Reliable and high resolution information about the current state of radioactive waste repositories and their casing can only be gained on site with borehole tools. Ultrasonic sensors are such a tool, making it possible to characterise the surrounding ground (salt, clay, granite) by measuring the elapsed times of high frequency seismic signals. The arising data must be transmitted to an above ground or to a subterranean observation point. There, the data (mostly still analogue signals) are collected, plotted and quantitative evaluated. The quality of the transmitted analogue signal is, however, reduced through, amongst others, the excitation signal (which is usually transmitted through analogue means). This problem becomes more severe as the distance of transfer increases. We are currently developing a digital data transfer system, which will span a distance of up to 2 km. This system allows 7 analogue signals, with a bandwidth of 250 kHz each, to be transmitted within a single optical fibre at the same time. The same single fibre is used in the opposite direction to remote control gain settings and others. Further areas of use lie where rapidly changing analogue signals must be transmitted in real time over large distances.


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