Internal combustion impulse source vs. mini-vibrator vs. nomad65 vibrator shot tests

Abstract

Over the last two decades 3D reflection seismic has been applied for mine planning in South Africa. For coal exploration, the mining targets are at shallower depths of less than 400m. The challenge set by the mining companies is to obtain high-resolution seismic data with maximum vertical resolution at this depth of investigation. The cost of geophysics must also be more attractive than that of the 'total drilling' alternative. Seismic source trials were therefore conducted at one mine site. The ICIS (Internal Combustion Impulse Source) has been developed by CGG to provide a solution for shallow target surveys and for in-fills where access for conventional sources is restricted. The source is self-contained, powered by readily available propane gas and oxygen, capable of 3,000 impacts per 16 oz. standard propane canister. A GPS-driven high-accuracy clock and a specially developed impact sensor provide accurate time-stamping and recording of time-break information to better than 1 μs. The shooting system integrates seamlessly with Sercel SN408 or SN428 recording systems, using an additional system process that allows the observer to switch rapidly from explosive or vibroseis mode to ICIS on a shot-by shot basis. The ICIS and Mini-vibrator performance were tested in shallow coal prospects, where existing 2D data have been acquired by Nomad 65 heavy vibrator. The tests demonstrated that ICIS is well suited to image target depths less than 500m with frequencies up to 100Hz. The comparison with the vibroseis sources showed that ICIS i) Delivered very good near offset data (> 50m) and for up-hole/low velocity layer survey ii) Required more shots per shot point in the medium offset range (1000m) due to its low energy iii) Not designed for far offset data (3000m). ICIS was found to be a viable geophysical and logistic solution for certain high-resolution mineral surveys.