OCTOBER, 2010

HO CHI MINH CITY UNIVERSITY OF TECHNOLOGYRESEARCH CENTER FOR TECH. & INDUSTRIAL EQUIPMENT

Sonic Calibration and Synthetic SeismogramVIETSOVPETRO PETROLEUM

JOINT VENTURE

VERTICAL SEIMIC PROFILE PROCESSING AND INTERPRETATION training course

Borehole Seismic Survey

1 Borehole Seismic Introduction

2 Borehole Seismic Tool and Acquisition

3 VSP Processing

4 Sonic Calibration and Synthetic Seismogram

5 VSP Examples

Kieu Nguyen Binh

HCMC-2010

# 4

Sonic Calibration and

Synthetic Seismogram

What is it?

Sonic Calibration

Sonic times are corrected with borehole seismic times

Drift = (Vertical Seismic time - Integrated Sonic time)

Seismic & Sonic measurements:

1. Measure at different frequency range

2. Have different lateral investigation

3. Are direction dependent measurements

Sonic Calibration

Why do we need it?

Upscale borehole time measurements from cm to Km

Absolute reference

Creation of:

(1) A continuous calibrated time/depth relation at the well

(2) A calibrated synthetic seismic answer at the well

• Noise, stretch,cycle skipping, hole conditions such as rugosity, borehole enlargement

• Formation alteration

• Noise, negative stretch, cycle skipping, velocity inversion due to gas, High dips

• Dispersion

• Time picking precision

• Different raypath

• Different raypath (Anisotropy, high dip formations relative to borehole, lateral

formation changes)

Sonic times too long:

Sonic times too short:

Checkshot times too long:

Checkshot times too short:

What are the causes for drift?

Synthetic Seismogram (Geogram) Processing

• Editing of logs

• Drift computation

• Adjustment of drift (sonic

calibration)

• Time to depth conversion

• Reflection coefficients

• Attenuation coefficients

• Convolution

Interval Velocity computed from VSP

INTV = delta Depth / delta Time

Interval Velocity computed from Sonic Log

INTV (ft/sec) = 1,000,000 / DT (usec/ft)

DT INTV

INTV from VSP (black)

INTV from DT (red)

Positive and Negative Drift

Positive Drift: VSPTT > ITT

Positive drift is usually associated with the dispersion effects between the higher

frequency bandwidth of the sonic 10kHz as compared to the lower frequency seismic

bandwidth below 100Hz

Correction: block shift method was applied to the sonic

Negative Drift: ITT > VSPTT

Negative drift is usually associated with formation alteration or damage. As formation

alteration generally affects mechanically weak formations, and cause slower sonic

velocity

Correction: ∆T minimum method was applied to the sonic

VSP transit time

(black)

Integrated Sonic

Transit Time, ITT

(red)

Drift = VSPTT – ITT

(blue) is 5 msecs at TD

Sonic Calibration = adjust the sonic log, so that the drift curve is zero.

After adjustment, the ITT is the same as the VSP-TT

The ITT from the calibrated sonic log provides a high resolution depth-time curve

After calibration, all the log data is

converted to a time index, using

the time-depth relation from the

calibrated sonic log

Sonic Calibration

Original drift before calibration

(scale is -20 to +20 msec)

Drift after calibration

(scale is -10 to +10 msec)

Time index

data

Z=V*Rh

o

Convolution - from reflectivity to synthetic seismic

Zero Phase

Wavelet

Multiply each wavelet

by the size of the

reflection coefficient

Sum all the

wavelets

Synthetic

Seismogram

Reflectivity

Coefficient

VSP – Surface Seismic merge (this slide shown earlier)

Good match at 1300 msec. Not so good deeper down.

VSP is 8-75 hz. Using lower frequency VSP decon does not improve the match

VSP is the correct answer. This can be confirmed with a synthetic seismiogram

VSP & Synthetic

C-Stk Synth

Apparent dipping

reflection seen away

from the corridor

stack window

with attenuation

The corridor stack and Synthetic are

independent measurements. If they

agree, then this confirms they are both

correct.

VSP verses Checkshot survey

~ 20 m level spacing ~ 100 m level

spacing and key

depth points

If Sonic Calibration is the only objective of a borehole seismic survey, then the levels can be spaced further apart.

The sonic log supplies the “higher resolution” part of the interval velocity.

Drift – Formation Alteration

Wireline ISONICISONIC / Wireline differences in shales come

from formation damage (time-lapse sonic effects

characterize mechanical state of rock)

Dtc 3’ – 5’

(us/ft) 90

3900

De

pth

(ft

)140

Dtc 8’ – 10’

(us/ft) 90140

GR

(API) 1000

4000

Drift – Formation Alteration

1 w

ee

k la

ter

1 w

ee

k la

ter

De

pth

(ft

)6

40

00

.

................

0 20-20

Raw drift

Sonic 5’ – 7’

(ms)

.............

....

0 20-20

Raw drift

Sonic 7’ – 9’

(ms)

.....

.....

.......

0 20-20

Raw drift

Sonic 3’ – 5’

(ms)

Drift - Raypath

Sonic log

Rig Source VSP

Rig Source

Rig source

Boat Sources positions

Vertical Incidence

Vertical Incidence VSP

Anisotropy

Formation lateral variations

Ray bending

anhydrite

Tie Geological Markers

Surface Seismic Surface Seismic

m

m

m

R

R

SYN SYN A.I. Den.Vel. tim

e

de

pth

A successful synthetic match clearly establishes the relationship between a change visible on the log

data (at which a marker can be interpreted) and the response to that change on the seismic data - it

clearly establish what the events on the seismic data represent in geological terms

Are they really multiples?

Could it rather be related to a

lateral investigation issue?

SYNSurface Seismic Surface Seismic

Tie Geological Markers

VSP

Yes, it is confirmed by

the VSP which has larger

lateral investigation than

other openhole logs

m

m

m

R

R

Are they really

multiples?