In recent years, vertical seismic profiling (VSP) has become an important tool for geophysicists involved in the exploration for oil and gas. VSP’s can provide precise time-depth relationships for seismic reflections. VSP’s frequently have better resolution (wider frequency bandwidth) than surface seismic. Unlike most surface seismic, the VSP corridor stack is a zero phase, multiple free seismic response at the well location. VSP’s recorded with the source close to the borehole may, however, be affected with tube wave noise. The good news is that tube waves can usually be prevented by certain practices in the field. If this fails, there are processing techniques which can cure VSP data afflicted by tube waves.
Tube waves are generated in several ways. The primary tube wave generator is ground roll from the source travelling across the wellhead, exciting tube wave motion in the well bore fluids. Tube waves can also be generated by body waves crossing discontinuities in the borehole, such as at casing size changes and washouts.
There are several procedures which can be followed in the field to prevent the acquisition of a tube wave. Choose a downhole tool which has been carefully engineered to minimize the effect of tube wave motion. Locate the source in a position such that ground roll will not reach the wellhead. A physical barrier such as a mud pit, a ditch, a hill or a lease road will reduce the amount of ground roll reaching the well-head. Fluid levels in the well bore can be lowered to a level where the fluid will not be affected by ground roll. The density of the well fluid can be increased to decrease the tube wave velocity. The last two options are not usually realistic, for reasons such as cost and safety.
A new generation of downhole tools has been designed to reduce the tube wave noise recorded in a zero offset VSP. These tools achieve acoustic isolation of a three-component geophone package from the tool body by decoupling the package from the main tool body. The large heavy bodies of monolithic tools need very large clamping forces to prevent movement caused by tube waves. Monolithic tools generally have resonant frequencies in the seismic band.
The Combinable Seismic Imager is a mechanically clamping open and cased hole tool. To decouple the geophones from the tool body, the sensor module containing the geophones is extended outside the tool body. The sensor module is light and clamped with proportionately large force, which reduces the sensitivity of the sensor module to tube wave motion. The sensor module has resonant frequencies outside the YSP band when it is extended outside the monolithic tool body.
The Array Seismic Imager is a multi-level cased hole tool. It consists of five shuttles, interconnected by a rubber bridle. Each shuttle contains three orthogonal geophones and a magnetic clamping system. The shuttles are light (9.5 Ibs.) with a small cross section and streamlined shape, and clamp with high force (135 Ibs.) to resist motion induced by tube waves. To decouple the geophones from the shuttle body, the geophones are suspended by an elastic material inside the frame of the shuttle. This acoustic isolation attenuates the small effect tube wave motion may have on the shuttle.
If tube waves do manage to infect an otherwise healthy YSP, there are several cures which can be tried in the processing centre. Methods such as F/K filtering and median filtering have been used to remove tube waves from YSP data. However, tube wave amplitudes may vary greatly from trace to trace and the slow apparent velocity of the tube wave often leads to spatial aliasing. These problems make F/K and median filtering ineffective and more noise may be introduced than is removed.
It has been observed on many VSP’s that tube waves, when measured by a three component geophone package which is well clamped to the borehole wall, have a particle motion in the radial direction or across the borehole axis. The horizontal component of a tube wave infected YSP contains most of the tube wave energy, an indication that the tube wave is nearly horizontal in its particle motion. Hodograms computed on windows which cover the tube wave show particle displacements which are rectilinear and almost horizontal in orientation. A filter can be determined which discriminates between events based on these particle motion characteristics. The polarization filter operates on any sample which ha an associated polarization angle of greater than 45 degrees from vertical. Rectilinear particle motion is also required for the filter to operate. The filter has been used successfully to reduce the amplitudes of the nearly horizontal polarized data associated with tube waves in zero-offset VSPs.
Tube waves in VSP data can be a serious problem. Careful location of the source in the field can prevent tube waves from being acquired. New generation tools have much reduced sensitivity to tube waves. Several remedies for tube waves are available in the processing centre. These remedies include the old standbys, median filtering and F/K filtering. Both of these remedies have been shown to have severe drawbacks. A new method, polarization filtering, has been used successfully to attenuate tube waves.
About the Author(s)
From 1979 to 1982, Allan Campbell worked at various jobs in the oilfields of northeastern B.C. He worked as a labourer on several pipelines including the Alaska Highway pipeline prebuild and on drilling rigs between pipeline jobs. In 1983, Allan attended the Northern Alberta Institute of Technology where he earned a diploma in Petroleum Technology. Dresser Atlas then hired him in 1984 as a cased hole engineer.
After the slowdown in the industry in 1986, Allan returned to university in Calgary and received a B.Sc. in Geophysics in 1990. His current employer is Schlumberger of Canada, where he works as a geophysical processor in a group specializing in VSP processing and associated work. Allan’s interests lie in all facets of borehole geophysics and three component seismic.