Characterising borehole radio antenna performance using finite-difference time-domain modelling

摘要

Radio Tomography (RT) has proven itself as an imaging tool for base metal orebody delineation. To date, theoretical considerations of the imaging technique and inversion algorithms have concentrated on the propagation of energy from the transmit antenna to the receive antenna, while ignoring the antennas themselves. The Finite-Difference Time-Domain technique for modelling antennas has been extended to efficiently model antennas embedded in arbitrary media such as rock. The model is set up with body-of-rotation symmetry to produce models that have three dimensional accuracy, while only having two dimensional computational cost. Wire dipole antennas are efficiently modelled by the addition of a subcell extension for a think wire coated with a thin layer of insulation. The extended code is used, both to aid in the design of an improved antenna, and to investigate how the performance of the antenna affects the imaging of RT data in particular circumstances. A completely insulated antenna is preferred because its performance is more independent of the surrounding rock. The numerical model aids in the design of an improved antenna, with the optimum combination of performance features in a physically realizable antenna. If the electronics package is placed at the end of the dipole, the electronics package can be housed in a bare metal pressure casing without significantly affecting antenna performance as a function of rock type. The model also shows how the use of RT can be influenced by the geometry of the system and particularly by the use of conductors to suspend the RT antenna: wire cable support is not recommended until full waveform inversion techniques can take into account the presence of the wire. Antenna arrays appear to be viable, but if antennas without insulation are used, the spacing between the antennas should be at least as great as the length of each antenna.