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over a distance of 330 m [ 1,100 ft] . Waveforms from each source were processed in a sequence that started with separating reflected P-waves from Stoneley and refracted P-waves. The azimuthal distribution of sensors at each receiver station allows identification of the direction to the reflector. Then, traces from each receiver station were depth-migrated using formation velocities measured by the Sonic Scanner logs from the earlier logging pass.2 7


To


account for tool rotation and the azimuthal distribution of sensors, the image at each receiver station was reconstructed by depth- shifting


and stacking images from each


azimuthal channel. Finally, the depth-migrated images were stacked. Images were obtained within 48 hours.


The results show a 5 -degree dipping event that extends at least 13 m [ 43 ft] into the formation (right). The dip of the event is in agreement with the expected geology at the well location. The high-resolution event can be correlated with a 1-m [ 3.3-ft] coal bed at the same depth position indicated by petrophysical logs (next page). The identification of a 1-m coal bed indicates the potential to obtain high- resolution images from a sonic-imaging survey. The resolution is far better than can be obtained from any surface or borehole seismic survey (below right).


Another potential application of sonic imaging is the detection of vertical fractures near but not intersecting vertical boreholes. Current techniques such as borehole image logging and fracture identification from Stoneley reflections work only if a fracture intersects the borehole. In many cases, a vertical well will miss vertical fractures. Deep imaging with the Sonic Scanner tool expands the volume of investigation to enable the identification of features that may delineate reservoir extent or the state of stress away from the borehole.


27 . Migration is a data-processing step that aim s to sharpen, shift and relocate re ectors to their true locations.


X ,3 0 0 X ,3 2 0


X ,2 4 0


X ,2 6 0


X ,2 8 0


X ,3 4 0


X ,3 6 0


X ,3 8 0 X ,4 6 0 X ,4 8 0 X ,5 0 0 X ,5 2 0 Source-receiver midpoint, ft


> A gently dipping re ector im aged far from the b orehole. The b orehole traj ectory is show n in red. The high-resolution event detected b y sonic im aging can b e seen ab ove and to the right of the b orehole, near the center of the im age. The re ector correlates w ith a coal b ed at the sam e depth position indicated b y petrophy sical logs.


X ,5 4 0 X ,5 6 0 X ,5 8 0


Sonic-imaging results on seismic scale


> Com paring high-resolution sonic-im aging data w ith a surface seism ic survey . The 1-m coal b ed resolved b y Sonic Scanner im aging ( inset) cannot b e seen in the surface seism ic survey .


30


Oilfield Review


V ertical depth, ft


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