Geomagnetic Methods:
Geomagnetic methods have been in
existence since the second century BC. They have been deployed in
various applications such as locating buried pipes, drums and other
objects, archaeological investigations, igneous dykes, and large-scale
geological structures such as impact craters. Magnetic anomalies in
impact craters are complex due to significant variations in the magnetic
properties of rocks (Pilkington and Grieve, 1992 ). In this
section on the geomagnetic methods used in the papers retrieved, we
focus on both magnetic measurements of core samples, borehole
measurements, and magnetic measurements over the crater either through
airborne, marine or ground-based
measurements.
Plado et al. (2000 ) proposed a magnetic model which showed
a circular magnetic halo at the crater rim. Central negative and smaller
positive anomalies were observed at the lake’s central north, north and
south. Together with weaker negative anomalies observed in the eastern
and western parts of the lake, they envelop a central uplift. Their
explanation of the magnetic anomalies suggested they were due to several
relatively strongly remanently
magnetised impact-melt rock or melt-rich suevite bodies. In the study,
the magnetic survey was conducted at every ~6.25 m along
the flight lines with the survey positions determined using Global
Positioning Systems. Data was recorded along 30 profiles with an average
length of 22 km. The
instrumentation used was a Scintrex CS-2 magnetometer at a resolution of
0.001 nT with a nominal flight altitude of 70 m, flight directions
north-south, and line spacing of
500 m. The aeromagnetic survey
was conducted in 1997.
Pilkington and Hildebrand (2003 ) used magnetic data to estimate
the diameter of central uplift (DCU), the diameter of
melt (DM) and the diameter of the innermost slump block
(DS). Ugalde et al. (2007c ) reported new
experimental methods for conducting magnetic surveys, which were
trialled in the Bosumtwi Impact Crater.
Due to the existence of previous
airborne and marine magnetic surveys in the crater, they attempted to
introduce an innovative way of conducting magnetic surveys. They
acquired, processed, and interpreted 3-D vector magnetic data. Even
though the Earth’s magnetic field is a vector quantity with both
amplitude and orientation, currently, magnetic surveys only record the
amplitude, i.e., total magnetic intensity. Conducting magnetic surveys
with the full magnetic vector provides many advantages for mapping
anomalies in the subsurface.
Elbra et al. (2007 ) measured the paleomagnetic properties
of drill cores from LB-07A and LB-08A in the Bosumtwi Impact Crater. The
measured magnetic susceptibility, the intensity of natural remanent
magnetisation (NRM) and the Koenigsberger ratio (Q ratio; representing
the ratio of remanent to induced magnetisation) among other
petrophysical properties. Measurements of magnetic susceptibility of the
drill cores show mostly paramagnetic values (200–500 ×
10–6 SI) throughout the core, except for a few
metasediment samples, and correlate positively with natural remanent
magnetisation (NRM) and Q values. They inferred that magnetic parameters
are related to inhomogeneously distributed ferrimagnetic pyrrhotite. The
paleomagnetic data show that NRM has shallow normal (and in some cases
shallow reversed) polarity, which is consistent with the Lower Jaramillo
N-polarity chron direction and is present in ferrimagnetic pyrrhotite.
Kontny et al. (2007 ) investigated the magnetic properties and
mineralogy of drilled lithologies to understand and interpret magnetic
anomaly patterns. The authors concluded that ferrimagnetic pyrrhotite is
the most important carrier of remanent magnetisation. They observed
somewhat surprisingly that the
prediction of previous researchers of a strong magnetic impact melt body
underneath Lake Bosumtwi interpreted from airborne magnetic data and
numerical modelling were not confirmed in their study.
Danuor and Menyeh (2007 ) reported on the potential field
measurements made in the Bosumtwi impact crater before the International
Continental Drilling Program in 2004. Magnetic measurements were made on
the lake with two proton precession magnetometers. The profile spacing
on the lake was 800 m with
station intervals of 10 m. The results revealed a large negative anomaly
with a minimum value of 55 nT with less notable positive anomaly to the
south. They also report other weaker negative anomalies in the southeast
and southwest. The authors attributed the cause of the anomalies to
magnetised bodies in the central northern area of the lake. Modelling
suggested the most likely source of the anomaly as a body between
250 m and 61 0 m below the lake’s
surface.
In order to determine whether the strong magnetic anomalies observed
within the Bosumtwi Impact Crater originate from strongly magnetic
material in the form of impact melt (Plado et al., 2000 ) or
weakly magnetic layers of impactite material overlying the crater floor
with the larger magnetic anomalies being attributed to granites and
other intrusives in the Proterozoic basement (Ugalde et al.,
2007c ), further investigations by Morris et al. (2007 ) focused
on gathering additional rock property and borehole information. Magnetic
susceptibility measurements were made with the Bartington MS-2 meter
with an E probe attachment at
~10 cm intervals
on the core. They concluded that there is no evidence of a strongly
magnetic impact melt within the layers of the derived sediments in the
crater. Danuor et al. (2013 ) summarised the geophysical
studies conducted in the Bosumtwi Impact Crater. In contrast, they
report the presence of magnetised bodies between
250 m and
610 m below the lake.