The Piper Navajo aircraft, used for the GeoCentre airborne geophysical survey, preparing for takeoff at the Aubigny-sur-Nère airfield with its three sensors for magnetic field measurements (France, 2008). © BRGM - José Perrin

Airborne geophysical surveys: FAQ

02.01.2013
Airborne geophysical surveys produce 2D and 3D representations of the surface and subsurface. The BRGM conducts acquisition campaigns in the French regions. Below are answers to frequently asked questions on this subject.

What is the purpose of airborne geophysical surveys?

The airborne geophysical surveys conducted by BRGM and its partners are supporting infrastructure modernisation across France. Acquiring innovative date of this type contributes to more in-depth knowledge of the subsurface. This is one of the BRGM’s key roles as the French Geological Survey.

Depending on the methods used, airborne geophysics can produce information from the surface down to a depth of several kilometres.

Airborne geophysics produces additional data supplementing other available information (geological maps, drilling and seismic data, etc.) to support better descriptions and a better understanding of the subsurface, with applications in the fields of planning, water and materials resources, geothermal energy, underground storage and geological knowledge in general.

Have surveys of this kind already been made in France?

A great many airborne geophysical surveys have been conducted France since the late 1960s. Until the 1980s, these were mainly for mining purposes, but the aims are now also geo-environmental.

A simplified overview of the coverage of airborne surveys in mainland France may be viewed online on the BRGM’s dedicated mining GIS site. 

A more detailed description of surveys in France compiled by the BRGM is available through the BRGM's InfoTerre portal, by downloading the data layer on "geophysics/airborne magnetic campaigns" 

What do the measurements involve?

Many different methods are used in airborne geophysics, each designed to produce information on one or more surface or subsurface parameters, at different resolutions and penetrating to different depths.

The methods most frequently used by the BRGM for subsoil imaging are:

  • magnetism: this method records variations in the Earth's natural magnetic field (with no emissions from the aircraft). These variations are plotted onto maps. Sophisticated processing methods are then used to build up 3D models of the magnetic properties of rocks, from the surface down to depths of several kilometres.
    This is the classic method used to supplement other information used to model geology and geological structures down to very deep layers.
  • spectral radiometry or gamma spectrometry: this method records natural radioactivity in the first metre down from the surface of rocks and soils (with no emissions from the aircraft). All rocks naturally contain isotopes with low levels of radioactivity, which are not harmful to health. Highly sensitive onboard spectrometers are used to make measurements from the aircraft to detect gamma rays released from the soil. Gamma radiation varies depending on the rock mineralogy. It is measured and analyzed by sensors and high-tech software that extract soil concentrations of uranium (U), potassium (K) and thorium (Th).
    The results from this type of acquisition are plotted onto maps as U, K and Th. These data supplement geological and soil maps and are analyzed to refine surface and near-surface geological mapping in order to produce more precise picture of the geochemical status of the environment.
  • electromagnetism: this method produces images of variations in the electrical conductivity of the subsurface, from the surface down to a few hundred meters in depth depending on the material. This is an active method, meaning that electromagnetic waves are emitted, propagated into the air and then into the soil, and eventually partially reemitted from the soil, when it can be detected by measuring instruments.
    The signal thus measured will vary depending on the conductivity of the subsoil. Sophisticated analysis of the signals produces 3-D models of subsoil conductivity to a surface resolution of a few meters. This method is sensitive to the porosity of rocks and their fluid content (and to the salinity of these fluids), and to the presence of clay.

What are the advantages of airborne geophysical surveys?

The main advantages are that these surveys are able to cover very wide areas in a short time and thus avoid lengthy prospecting campaigns on the ground. The entire territory can be covered by type of survey, except for large urban areas and some restricted sites (nuclear power stations, strategic sites, etc.).

Flight permits must be systematically requested prior to any survey flight.

Have you requested the necessary permits?

Airborne geophysical surveys to acquire high-resolution data require flights at very low altitudes, from about 80 to 120 m above the ground.

All requests for low-altitude flights in France are subject to authorisation from the DGAC, the French airspace regulator, and from the Prefect of the départements concerned. Flights take place only after an official Order has been issued by both authorities. These Orders concern one aircraft and its pilot only, for a specified period.

The companies involved in these data acquisition campaigns ensure strict compliance with these Orders.

Specific requests have to be made for sensitive sites such as nuclear power plants, military zones, etc.

Why do survey aircraft fly so low?

The optimum altitude for successful data acquisition ranges from 80 m to 120 m. This is actually a compromise between the demands of the measuring techniques used, which require the lowest possible altitude, and safety and environmental considerations.

Is financial compensation provided for?

Airspace is free, and all necessary authorisations from State authorities for surveys of this kind are obtained before the flight takes place. In the unlikely event of an incident occurring as a direct result of a survey flight, an in-depth investigation is rapidly conducted. The operating companies are fully insured for this type of work and will address any legitimate claim they may receive.

CONTACT

BRGM

José Perrin
Direction des Géoressources

3, avenue Claude-Guillemin
45060 Orléans Cedex 2
France
Tel.: 33 (0)2 38 64 31 09
Fax: 33 (0)2 38 64 38 92

Reach the contact form

BRGM - 3 avenue Claude-Guillemin - BP 36009 45060 Orléans Cedex 2 - France Tel.: +33 (0)2 38 64 34 34