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VERTICAL SEISMIC PROFILING


Vertical seismic profiling commonly abbreviated as vsp is a major technique in geophysical exploration which is used for measuring down going and up going wave field through a strati-graphic sequence shortly after the first attempt of developing surface reflection seismology where both source and receivers were at the surface, the line of research set out with the idea of buying either sources or receivers in boreholes.The geometry forms the basic principles of determining seismic wave

propagation velocities from vsp measurement even today.The technology of vsp acquisition and processing was initially developed by research in 1950's and 1970's.Today the fruit of these effort have served not only in the improvement of velocity analysis but have materialized numerous other application ranging from structural imaging of complex 3D structures such as faults and salt domes in exploration studies.

A variety of data acquisition techniques are currently employed, depending on the survey application:Zero-offset vsp,offset vsp,multi-offset vsp,walk-away vsp and seismic while drilling technique.

BASIC PRINCIPLES OF VSP

Vertical Seismic profiling is a form of seismic reflection technique that utilizes boreholes.Shots are usually fired at the surface,at wellhead or offset laterally from it, and recorded at different depth within the borehole wall.In the conventional seismic method, the source and the receivers are generally in line placed horizontally along the earth surface.While in the case of vsp the detector are placed in the well bore vertically.The seismic sound generator is located near the well bore while the geophones are deployed down the well.

ZERO-OFFSET VSP

This has to do with the distance between the source and the geophone in the well bore. Zero-offset vsp means that the source is placed directly above the geophones in the wellbore such kind of vsp can help identify a reservoir target has been drilled in deviated well bore and complex structures.The higher frequency of zero-offset vsp produces greater vertical resolution than surface seismic method for a much higher resolution. providing precise velocity information as well as tru seismic wavelet well tie with phase frequency and time/depth,zero-offset vsp can help identify a reservoir target "head of bit" or confirm that the target drilled. Even in deviated well bore and complex structures,the higher frequency of zero-offset vsp produces greater vertical resolution than surface seismic for a much higher resolution subsurface image of the near-well corridor stack.

BENEFIT OF ZERO-OFFSET VSP

Provides greater vertical and lateral spatial resolution
Improves velocity analysis for surface seismic processing with accurate velocity model
provides high resolution imaging and attributes beneath and away from the well for well reservoir imaging.

Provides rock properties/pore pressure indication and estimation of AVO (amplitude variation with offset) calibration.

OFFSET VSP

Offset vsp is a type of vertical seismic profile in which the source is located at an offset from the drilling rig during acquisition.This allows imaging to some distance away from the well bore. By moving the source distance away from the receiver.The offset vsp can be used to image the subsurface away from the well.The acquisition of the offset vsp should always include some types of pre-survey modelling to determine the proper source location needed to achieve the desired objective.

BENEFIT OF OFFSET VSP

Provides good lateral and spatial resolution than surface seismic.
It improves velocity analysis for surface seismic processing.
It is used for good structural interpretation like faults and anticline.

WALK AWAY VSP

This type of vsp in which the source is moved to progressively farther offset at the surface and receivers are held in a fixed location,effectively providing a mini-2D seismic data with more continue coverage.

SEISMIC WHILE DRILLING

This type of drilling uses drill bit as the source and  geophones placed at the surface.The bit tooth of the roller cone bit drills the rock strata which propagates along direct path to sensors deployed on the earth surface or on the sea floor if the well is in offshore location that allows specific target geology to be imaged.

An imaging capability is created by the drill bit wave field that propagates downward and upward reflection of waves below the drill bit as depicted by the ray path.A sequence of seismic traces can be created as the drill bit transverses equally spaces.Depth interval equal to a length of a drill pipe.
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