Research On Drill-Bit Signals Extracting Methods Of Seismic While Drilling

Seismic While Drilling (SWD) is a new borehole seismic technique,which uses the vibrations produced by a drilling bit as a downhole energy source.Its signals are recorded by pilot sensors attached to the top of the drill string and by receivers located at selected positions around the borehole,so SWD can also be called Vertical Seismic Profile While Drilling or Drill Bit VSP. SWD has not only the advantages of the Surface Seismic Survey and VSP,but also its own advantages of no interruption to drilling and no risk to the borehole,and especifically it can predict the subsurface conditions ahead of the drill bit without drawing the bit out of the hole,and can also provide the seismic images for real-time survey,so it is a very good way to reduce the drilling risks and to enhance the economic benefit in petroleum exploration and exploitation.In the present SWD survey,roller-cone bit is widely used because of its advantages in signals recording and processing.The axial force of roller-cone bit generates P wave and SV wave,while the horizontal force generates SH wave, whose amplitude is much smaller than that of P wave and SV wave by three orders for typical bit size and frequency.The signals we get at the top of the drill string and at the surface are continuous,so only after crosscorrelation can the useful information be available.But in fact the pilot signals recorded at the top of the drill string are inescapably influenced by the noises and the transmitting effects of the drill string which perform as frequency abnormality and high-velocity multiples.So how to attenuate the noises and eliminate the transmitting effects are the key problems in SWD signals processing.According to noise survey,the noises at well site have some characteristics which can lead to attenuating methods.The transmitting effects of drill string are also important factor which include the drill string's absorption to the drill-bit signals,the ghost reflection at the top of derrick and the multiple reflection in the drill string.Based on the characteristics of the transmitting effects,this paper develops the pilot deconvolution to eliminate the transmitting effects,which can get good results in both simulating data and real data.Meanwhile,in the examination and the application of the drill-bit signals,a more flexible correlating method which includes division,correlation and stack is proposed and verified by simulating data.