A helicopter-borne ground penetrating radar system for geological and glaciological applications

摘要

Ground penetrating radar is a widely used technology in geology for the detection of near surface objects and for measuring ice thickness in glaciology. A wide spectrum of ground based systems are available for special tasks in geology. Airborne system operating from a fixed-wing aircraft are preferably used for long range profiling over large scaled glaciers and cannot perform local surveys with high lateral resolution. The lower speed of a helicopter and its high agility providing a free choose to increase the density of data points are of special interest to perform rapid surveys with high lateral resolution. A helicopter-borne radar system was developed in co-operation with the Technical University Hamburg for detailed surveying of limited areas. Two options of operations allow to measure ice thickness of up to 3,000m with a depth resolution of 5 m or to detect near surface objects deeper than 2 m with a depth resolution of 1 m. The maximum depth penetration in this case is 300 m (depending on the properties of the host material). The operating frequency is 150 MHz with a pulse repetition frequency of 30 kHz and 1.6 kW peak power. The pulse length of the signal can be set to 12 ns, 60 ns or 600 ns according to the depth of the target object and the desired vertical resolution. The receiver signal may be amplified either logarithmic or linear covering a dynamic range of 120 dB. For each of these signals two output channels are available for subsequent digitisation of the reflection signal. The data acquisition system is operating with a sampling frequency of 1 GHz. The registration rate of the optional stacked signals is 10 traces per second. Together with high precision DGPS navigation data each data point may be positioned to an accuracy of 1 to 3 m depending on the speed of the helicopter. The electronic part of the radar system and the data acquisition unit are installed inside the helicopter. A corner reflector antenna (size 5 * 4 m) with two dipoles is attached to the helicopter via a towing rope and a data cable. It is carried as a sling load 15 m beneath the helicopter. The non-permanent installed system (installation within 20 minutes) allows to use any type of helicopter without additional logistic support. This radar system was firstly successfully used during a German Antarctic expedition in 1995/96 to survey the thickness of the shelf and inland ice sheets covering the central Queen Maud Land/Antarctica. Recently improvements of the data acquisition algorithm focussed on increasing depth resolution to open future applications in hydrology, in mapping permafrost thickness and in various branches of applied geology were firstly tested during a field survey for mapping high mountain glaciers in Chile.