Whirlybird Data Have Advantage

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 Basin (WCSB).

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 1).

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 2).

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 Low.

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 feature
(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.

Conclusions

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 techniques.

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.