Last month's Geophysical Corner illustrated
some of the exploration applications of high-resolution aeromagnetic
(HRAM) data in the fold belt region of the Western Canada Sedimentary
In it, we demonstrated that typical HRAM data acquired
with fixed-wing aircrafts flying 125 meters above the ground can
image deformed lithological units, which allows the recognition
of key geological structures within the detached sedimentary section.
We also illustrated that an important contribution
of HRAM surveys is the detection of reactivated basement faults,
which either enhance the reservoir potential of rocks or result
in the compartmentalization of certain hydrocarbon traps.
Finally, we indicated that the quality of fixed-wing
surveys might deteriorate in areas with rugged topography.
This article illustrates the benefits of using helicopter-mounted
systems to acquire HRAM data. These surveys are normally collected
with 50- to 100-meter flight-line spacing by a sensor hanging from
a helicopter at a mean altitude of 30 meters (figure
The helicopter-mounted system has the advantage of
increased resolution as well as the ability to perform draped surveys
over rugged terrains -- however, the tight flight-line spacing requires
that these surveys be flown over site-specific areas, such as existing
fields or prospective regions.
Geological Mapping: Coleman Gas Field
In this example, the helicopter-mounted system was
flown over the Coleman gas field in the WCSB's southern foothills,
approximately 150 kilometers south of Calgary (figure
The Coleman Field produces gas from fractured carbonates
along the leading edge of thrust sheets that are detached from the
shallow structures outcropping at the surface. The field is also
located within a major structural discontinuity known as the Vulcan
The survey's objective was to identify faults that,
due to their slight offsets, could not be seen on seismic data but
still compartmentalized the reservoir.
Figure 3 shows some of
the datasets used in the interpretation of the Coleman survey. These
include HRAM imagery, digital elevation model (DEM) data and surface
geological information derived from published geology maps.
The figure 3 imagery
illustrates that the helicopter-mounted system produces a "smooth"
magnetic image that is generally not affected by the strong topographic
relief present in the Coleman area. The strongest anomalies visible
in the HRAM data arise from contrasts in the magnetic susceptibility
of outcropping units.
These anomalies act as stratigraphic markers, outlining
near-surface geological structures found in the survey area.
The internal geometry of anticlines and synclines
is characterized by the presence of several elongated and asymmetrical
"magnetic ridges," which reveal the attitude of inclined bedrock
strata and outlines the location of folds and plunging noses.
The HRAM imagery also illustrates that several magnetic
markers are cut and offset by linear features, which are believed
to represent fault systems.
We postulate that the major northeast-trending magnetic
(A-A'), which corresponds to the field's southern boundary, reflects
late reactivation of a major fault or zone of weakness in the basement.
This feature is coincident with a prominent gravity and magnetic
low in the basement referred to as the Vulcan Low (see figure
2), and as such, has been coined the Vulcan Low Fault Zone (VLFZ).
On the other hand, the northwest-trending features
(B-B') appear to reflect shallow "tear faults" and do not correlate
with deep-seated basement structures. These faults seem to strongly
affect the internal geometry of the producing field.
It is interesting to note that the latter fault system
appears to manifest subtle topographic expressions that can be detected
on the DEM imagery, suggesting recent motion along these faults.
Since the 1999 acquisition of the HRAM data over
the Coleman Field, several similar surveys have been collected over
developed and undeveloped gas fields in the Canadian Fold Belt region.
In all cases, the helicopter-mounted systems proved very useful
to map near-surface geological structures, which are nearly impossible
to image with conventional fixed-wing surveys due to strong topographic
effects during data collection.
The main contribution of helicopter-borne HRAM data
to exploration and development activities is the detection of reactivated
basement faults and detached "tear faults" that have not previously
been recognized through conventional surface and subsurface mapping
It must be emphasized that these surveys only improve
the recognition of near-surface structures within the first kilometer
below the surface. Unfortunately, they do not provide significantly
more information regarding deep-seated structures, which can be
detected with less expensive conventional fixed-wing HRAM surveys.