m at station spacings of 7.65 m. A
proton precession magnetometer was used, which only had a resolution of 10 nT, although this is adequate to
characterize the large anomalies that were observed. A base station was used so that the temporal diurnal
variation, occurring over the course of the day because of ionospheric effects, could be separately recorded.
Removal of this pattern from the data set isolated the actual spatial variation in the magnetic signal. Anomalies
were as high as 1500 nT.
-
Lindqvist et al. (1985) made six ground magnetic traverses over clinkers at three localities in New Zealand. They
used a proton precession magnetometer, making measurements every 5
-
20m along east
-
west profiles 200
-
400m
in length. Anomalies of 1500 nT were observed.
Sternberg and Lippincott (2004) reported on magnetic surveys in the vicinity of two recently extinguished
coal-seam fires in western North Dakota. Sixteen profiles were run at these two sites, using a cesium vapor
magnetometer and GPS positioning. Measurements were made every 0.5 seconds at walking speeds, with profiles
at variable spacings. Anomalies over 1000 nT were typically observed when clinker outcrops were crossed, with a
peak anomaly over 2800 nT.
Sternberg et al. (2008) reported on magnetic surveys over active and recently extinguished Tertiary lignite
coal-seam fires in western North Dakota, USA. Five sites were surveyed with a cesium vapor magnetometer.
Anomalies over 1000 nT were observed.
Lindner and Rüter (2007, 2008) described magnetic surveys over burning coal as part of the Sino-German Coal
Fire Research Initiative. In the Xinjiang Autonomous Region, two fire zones were surveyed, with measurements
made every 1
2 seconds at walking speed with an Overhauser survey magnetometer, along with a proton
precession magnetometer base station. Sub-centimeter positioning by differential GPS allowed measurements to
be made in
-
“
random walk-mode,
”
which was advantageous in the rugged terrain. Grids were 200
-
300 m long by
40
-
80 m wide. Anomalies over the burning Jurassic strata exceeded 500 nT (Lindner and Rüter, 2007).
Lindner and Rüter (2007) also cite a study by Loga
ev and Zacharov (1973) of magnetic measurements over
steeply dipping burning coal seams in Siberia, with anomalies over 800 nT.
ĉ
Thus, magnetic surveys have been reported over ancient as well as current coal fires in North America, Europe,
Asia, and Oceania, and over similar combustion metamorphic rocks in South America. The general amplitudes of
the strongest anomalies seem quite similar, so the next section will graphically present some of the profiles.
