The technical Approah thatr will be employed in this effort is the same that has proven effective for clients in the past to locate and evaluate tunnels and other under ground voids.

The approach consists of the employment of a multi-spectral Airborne Electromagnetic Reflectometer using a proprietary modification of the Global Positioning System (GPS) to provide sub-meter location capability. This technology is subject to North American and European Patents.

 Airborne Electromagnetic Reflectometry

 This technology was originally developed as a geophysical exploration technique to obtain continous high quality geotechnic data along proposed oil and gas pipeline routes where conventional test boring programs had proved inadequate, especially in difficult terrain found in the Arctic and Arabian Desert environments. The system is capable of acquiring greater than 600 kilometers of data per day. When data acquisition capabilities of this system are combined with proprietary advanced signal processing technology, it becomes possible to analyse and display subsurface features which can be detected by the interaction of electromagnetic waves with subsurface materials. This discussion is intended to be a brief explanation of the operating principles which allow the system to detect and delineate subsurface features, particular tunnels.

 The physical principles by which electromagnetic reflectometry operates are analogous to seismic reflection system.  In the case of seismic system, acoustic energy is radiated into the ground. Physical discontinuities that affect the velocity of the acoustic wave result in a partial reflection of the wave. If the velocity of the wave is known, the depth to the various interfaces may be caøculated. With Airborne Electromagnetic Reflectometry, single cycle pulses of electromagnetic energy are radiated into the ground from an aircraft mounted antenna. Energy is partially reflected to the antenna by changes in the electrical properties of the ground, primarily changes in the dielectric constant.

 Unlike seismic waves, it is relatively easy to vary frequences of the electromagnetic wave emitted. When electromagnetic energy interacts with matter, it does so in a frequency dependent manner. For example, a sunset appears red because blue light (shorter wavelength) is scattered more strongly than red light (longer wavelength) which penetrates the duty atmosphere to the observers position. This basic effect, but at radar frequencies, may be used to yield information about the subsurface structure or objects.

 Taking full advantage of the frequency dependent of elwecteomagnetic energy with matter, the system has the ability to measure the "signature", ie a set of electrical properties characteristic for a particular material. This allows Measurement and Signature Analysis to be applied in order to delineate the occurrence of a specific material. For example, once the signature of a material is mathematically determined, a single color may be assigned to the area of the subsurface map having that signature. The process may be carried one step further. The strength of the signature may be plotted as a range of colors, with the resulting map indicating both position and relative concentration of the material.

Read an Norwegian article from "Forsvarets Forum"