One of the main advantages of our modern-day approach to gamma-ray surveys and data processing is the much simpler sensor calibration and survey preparation. Below we will describe the steps that need to be taken in order to perform proper airborne survey work with one of our Medusa detector systems - more specifically the MS-4000 system.
Background - Medusa vs classical approach
In the "classic" approach to airborne gamma-ray survey work (described in several publications, a number of steps must be taken before take-off and also post-survey. Below a table that compares these steps between the "classic" approach and the "Medusa" approach.
Comparison of pre-survey activities for different systems:
Sensor packs are "calibrated" using calibration pads. These pads are (often) concrete structures of about 2m diameter and 30-60cm thick. Normally 4 pads are used; 3 with added 40K, 232U and 238Th sources and one background pad.
The results of the calibration are the stripping factors that are used in the "windows" analysis.
Before shipment, each Medusa sensor is calibrated in the Medusa Stonehenge calibration facility. This calibration is absolute, and based on the use of nuclear particle modelling of the detector response.
The result of the calibration is a set of "standard spectra". These are response curves of the system to a source of unit strength (1Bq/kg or 1 %/ppm) of 40K, 238U, 232Th and - if requested - other nuclides.
The calibration data is uploaded to the detector and is used during the survey to extract nuclide concentrations on the fly. The calibration data is also provided as a calibration file that can be used in the Medusa post-processing software GAMMAN
Pad calibrations are intrinsically difficult to interpret and use. The absolute radionuclide concentration in the pads is not well-known, their geometry is very limited, and the pads are not pure sources of K, U or Th but will always contain a mix of all three nuclides.
The "modelling" approach Medusa takes delivers response spectra of a system to pure sources of 40K, 232Th and 238U. And if needed other sources like 137Cs can be modelled easily.
For more info on Medusa calibration see Calibration procedure
|Before (and sometimes after) each flight the linearity of a system is checked by using a source (often Th-welding rods). To do this, a series of spectra is recorded and checked on the position of the known Th peaks.
|The Medusa system is self-stabilising. During the measurement, the spectra are energy-stabilized using a matching algorithm that shifts the spectrum to the proper peak positions. This can be done during take-off or during the survey itself.
|This is an action often asked for by clients as "proof" of proper system operation. The embedded logging software on the medusa sensors allows storing such a measurement as a "sample" that can be used as proof of proper system functioning.
A series of flights over a calibration range with well-known radionuclide concentrations is needed, both to establish absolute ground concentrations and to find the elevation-induced attenuation of the sensor.
|The elevation corrections are part of the calibration procedure. The Medusa post-processing software comes with a database of response spectra for each elevation between 40 and 160m. There is no need for flights over calibration ranges
|The main drawbacks of the calibration flights over a test range are the daily variations due to radon and soil moisture and the rapid increase in footprint that occurs when flying higher. The detector sees much more than the "strip" that is often used for the calibration flight.
|Once every while, typically once per survey, a cosmic calibration flight has to be done. In the "classic" scenario, a flight is done at several elevations, typically from 500m to 2000m. At certain steps (every 250m) one stays for a minute or two, collecting enough data. The spectra acquired contain contributions from the cosmic radiation and from the plane background (plane material, crew). During post-processing, the data retrieved at several elevations is used to construct a plane background spectrum and a cosmic spectrum.
In principle the Medusa method is very similar to the classical method described.
Medusa have, however, developed a new approach in which the tedious hovering at high altitudes is not needed. One can use data for instance taken while flying back to the base. One has to fly at a high elevation though, at least 500 above ground.
|The new algorithm will be part of the latest GAMMAN software versions (1.49 and up). See also the description in 1. Cosmic background spectra in Gamman
Some traditional packs of detectors contain an upward looking detector. This detector is shielded from below by the other crystals and can be used to monitor the radon in the air surrounding the survey plane.
The procedure to use the upward data to estimate radon is not trivial and prone to errors. Therefore, other algorithms have been developed that compare one of the low-energy peaks of 238U (mostly the 609keV peak) to the high-energy window. For radon, the ratio between these peaks is different from the ratio one would expect for 238U in the soil under the plane. This way an estimate can be made of the radon contribution
The Medusa approach to estimate presence of radon is similar to the peak ratio method mentioned.
The difference is that the Medusa method compares a complete radon spectrum to a 238U spectrum.
The calibration data of a Medusa detector contain a modelled radon spectrum.
Contributions of radon (and cosmics) are usually studied during post-processing of airborne survey data.
The Medusa software package GAMMAN allows doing just that. The software allows performing full spectrum analysis (FSA) of the measured spectra. The FSA method uses virtually all spectral information instead of the data from 3 (or 4) peaks as done in the Windows method.
Check for instance the Gamman Getting Started
Step by step
Below, we will focus on several aspects of doing airborne work using the MS-4000 or other Medusa sensors.
Detector use and checks
It is virtually impossible to describe a "generic" recipe on how to do an (airborne) gamma-ray survey, The choice of flying speed, height, distance between survey lines, tie-lines etc, depends heavily on the area you aim to survey and on the phenomena you aim to map. A regional survey mapping larger scale geology requires an approach different from a survey looking for Uranium hot-spots for instance. Despite the impossibility of determining a generic recipe, there are a few things to keep in mind while deciding on your survey program.
Rule of thumb is to fly as low and slow as possible within the survey boundaries.