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Vertical

SeismicProfiling

VSP

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Offset VSPThe standard VSP is zero-offset, but

for more detailed work the sourcescan be offset from the well.

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VSP can be used to image vertical

structures.

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Vertical Seismic Profiling

VSP uses this geometry:

One or more shots at the surface

Geophonesare in the well

holeRed rays are the direct rays

Black rays are the reflected rays

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Ocean VSP experiment. It may

have a single geophone which

is moved while the shot is

repeated at the same point.

Economically, this is very

inefficient.

Sometimes, multiple geophones

can be put down the well. Butthis is more difficult since all the

geophones must be

mechanically locked to the well

bore.

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Land VSP experiment. It may

have a single geophone which

is moved while the shot is

repeated at the same point.

Economically, this is very

inefficient.

Sometimes, multiple geophones

can be put down the well. Butthis is more difficult since all the

geophones must be

mechanically locked to the well

bore.

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Multilevel receivers

VSP

geophones

A single receiver

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Some sources from the Baker-Hughes website!

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VSP parameters

About 50 to 100 feet between geophones.

Can be adjusted for non-zero offset VSP.

Usually a mechanical source (not shot) so

that it can be repeated and the datastacked (summed).

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Requires a well, so its usually done to

further delineate a prospect. Often done with a well-logging survey by

the same contractors (Halliburton,

Schlumberger). Expensive.

Because the receivers are at depth, bettersignal and resolution is obtained.

It can see reflectors below the well-bottom.

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VSP applications

Velocity estimation. This is usually called acheck-shot survey.

Precise correlation of surface seismic data withdepth and the well-logs.

Separation of primary reflections from multiplesfor guidance in seismic section.

Calibrate reflectivity coefficients from well log

data. Provide seismic data processing parameters

velocity, multiple suppression, deconvolution...

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Complex structure imaging

Here, VSP is used to delineate the vertical edges of a salt dome.

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So what does this look lie when we plot it?

One or more shots at the surface

Geophonesare in the well

holeRed rays are the direct rays


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Lets assume that the source is very very close to thewell, so the actual geometry looks more like:

Red rays are the direct rays


This is called a zero-offset VSP

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Now we make a plot of the expected travel times, note we

can use the same plotting software:

Geophone Depth

time

Direct ray

reflectionsReflection curve meets the direct

ray curve when the geophone

depth is the same as the reflector

depth.

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Theres multiples too! Predictive deconvolution can be used to

suppress them just like for the regular reflection data

Geophone Depth

time

Direct ray

Reflections and

multiples

Direct ray multiples

Surface is here

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Reflections

Tube wave T

Refelcted tube wave T

Reflection multiples

Direct waveMultilples

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Downgoing multiplesDirect wave

Reflections

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We can find the layer velocity from the line slopes.

Here is the constant velocity case

Geophone Depth

time

Direct ray has slope: time/depth = 1/velocity

Reflected rays have slope: time/depth = -1/velocity

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Here is a variable velocity case! By measuring the slopes of

the individual segments we can get the interval velocities!

There is no way to directly get interval velocities with regular

reflection data!

Geophone Depth

time Direct ray has slope: time/depth = 1/velocity

Reflected rays have slope: time/depth = -1/velocity

W t t i d d i b d

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We can separate out upgoing waves and downgoing waves based

on their slopes. We are mainly interested in the upgoing reflections.

Geophone Depth

time

Direct ray is a downgoing ray.

Reflections areupgoing waves Direct ray multiples are

downgoing waves.

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VSP processing

The goal of VSP

processing is often tomake it look like part

of a standard

reflection survey.Then it can be spliced

into the seismic

section.

Offset-VSP is processed and spliced

into section.

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VSP Processing

Much of the same preliminary processing

as with reflection data is the same: Correlation if vibrator source.

Deconvolution for multiple suppression and

wavelet shaping.

Stacking over repeated sources.

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VSP Processing

The rest of the processing generally does

the following: Filter out downgoing waves so that only

reflected waves are left

This is dip-filtering, it removes waves with positive

slopes

Flatten the downgoing waves

Stack the traces by summing them

original Downgoing Flatten

fl ti

Corridor

t k

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waves removed reflections stack

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VSP corridor stack has been inserted into reflection section.

This gives increased resolution and a better tie-in to the well.

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Upgoing waves.Note curvature due to

non-zero offset

Downgoing wavesOriginal offset VSP

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After decon for multiple

suppression and

sharpening.

Corridor stack inserted

into reflection section.

Note improved

resolution. Curvature dueto non-zero offset and

deviated well.

Upgoing reflections

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This is an L-plot which compares the synthetic

seismogram produced from the well logs tothe VSP panel (from VSFusion website).

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