| The application of aeromagnetic data to hydrocarbon
exploration has moved from primarily mapping basement structures and
lithologies to imaging and mapping structures within the sedimentary
section. High-resolution aeromagnetic (HRAM) surveys are now relatively
inexpensive tools for 3-D mapping of faults and fracture systems propagating
through hydrocarbon-bearing sedimentary levels.
Advances in data acquisition techniques (enhancements
in magnetometers, compensation software/hardware for suppressing
airplane noise, positioning utilizing GPS systems, pre-planned drape
surveys, gradiometers measuring horizontal and vertical total magnetic
intensity gradients, etc.), as well as data processing and displaying
procedures (such as micro-levelling), have significantly improved
data quality and resolution, providing levels of detail that are
compatible to those derived from seismic, well and surface geological
Current industry standards view "high-resolution"
aeromagnetic data as fixed-wing surveys with flight-line spacing
of 400-800m and tie-lines every 1,200-2,400m, and with mean flight
clearance of 100-125m.
The Canadian petroleum industry has been receptive
to the introduction of HRAM data as a tool for exploration. Consequently,
a large volume of HRAM data has been collected over both mature
and frontier portions of the Western Canada Sedimentary Basin (WCSB),
including part of the highly deformed Canadian Fold Belt.
HRAM Data In Western Canada
A portion of HRAM data collected in the WCSB is shown
in the inset of figure 1. The imagery
represents an amalgamation of several different speculative HRAM
surveys that were collected over the past decade by industry vendors
and the Geological Survey of Canada (GSC).
The area contains approximately one million line-kilometers
of HRAM data (representing about a third of the total data collected
in the entire basin), covering parts of the WCSB and Mackenzie Corridor.
We focus first on the Canadian Fold Belt region because:
- The geological structures that will be shown are clearly detached
from the basement, and thus illustrate that HRAM data can image
- Fold belt regions are usually characterized by extremely complex
structures that are difficult (and extremely expensive) to image
with seismic data.
- The collection of HRAM data is done in a non-invasive manner,
and therefore can be collected in environmentally sensitive areas.
- The rugged topographic relief of fold belts allows us to demonstrate
the ability of the new surveys to overcome the effects of topography
on the magnetic signature of structures.
- Fold belt regions appear to represent a significant portion
of the remaining uncharted areas for frontier oil and gas exploration.
HRAM Mapping -- Fort Norman Area, Northwest Territories
In 1998 the GSC began to collect a series of non-exclusive
HRAM surveys along the Mackenzie Corridor. HRAM data was collected
at 800m flight-line spacing and mean elevation of 125m above ground.
One such survey was collected over the Fort Norman
area, along a partially exposed portion of the Mackenzie Mountains
thrust belt (figure 2). In this area,
the structural style of the fold belt is further complicated by
the presence of a major northeast-trending tectonic element known
as the Gambill Shear Zone (GSZ).
Figure 2a illustrates
that in the subsurface, the Gambill Diapir acted as a transfer zone
linking the GSZ with the Norman Wells Thrust (MacLean and Cook,
1999). Geological mapping (2b) and
the digital elevation model (2c)
show that the structural features associated with these tectonic
elements are only partially exposed at the surface.
However, the HRAM data (2d)
clearly demonstrate that the transfer zone reflects the presence
of a complex shear element that exhibits basement-involved, right-lateral
faulting, which resulted in a complex pattern of surface and near-surface
In addition, one can notice that the overall magnetic
intensities do not reflect the rugged topography found in this region.
The block diagram shown in figure
3 illustrates that the GSZ consists of high-angled wrench faults
and a series of tight salt core anticlines that developed and wrapped
around a structural high to the north. Strike-slip reactivation
of northeast-trending basement faults may have triggered the development
of salt diapir structures along this fault zone during Laramide
Magneto-Stratigraphy In the Coleman Area
Our experience in fold belt regions shows that HRAM
data collected at sufficient resolution and flown at low altitude
can resolve magnetic signatures that are associated with structures
found within the sedimentary section.
Studies have shown that HRAM data can image the internal
geometry of folded strata, given that they detect minute variations
in the magnetic response of near-surface deformed rock units.
For example, an HRAM TMI (total magnetic intensity)
profile nearby the Coleman gas field in the southern Canadian Fold
Belt reveals that lithostratigraphic units produce magnetic "highs"
and "lows" (figure 4). In this case,
the diagram illustrates that the Crowsnest Formation (volcanic sediments)
is associated with a magnetic high, suggesting that it contains
minerals with relatively higher magnetic susceptibilities than the
surrounding rocks of the Blackstone Formation and Blairmore Group.
Consequently, this formation forms a "magnetic marker
bed" that may reveal the size and shape of folds at a consistent
Since HRAM surveys are strongly influenced by near-surface
structures, they are particularly useful in areas where the reservoir
targets are located within the upper thrust sheets, where there
is significant correlation between surface and subsurface structures.
We have concluded that HRAM data also can detect
the presence of reactivated basement faults as well as "tear faults."
The recognition of these faults is crucial in exploration, since
they can either enhance the reservoir potential of rock units or
raise concern about the presence of structural compartments within
a targeted fold.
The quality of HRAM surveys in rugged areas, however,
may deteriorate due to strong topographic effects, which are very
difficult to remove with current processing techniques.
The best way to overcome this problem is to collect
data with helicopter-mounted systems. Although significantly more
expensive, these systems are flown while draping the landscape,
and as such, minimize the effect of topography.
The benefits of this technology will be illustrated
in a future article.