Walk-Above Vertical Seismic Profile
A walk-above vertical seismic profile (also called a walkabove VSP or walk-above VSP) is a seismic acquisition geometry in which the seismic source at the surface is progressively moved to maintain a position directly above the downhole receiver as the receiver steps through a deviated or horizontal wellbore, recording seismic traces that illuminate the formation directly surrounding the wellbore; as the wellbore deviates from vertical, the surface location directly above the downhole receiver changes with each receiver depth station, so the source position must "walk above" the receiver to preserve the near-vertical ray path geometry that provides focused illumination of the subsurface immediately around the wellbore rather than beneath a fixed surface point; the walk-above VSP is distinguished from standard zero-offset VSP (source fixed at the wellhead for a vertical well) and from conventional offset VSP (source fixed at a set distance from the wellhead while the receiver steps through the well) by the intentional source relocation to follow the deviated wellbore's surface projection; the technique provides a seismic image of the formation in the near-well volume that is used to calibrate surface seismic reflection character to downhole geology, to detect formation boundaries ahead of or beside the drill bit during extended-reach drilling and geosteering applications, to characterize the azimuthal anisotropy of fractured reservoirs by analyzing the shear wave splitting in the downgoing wavefield, and to image near-well structure that surface seismic cannot resolve adequately due to acquisition geometry limitations or shallow velocity model uncertainty.
Key Takeaways
- VSP geometry advantages over surface seismic for near-well imaging stem from the downhole receiver's position within the formation being imaged, which eliminates the weathering layer and near-surface velocity anomalies from the upgoing reflection path, improves the signal-to-noise ratio of reflections from deep targets, and provides a direct calibration of the seismic reflection character at a known geological depth: in a walk-above VSP, the receiver placed at a specific depth in the wellbore records both the direct downgoing wavefield (the wave that traveled directly from the source to the receiver through the formation) and the upgoing reflected wavefield (waves that traveled from the source, reflected from formation boundaries in the vicinity of the well, and returned to the receiver from below); the upgoing and downgoing wavefields are separated by the difference in apparent moveout across the receiver array (upgoing reflections have positive velocity moveout — they arrive later at shallower receivers — while downgoing direct arrivals arrive earlier at shallower receivers); the separated upgoing wavefield, when migrated to its correct subsurface position, provides a seismic image of the formation around the wellbore that is calibrated to the known geology at the receiver positions, allowing the interpreter to tie surface seismic reflections directly to formation boundaries identified in the well logs at the same depths.
- Look-ahead capability of walk-above VSP during drilling allows detection of formation boundaries ahead of the bit before the drill string reaches them, providing early warning of unexpected geological targets including overpressured zones, fault planes, fluid contacts, and reservoir boundaries that may require changes in the drilling program: in a pilot hole drilled in advance of the planned horizontal well, a walk-above VSP with the source walking above the deviated pilot hole receiver provides a seismic image that extends ahead of the receiver's current position by the distance illuminated by the reflected wavefield from deeper reflectors; the look-ahead distance depends on the geometry of the reflector illumination and typically extends 200-500 meters ahead of the deepest receiver station; the look-ahead image is particularly valuable in exploration wells where the pre-drill seismic model of the subsurface may be uncertain, and in appraisal wells where the exact position of reservoir boundaries and fluid contacts needs to be confirmed before committing to the full horizontal well design; the real-time acquisition and processing of walk-above VSP data during drilling (rather than waiting for post-drill acquisition on wireline) has been enabled by the development of downhole seismic-while-drilling (SWD) receivers that can be incorporated into the BHA and transmit data to surface via the mud pulse telemetry system.
- Shear wave analysis from walk-above VSP data provides information about formation anisotropy (directional variation in seismic wave velocity) that is particularly diagnostic of natural fracture orientation and intensity: natural fractures aligned with a preferred orientation create seismic anisotropy where shear waves polarized parallel to the fracture strike (the fast shear wave, S1) travel faster than shear waves polarized perpendicular to the fracture strike (the slow shear wave, S2), and the time delay between S1 and S2 arrival at the downhole receiver (shear wave splitting) is proportional to the fracture intensity and the thickness of the fractured zone traversed; the walk-above VSP geometry with multi-component (3C) geophones that measure ground motion in three orthogonal directions provides both the P-wave (compressional) and S-wave (shear) response, and the shear wave splitting analysis from the downgoing shear waveforms gives the fracture strike and the degree of fracture intensity in the formation around the well; this fracture characterization from the walk-above VSP is used to optimize the orientation of a subsequently drilled horizontal well (which should be drilled perpendicular to the fracture strike to intersect the maximum number of natural fractures during hydraulic fracturing) and to predict zones of high natural fracture intensity that may be productive or may cause wellbore stability problems during drilling.
