Tube wave suppression: A comparison of median filtering and nearest‐neighbor subtraction approaches

Abstract

Tube waves are guided waves which travel in a borehole after a seismic disturbance. Tube wave noise appears as strong amplitude linear events which can override desired signal. Median filters tend to reject spikes and pass uniform data. The median filter technique can be very effective at removing tube wave noise, but it can also attenuate or distort desired signal such as direct arrivals or reflections. These signals generally appear as dipping events after tube waves are horizontally aligned with static corrections. The median filter, applied along the horizontal (depth) axis, theoretically rejects the dipping events as spikes and passes the horizontal tube waves. Subtraction of the median filter’s output from the original data then leaves only the dipping events. In practice, borehole data are not spikes, which causes the median filter to only partially reject dipping events. Subsequent subtraction then leaves these events attenuated or distorted. An important observation is that the median filter technique should work better for spatially undersampled data, since the median filter sees the wavelets of dipping events as spikes only if neighboring wavelets do not overlap in time The purpose of this work was to replace the median in the described subtraction process with another filter which yields better rejection of dipping events. The replacement filter measures the coherence of neighboring signals using a normalized, time-windowed, zero-lag crosscorrelation. If the coherence is at or above a certain threshold. subtraction is performed. If the coherence is below the threshold, no subtraction is performed. The filters were tested on borehole data collected in Loudon, Illinois, a strong tube wave site. Results yielded near 9-to-1 or 19 dB tube wave attenuation using the median filter and near g-to-1 or 18 dB attenuation using the nearest-neighbor crosscorrelation filter. The median filter caused significant distortion of the direct arrival wavelets while the correlation filter caused little or no distortion.