CAVEM

The CAVEM project aims at studying the feasibility of a geophysical prospecting device (at MF and HF bands, 300 kHz-30 MHz) for identifying and characterizing near surface cavities (for civil or military applications). The project purpose is to create new metrological and interpretative solutions for high-resolution electromagnetic prospection. We will consider a wide range of natural and man-made cavities: empty cavity, water-filled karstic channels, empty or metal-equipped tunnels. From a methodological point of view the project aims at demonstrating, through the use of numerical modeling and a proof-of-concept device, that using a wider frequency range including intermediate frequencies (MF-HF bands) would allow better cavity detection and characterization, whatever the cavity type though with a special focus on tunnel types. We will compare the two most-used configurations in geophysical EM measurements: a) fixed transmitter with a mobile receiver and b) mobile transmitter-receiver couple. From the metrological point of view the main objective is to study the feasibility of a wireless connection between the transmitter (Tx) and the receiver (Rx) in order to facilitate in situ data acquisition. This would allow a better field work productivity as well as the possibility of future developments such as data acquisition with unmanned vehicles. Current state-of-the-art low frequency Slingram devices use link cable for synchronization and data-transfer between Tx and Rx. We will study the possibility the replace such a link with a wireless connection at least for data-transfer purpose. Considering the MF-HF band studied in this project, the synchronization between Tx and Rx should achieve sub-nanosecond accuracy. We will also study a way of precise relative positioning of Rx with respect to Tx and will try to achieve a relative positioning with centimeter-level accuracy. Finally we will try to use pitching and rolling angle of both transmitter and receiver loops to apply corrections to the measured EM field. In the third part of this project we will try to apply to cavity and tunnel detection various interpretation strategies used in other EM prospecting applications : a) inversion methods using equivalent sources (dipole or filaments), as used for polymetallic ore deposits characterization in mineral exploration with fixed Tx; b) library-based methods used for identification and classification of UXO with mobile Tx.