High Resolution
Gamma-ray Spectrometer Surveys

Sander Geophysics has over forty years of experience conducting high resolution gamma-ray spectrometer surveys in Canada and internationally.  The company owns and operates 11 fixed-wing aircraft for airborne gamma-ray spectrometry, as well as two Airbus Helicopters AS350 B3s.  SGL and has successfully flown numerous helicopter-borne spectrometer surveys.   Surveys using a fixed-wing aircraft offer both speed and lower operating costs, however helicopter-borne surveys are capable of defining small anomalies more accurately and result in data of higher resolution and sensitivity.

SGL's fixed-wing aircraft includes eight Cessna 208B Grand Caravans, a de Havilland DHC-6 Twin Otter, and two Britten-Norman BN2B-21 Islanders each of which can accommodate 50.4 litres (3000 cu in) of downward facing crystals and 8.4 litres of upward facing crystals.  Helicopter-borne systems accommodate up to 33.6 litres facing down and 8.4 litres facing up.  All our aircraft are equipped with an integrated navigation system, SGNAV utilizing a NovAtel 24-channel dual frequency GPS (Global Positioning System) receiver for precise navigation and accurate flight path recovery.

Sample Configurations

  Detectors (litres) Crystals
Helicopter up to 33.6 downward 4 - 8
up to 8.4 upward 1 or 2
Fixed-Wing up to 50.4 downward 8 - 12
up to 8.4 of upward 1 or 2

High resolution aeromagnetic data can be gathered simultaneously with either system.  In the fixed-wing aircraft, the cesium magnetometer sensor is mounted in a stinger on the tail of the aircraft with the capability to include sensors in a horizontal and/or vertical gradiometer configuration.  The helicopter has either a stinger or a towed bird with cesium magnetometer sensors, singly or in a gradiometer configuration.

SGL has implemented a spectral component analysis noise reduction processing technique, based on the method of Hovgaard and Grasty (of Radiation Solutions and Gamma-Bob respectively).  The noise reduction technique, called Noise Adjusted Singular Value Decomposition (NASVD), uses the full spectrum data to enhance the resolution of radiometric data.  The reduction in statistical noise is equivalent to increasing the detector volume by a factor of between 3 and 4.  SGL also uses a combination of spectrum fitting and NASVD to produce maps of cesium, and other man-made nuclides, from the 256 channel radiometric data.

SGL uses Exploranium GR-820 and Radiation Solutions Inc. RS-500 spectrometers which are recognized as the most advanced airborne spectrometer systems available.   They include an on-board computer which allows real-time signal processing analysis previously available only in laboratory instruments.  The combination of automatic gain control for individual crystals, multi-channel analysis, and full spectrum recording removes many of the limitations inherent in the older airborne systems.

Unlike older systems which heated the detector packages to provide gain stabilization, the GR-820 uses spectrum analysis techniques to control the gain of the system.  The system constantly monitors the natural thorium or potassium peak of each crystal detector, and using a Gaussian least squares algorithm, adjusts the gain of each crystal individually.  In addition, each crystal resolution is calculated in real-time and the operator is informed if the crystal is out of specification.

The GR-820 and RS-500 is designed to provide high quality signal processing with a minimum of operator intervention.  This enables the spectrometer to be operated in a variety of conditions and minimizes operator error.

Gamma-ray Spectrometer Survey System

Crystal Detector Controller
Real-time spectrum analysis for each individual-crystal ensures optimum stabilization and resolution. This is achieved by the use of a sophisticated Gaussian curve fitting algorithm for centroid analysis of the natural thorium or potassium peak.
Each crystal has individual pole-zero cancellation, semi-Gaussian shaping and an advanced baseline restorer.
High energy cosmic pulse rejection.
Real-time graphical display for system and spectrum monitoring.
Accurate pile-up rejection for simultaneous pulses allows quantitative gamma-ray spectrum analysis almost independent of system count rate.  Special circuitry analyses for pulse pile-up and permits only detector signals from single events to be analysed.  Simultaneous events in adjacent crystals are summed to reduce the Compton effect.
Residual pulse pile-up at 100,000 cps - less than 2%
Analog to Digital Converter - ADC
Two Individual A/D Converters one for upward detectors and one for downward detectors
Number of Channels 256 for downward detector, 256 for upward detector
Type Wilkinson ramp, 50 MHz
Linearity integral:  less than 0.2%
differential:  less than 1%
Average System Dead-time less than 5 microseconds/pulse
Live Time Channel actual system live time output with digital data allowing post correction of system dead-time to an accuracy of 0.1%
Maximum Number of Counts/Channels 65,535 [16 bit]
Thresholds lower:  software selectable from channel 2-50 in 1 channel steps
upper:  3 MeV, A/D offset is software selectable
Cosmic Channel all pulses above 3 MeV are summed and recorded in the cosmic channel as a direct measure of cosmic ray activity
Maximum Input Count Rate 100,000 cps
Digital Data Recorded
System resolution and detector resolution are automatically computed for each crystal (and summed crystals) and are recorded to provide accurate in-flight and post-flight quality assurance.)
Window Mode Eight user selectable windows typically corresponding to the total counts, potassium, uranium and thorium.   The eight windows allow recording of these 4 parameters in both the upward detectors and downward detectors, plus one cosmic channel for pulses >3 MeV.
Full Spectrum Mode Full 256-channel spectrum with user selected end points.  Individual detector and the system resolution also recorded for both upward and downward crystals providing accurate post-flight data quality.