Airborne Gravity with AIRGrav
Sander Geophysics Limited (SGL) offers airborne gravity surveys using our
Airborne Inertially Referenced Gravimeter - AIRGrav. SGL´s AIRGrav
is the only purpose-built airborne gravimeter, and was designed specifically
for the unique characteristics of the airborne environment. This design
approach has resulted in a superior gravity instrument which can be flown in
an efficient survey aircraft during normal daytime conditions. In addition, AIRGrav
can easily be flown in combination with magnetic and/or radiometric
instruments to increase the survey benefits. AIRGrav was designed
primarily for petroleum exploration, where it is an economical alternative to
ground and shipborne surveys. AIRGrav also has exciting application
in regional geophysics, mineral exploration and geodesy.
The AIRGrav system includes an all new gravimeter on a three-axis
inertially stabilized platform, combined with high resolution Differential GPS
(DGPS) to correct for aircraft movements due to turbulence, aircraft
vibrations and drape flying. The gyro stabilized inertial platform makes the
gravimeter much less affected by horizontal accelerations than other systems,
which use modified sea gravimeters. This allows AIRGrav to achieve
consistently higher resolution. The AIRGrav system is currently
available to fly fixed-wing or helicopter surveys.
Terrain corrections are performed using either an existing digital terrain
model or terrain data acquired in the course of the gravimetric survey.
Digital terrain models can be supplemented with remote sensing data if
required, depending on the nature of the terrain and the resolution of the
survey. Terrain corrections for airborne gravity surveys are in some ways
easier than ground surveys because airborne gravity does not require near
station corrections, and is not susceptible to errors due to anomalous
densities near a gravity station.
Production surveys have been flown under a variety of conditions including
offshore, at a constant altitude over rolling terrain, and with a loose drape
over high mountains (>3,000 m). Because the system is relatively tolerant
of turbulence, it works well on drape surveys and in moderate to severe
turbulence.
Sander Geophysics recently completed a combined gravity and magnetic survey
in the Turner Valley area, south of Calgary, Canada. The map above shows the
first vertical derivative of the terrain corrected Bouguer gravity with the
shadow of the first vertical derivative of the total magnetic intensity. The
data set consists of over 12,000 lkm of data flown on 250 m spaced east-west
lines, and 1,000 m north-south lines. The extent of the data set is 60 km from
north to south. Noise was calculated to be 0.3 mGal. Gravity anomalies of
less than 2 mGal can be seen clearly on the data set and correlate well with
known oil and gas fields.
Results from numerous AIRGrav surveys have been rigorously analyzed
to determine system performance and optimum processing parameters. The graph
below shows rms gravity noise plotted against filter length (20 s = 1 km).
The graph shows that the noise level drops off quickly as the filter length
increases, which indicates that noise is concentrated in the higher
frequencies. The graph also clearly illustrates the relationship between
accuracy and resolution, and how one can be traded off against the other
depending on the client´s objectives.
The following graph demonstrates the effect of averaging a number of lines (a
process which is similar to stacking seismic data), which can be achieved by
flying closely spaced lines and applying a filter with a wavelength longer
than the line spacing. The graph shows rms gravity noise plotted against the
number of lines averaged, for various filter lengths. Using this procedure,
the final grid data will have a noise level significantly lower than the
individual profile data.
Shown below are typical AIRGrav survey parameters. As described
above, the accuracy and resolution of the gravity data depends on the aircraft
speed and the line spacing.
Accuracy RMS (mGal) |
Resolution ½ sine wave (m) |
Aircraft |
Velocity (kt) |
Line Spacing
(m) |
Production Rate (km2/day) |
0.2 |
4,000 |
fixed-wing |
120 |
1000 |
500 |
0.2 |
2,200 |
fixed-wing |
95 |
300 |
150 |
0.2 |
1,100 |
helicopter |
50 |
200 |
100 |
0.2 |
700 |
helicopter |
35 |
50 - 100 |
30 |
AIRGrav services include survey planning, data acquisition,
preliminary processing in the field, final data processing to terrain
corrected Bouguer gravity and a comprehensive technical report. SGL also
offers integrated interpretation of AIRGrav, aeromagnetic and other
data sets.
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