Vertical Seismic Profiling (VSP) Part 1

Defenition to Near – long Offset VSP

• Most of this material is taken from Borehole Geophysics : Theory and Practice by DR. Ronald C.Hinds and Rick D. Kuzmiski

Introductory comments

• Borehole geophysics (like VSP) controls seismic (in depth) contribution in exploration

VSP Defenition

• Hardage : A vertical seismic profiling is a measurement procedure in which seismic signal generated at the surface of the earth is recorded by geophone secured at various depths to the wall of a drilled well

VSP Fundamentals and Acquisition

• VSP Survey Geometry

- defines the location of the surface source and the borehole sonde

- the placement of the source and receiver with respect to the target helps one to evaluate the seimic imaging of the target zone

VSP Fundamentals and Acquisition

• Surface seismic records upgoing waves, airblasts, ground roll, and refraction while VSP records upgoing, downgoing, and borehole waves.

• Stoneley wave and Tube wave are the major noise in VSP (high energy, propagate along solid – fluid interface such as along the wall of a fluid – filled borehole, low frequency)

VSP Fundamentals and Acquisition

• One of the ways to combat the stoneley wave and tube wave in VSP is increasing the offset distance

• Consider a conceptual model of how a wave travel through the media from the source to the geophone tools

VSP Configurations

• Near / Zero OffsetSource near wellbore of vertically drilled well

• Lateral / Far OffsetSource are far enough away (how far?) from vertical well

• Deviated BoreholeNear offset : source at wellheadFar offset : source over receiver

• Walkawayseveral offset placed from wellhead to offset ~ ½ depth

Near offset and Far Offset

Near offset and Far Offset in a Deviated Well

Near Offset VSP

• Source position at S1, rays travel straight down and up

• Uses : - linking seismic to geology - recognition of multiple events - estimation of reflector dip - determinging lithologic effect on wave propagation

(attenuation) - predicting lithology ahead of bit - sonic log correction

Surface seismic only receive upgoing wave energy

Surface seismic raypath are intersected by wellbore

UPGOING

DOWNGOING & MULTIPLES• Downgoing (DG) travel time increases with increasing receiver depths• First DG event is the first break event• First Break event is P – wave Primary event• DG multiples arrive in after the “first break” primary DG. The delay is

caused by the bounce.• Surface generated multiples appear on the entire VSP data set.

UPGOING & MULTIPLES

• Primary UG waves travel directly to reflector below sonde & bounce back up to sonde.

• Multiple UG take additional bounces beneath or above sonde depth.