With over 500,000 lkm’s of gravity survey completed, the system delivers by combining industry-leading Gravimetric Technology gravimeters mounted using NRG patented technology in a Pilatus PC6 aircraft renowned for its ability to fly slowly.
FIXED WING GRAVITY AND MAGNETICS
With over 500,000 Ikms of gravity survey completed, the Xtract™ system delivers by combining industry-leading GT gravimeters with an ideal slow-flying aircraft platform.
At the heart of the patented Xtract™ gravity system is a GT2A total field gravimeter. Initially designed and built by Joint Stock Company Gravimetric Technologies in the Russian Federation. The system has been employed in both military and civilian applications and includes extensive use in airborne geophysics as an airborne gravimeter.
The standard GT2A does however suffer from short-term, temperature related drift. This is not ideal when mapping large long-wavelength sedimentary basins.
NRG™ has developed the Xtract™ system which incorporates a patented thermal control system for the unit. This reduces the random drift present in standard GT2A systems significantly. The residual drift in the patented Xtract™ system is greatly reduced and linear. This results in previously unobtainable crossover results, which translates to exceptional accuracy and repeatability of the final product.
Standard GT2A meter
NRG’s patented Xtract™ thematically controlled unit
The graph below shows the difference between the pre and post flight readings for seven flights with the patented Xtract ™ thermal control system and seven flights with a standard GT2A gravimeter. The drift in all instances was determined by taking readings at the same base position on the ground before and after production survey flights. The drift envelope of the Xtract™ system is about 0.2 mGals whilst the drift envelope of the standard GT2A is significantly larger by a factor of three, at 0.6 mGals.
The gravity sensor, mounted inside the Xtract ™ system, is a custom-designed accelerometer mounted inside a gyro-stabilised unit. Inputs from the gyros, inclinometers, angle sensors and dual frequency GPS are used to drive servo motors which maintain the sensor in a vertical position. The entire assembly is mounted on a rotation table, maintaining the sensor orientation at the same heading.
Gravimetric Technologies GT meter at the core
As per all airborne gravimeters, the raw data is filtered with time based and user selectable filters. As a result, the resolution of gravity data is directly proportional to survey speed. The Pilatus PC6 is renowned for its STOL capability and exceptionally low stall speed, resulting in the highest possible resolution of data.
In addition to the Pilatus’s slow cruising speed, the aircraft has an exceptional endurance with long-range tanks and is ideally suited for the large prospect areas typically covered in airborne gravity surveys. The Porter is fully capable of operating from different types of unprepared, rough and short airstrips, in remote areas, at high altitudes and in all climates. A simple but solid construction, proven systems and features, combined with precision workmanship result in the highest reliability and lowest field maintenance possible.
Key features of the system include:
- Xtract™ patented thermal control system
- Low and linear drift rates, resulting in improved accuracy and repeatability
- Highest possible resolution ensured by a low flight speed
- High precision 3-axis Schuler-tuned platforms with vertically constrained sensing element limits contamination from horizontal accelerations
- High productivity and less susceptibility to turbulence – minimised horizontal coupling and accurate vertical velocity mapping allows for production in a larger turbulence window
- Safe operations from remote flight bases
- Minimal in field maintenance required
- Consistent high quality, accurate and repeatable data
The Pilatus PC-6 aircraft is owned by NRG™ and is a dedicated full-time survey platform. It has therefore been subject to an intense magnetic clean up and degaussing process. This ensures the collection of very low noise magnetic data. The magnetic sensor configuration in the Xtract™ system is a Caesium cell, fixed boom installation.
This magnetic data is compensated using our specially adapted and modified 27-term compensation algorithm. The raw data is collected as a base frequency of 1000 Hz enabling intelligent removal of external interference such as radar, radio transmissions and culture.
The resultant 20 Hz final processed data is extremely clean, resulting in the highest possible standard of magnetic grids and enhanced products.
|Typical noise envelope
|0.001nT/m SDEV 4th diff
|Pilatus PC6 (“Porter”)
|Pentium 4 800Mhz
|-10 to 65 Degrees C
|Standard Sampling Rate
|NRG RDAC ll
|Internal System Noise
|15 000 – 105 000 nT
|40 000 nT/m
|-40 to +50 Degrees C
|0.0006nT √Hz RMS
|+- 10 V
|<6 pTrms/√Hz @1Hz
|Base Station Magnetometer
|15 000 to 105 000nT
|0.0006nT √Hz RMS
|GPS Base Station
|Novatel DL-V3 OEMV L1,L2
|L1 C/A, L2 P(Y), L1 Carrier Phase, L2 Carrier Phase
|No. of Satellites
|± 0.25 g, ± 0.5 g and ±1.0 g
|Operating Pressure Range
|600 – 1060 mb
|0 – 762 m
|Accuracy 0 – 10 m
|Accuracy 10 – 762 m
|Novatel DL-V3 OEMV L1,L2
|L1 C/A, L2 P(y)
|Number of Satellites
|10 Hz Triggered
|Processing Software Platforms
|Geosoft Oasis Montaj, Waypoint GrafNav,GTGRAV and GTNAV, Propriety Software