Motivation

Commonly, GPR investigations are carried out with single-channel devices in a constant-offset setup with very small offset, so that it can be regarded as a zero offset section. This is an appropriate approach in most cases, however, multi-offset measurements can provide additional information on e.g. the velocity distribution in the subsurface or the direction of incidence if measuring in full space as is the case for mining environment. Multi-offset measurements can be collected with single-channel devices by sequentially measuring the profile with different offsets, but this technique is very time expensive and suffers from positioning errors.  Recently, commercial multi-channel devices are available but they are optimised to measure wide areas in a constant-offset configuration with parallel oriented antenna pairs but they are not flexible concerning antenna setups and not designed for multi-offset measurements. This is the reason why we developed an own multi-channel radar system together with the company “Hentschel Systems”  that allows a high flexibility concerning antenna configurations and that is compatible to the transmitter technology that has been developed at the institute for several years.

Design

Design
A multi-offset GPR system for use in rough terrain and in mining environment needs to meet the following criteria:

• lightweight
• small pack size
• easy to set up
• easy to pull
• flexible antenna spacings

To ensure these criteria pipe dipole antennas with a centre frequency of 80 MHz were used that are put on a tarpaulin. The transmitting antenna is fixed at the nose of the tarpaulin and up to 16 receiving dipoles can be fixed at various offsets. They are connected via coaxial cables to the central unit whereas the transmitting antenna is connected via a fibre optic cable for triggering. At the end of the tarpaulin a wheel is connected for distance measurement. The whole setup can be pulled by hand or a vehicle while measurements are carried out continuously. und the maximum speed depends on the repeating rate of the transmitter, the number of receiving antennas, the number of samples per trace and the stacking rate and is typically slow walking speed.