- Source positioning accuracy in walk-above VSP acquisition is critical because the geometry of the illumination depends on placing the source directly above (or at a known offset from) the downhole receiver position: in an onshore walk-above VSP, the source vehicle (vibroseis truck or dynamite shot locations) must be moved to a new surface position above each downhole receiver station, requiring accurate knowledge of the subsurface receiver position (from the wellbore survey inclination and azimuth measurements) and the ability to access the corresponding surface location; in rugged terrain or in areas with surface access restrictions (farmland, industrial infrastructure, sensitive environmental areas), the required source positions may not be accessible, limiting the walk-above geometry to the accessible portion of the wellbore and potentially compromising the illumination of the near-well formation; in offshore walk-above VSP acquisition, the seismic vessel must navigate to the surface position above each downhole receiver station, which requires vessel positioning accuracy comparable to the desired receiver spacing (typically 5-25 meters) and may be limited by water depth, vessel handling characteristics, and weather conditions; the GPS-based dynamic positioning systems on modern seismic vessels can achieve positioning accuracy of 0.5-2 meters, adequate for most walk-above VSP geometries, but environmental conditions such as strong currents or rough seas may prevent maintaining the required source position during acquisition.
- Walk-above VSP integration with surface seismic provides a calibration dataset that improves the accuracy of the surface seismic depth conversion and geological interpretation in the vicinity of the well: the walk-above VSP provides both a checkshot velocity function (the one-way travel time from the surface to each receiver depth station, used to compute the average velocity at each depth and to calibrate the velocity model used for surface seismic time-to-depth conversion) and a synthetic seismogram (computed from the acoustic impedance log and the source wavelet extracted from the VSP direct arrival) that can be compared directly with the surface seismic traces at the well location; discrepancies between the synthetic seismogram and the actual surface seismic trace at the well indicate either errors in the well log-to-seismic tie (phase reversals, polarity conventions, log quality issues) or genuine differences between the seismic response and the well-log-predicted response (thin beds below seismic resolution, lateral heterogeneity between the well and the surface seismic measurement location); the VSP direct arrival provides a clean recording of the source wavelet at depth (free of the near-surface reverberations and ghost reflections that contaminate the surface-recorded wavelet), which is used for deterministic seismic deconvolution and broadband wavelet extraction that improves the resolution of both the VSP image and the surface seismic data processed with the VSP-derived wavelet.
Fast Facts
Vertical seismic profiling as a field technique was developed in the 1950s and 1960s from the concept of uphole shooting (recording seismic waves in a wellbore with a surface source) that had been used since the 1930s for velocity measurements and weathering corrections. The systematic use of VSP as an imaging and calibration tool rather than just a velocity measurement was formalized by Galperin in the Soviet Union in the 1970s and adopted by Western oil companies in the late 1970s and 1980s following the publication of Galperin's work in English translation. The walk-above VSP geometry specifically for deviated wells was developed in the 1980s as extended-reach and horizontal drilling became common, requiring new VSP acquisition geometries that could accommodate non-vertical wellbore trajectories while maintaining the near-well illumination that made VSP valuable for formation evaluation and seismic calibration.
What Is a Walk-Above Vertical Seismic Profile?
A walk-above VSP is a specialized seismic measurement designed for deviated or horizontal wells, in which the surface seismic source moves to stay directly above the downhole receiver as the receiver steps through the wellbore. The "walking" refers to this deliberate movement of the source across the surface, tracking the position of the wellbore below. In a standard vertical well VSP, the source stays near the wellhead because the well goes straight down. In a deviated well that kicks out at an angle and drills for several kilometers along a nearly horizontal path, the position directly above the receiver moves continuously across the surface as the receiver moves along the wellbore. Keeping the source above the receiver maintains the geometry that illuminates the formation immediately surrounding the wellbore — the zone of most interest for calibrating well logs to seismic reflections, for detecting formation boundaries that the drill bit will soon encounter, and for characterizing the fracture system that controls reservoir productivity. The result is a seismic image focused on the near-well environment, calibrated to known geology at the well, and free from the surface noise that degrades deep reflection data in conventional surface seismic.
Synonyms and Related Terminology
Walk-above VSP is also called walkabove VSP or deviated well VSP. Related terms include vertical seismic profile (a seismic acquisition and processing technique in which the receiver is placed at one or more depths in a wellbore while the source is at or near the surface, recording both downgoing direct arrivals and upgoing reflected wavefields to provide depth-calibrated seismic images and velocity measurements in the vicinity of the well), checkshot survey (a simplified VSP acquisition in which one-way travel times from the surface source to downhole receivers at selected depth stations are measured and used to calibrate the surface seismic time-depth relationship and to compute interval velocities between receiver stations), seismic while drilling (the acquisition of seismic data using downhole sensors incorporated into the drill string BHA while the well is being drilled, enabling real-time VSP-like measurements and look-ahead imaging without interrupting the drilling operation), look-ahead seismic (the use of VSP or other borehole seismic methods to image the formation ahead of the drill bit, detecting unexpected geological targets including overpressured zones, fault planes, and reservoir boundaries before the drill string reaches them), and shear wave splitting (the seismic phenomenon in which shear waves traveling through an anisotropic medium such as a naturally fractured formation split into two orthogonally polarized waves traveling at different velocities, analyzed from multi-component VSP data to determine fracture orientation and intensity).