Beamform processing for sonic imaging using monopole and dipole sources

Abstract

New processing techniques are presented that enhance event signals for sonic imaging using monopole and dipole sources. The techniques use the azimuthally spaced receivers of a sonic logging tool. Sonic imaging, which is also known as borehole acoustic reflection surveys, uses a sonic logging tool in a fluid-filled borehole to image geologic structures. Signals from monopole and dipole sources are reflected from geologic interfaces and recorded by arrays of receivers of the same tool. Because the amplitudes of the event signals are very weak compared with the direct waves, borehole modes, and noise, the event signals are often difficult to extract. To enhance the weak event signals, beamforming techniques were developed to stack the waveforms from azimuthally spaced receivers of the tool for given azimuthal directions. For the incident P-waves from the monopole source, phase arrival times for the azimuthal receivers are time shifted for stacking using properties of wave propagation in the borehole. For the incident SH-waves from the dipole source, the signs of waveforms for the receivers are changed for specified azimuths. When the waveforms are stacked for the back azimuth of the event signals, the signal-to-noise ratio of the event signals is significantly improved because the event signals are enhanced whereas the direct waves are relatively smeared, and random noise is canceled. Therefore, the stacked waveforms also provide accurate back azimuths of the incident waves.