Virtual Source and all its offshoots are a breakthrough that will yield repeatable data for reservoir monitoring, enable proper shear seismic and allow us to “look ahead” as we drill, all under previously “no go” areas for surface seismic.

In settings where the overburden is complex or changing, traditional time-lapse signals are

The Virtual Source method focuses the surface energy to a point as a pulse with a desired waveform at time zero. (Image courtesy of Shell)
weak or non-repeatable, leading to ineffective seismic input to reservoir modeling and poor value realization from repeated seismic surveys. Virtual Source circumvents these problems by synthesizing controllable sources at the location of sensors placed underneath any complicated or changing overburden. This results in a buried seismic survey with fixed source and receiver locations that illuminate the underlying reservoir section.

These virtual sources can harness and focus all manner of scattered energy that illuminates them and as such are completely data-driven. We don’t need to know any details
of the overburden to create them.

In most situations one may also synthesize shear virtual sources, even when the real sources do not emit shear energy. Such is the case in marine environments, where shear (S) energy is generated from mode conversions in the overburden. Such shear virtual sources enable imaging with S-S reflections, in addition to conventional compressional (P-P) and P-S converted-wave imaging.

Downhole virtual sources need to be illuminated properly by real seismic sources on the surface. The simplest application is to illuminate along the borehole to estimate velocities in complex areas. These “virtual check shots” can be generated from traditional walkaway vertical seismic profile (VSP) acquisition and may be used where well logs are not available or practical and where traditional single-source check shots are inaccurate. These same sources can also look ahead with high resolution or be “steered” to characterize drilling hazards that are difficult to resolve from the surface.

Case study
A walkaway VSP was acquired while a well was being drilled through thick salt in a deepwater prospect in the Gulf of Mexico. The objective was to obtain a higher resolution image of the path ahead, which surface seismic suggested was plagued with intra-salt hazards and a complex base of salt interface with a potential for highly overpressured sands immediately below salt.

The VSP dataset consisted of 40 geophones that were transformed into 40 virtual sources, giving a total of 1,600 source-receiver pairs illuminating a narrow path ahead of the well. This high-fold dataset confirmed that a prominent seismic feature in the salt was indeed an intra-salt reflection and not a multiple from the surface seismic. Steering the virtual sources allowed the operator to estimate the dip and attitude of the intra-salt reflector, and a salt velocity estimate from the virtual check shot yielded a prediction to the depth of the reflector 2,000 ft (610 m) ahead of the well with 50 ft (15 m) of uncertainty; the prediction was communicated to the rig, which soon thereafter encountered the inclusion within 2 ft (.6 m) of prognosis.

A similar analysis yielded a prediction for the depth to base of salt 4,000 ft (1,220 m) ahead of the well with 70 ft (21.4 m) of uncertainty. This prediction was also validated by the well results. Further dissecting of the base salt reflection on the virtual source data revealed a 300-ft (92-m) non-reflective section immediately below salt, underneath which an amplitude was observed. The seismically quiet section suggested a salt exit into a benign drilling environment; the amplitude was suggestive of hydrocarbons. Both predictions turned out to be accurate.

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