REFLECTION AND POLARIZATION OF TUBE WAVES AS SEEN IN VSP DATA¹

Abstract

P‐wave and S‐wave data acquired with vertical seismic profiling (VSP) often include tube waves propagating in the borehole, although considerable efforts are generally made to ensure that these waves are not recorded. However, several theoretical studies have indicated that tube waves could provide important information about the rock formation and thus should not be considered as pure noise. In order to study some of these aspects experimentally, tube waves were acquired by VSP in a well in the Paris Basin both before and after casing. A sparker was used as source inside the borehole, which ensured that the data recorded contained high‐amplitude tube waves. It is shown that the casing is an obstacle which prevents the study of formation parameters, and thus further tube‐wave acquisitions should be carried out in open holes only. The before‐casing tube‐wave reflection log is compared to a synthetic log computed from the sonic log. The high resolution of the tube waves is of particular interest, revealing layers that are too thin to be detected in body‐wave surveys. It has recently been suggested that the projection of the tube‐wave polarization in the horizontal plane can be used to determine directions of stress‐induced anisotropy in the rock formation. Strong polarization anomalies are observed in the data sets but are attributed to tool problems rather than any rock‐formation feature.

Initial seismic observations from a deep borehole drilled into the Canadian Shield in northeast Alberta

“…The high-frequency tube waves have greater resolution than body waves, which make it possible to detect thin layers including fracture zones (Mjelde 1992). At 1,402 m depth, this set of tube waves does not correlate to any change in borehole diameter but is related to the presence of the fracture zones.…”