geop 501 chapter 2 borehole seismology -...

GEOP 501

Chapter 2

Borehole Seismology

Dr. Abdullatif Al-Shuhail (KFUPM)

Borehole seismology involves the recording of

seismic energy using one or more wells.

The main benefits of borehole seismology are higher

resolution and less near-surface effects.

Drawbacks of this method:

– Expensive because it requires drilling wells.

– Only near-well area (0.1 – 100 m) is sampled.

– Low spatial resolution because wells are usually drilled far

from each other.

Introduction

Uphole surveys

Check shot surveys

Vertical seismic profiling

Acoustic logging

Crosshole surveys

Seismic while drilling

Types of Borehole Seismic Surveys

It involves the recording of first arrivals along a

shallow well (50-100 m) that penetrates the

subweathering layer.

Objective is estimating the velocity and thickness of

the weathering layer for use in static corrections.

Two methods:

– Uphole survey: Sources are placed in the borehole at

known depths and a receiver is placed near the well head.

– Downhole survey: Receivers are placed in the borehole at

known depths and a source is placed near the well head.

Uphole Surveys

Weathering

layer

Subweathering

layer

R

S1

S2

S3

Z1 Z2 Z3

Z

T

Uphole Surveys Uphole survey

S

R1

R2

R3

Z1 Z2 Z3

Z

T

Uphole Surveys Downhole survey

Weathering

layer

Subweathering

layer

Interpretation of an uphole survey data includes the

following steps:

1. Picking the first arrivals from each depth level

2. Applying any necessary corrections to these times

3. Plotting the data on a T-Z plot

4. Identification of various layers

5. Fitting lines to the T-Z dataset of each layer

6. Computing the velocity and thickness of each layer

Uphole Surveys Interpretation

The following corrections are generally required:

– Conversion to absolute time

– Conversion to vertical time

Conversion to absolute time involves checking for

possible system delays as well as data

extrapolation.

Conversion to vertical time involves the

computation of vertical time from the measured

slant time.


Picked times represent rays that traveled along

slanted paths.

We need time along a vertical path.

The correction formula is:

– TV: vertical (corrected) time

– TS: slanted (measured) time

– Z: receiver depth, X: shot offset from well head

– This formula is correct for a flat surface and a

surface shot.


22 XZ

ZTT S

V

S

R

Z

X S


Z

T

Z

T

Z

T

Plot T-Z data and

identify layers by

grouping points lying

along a common slope

Fit lines to T-Z dataset

of each layer

Estimate thickness

and velocity of layers

Layer 1

Layer 2

Layer 1

Layer 2

T1 = Z / V1

T2 = T02 + Z / V2

Thickness

of layer 1

Uphole Surveys Examples

Raw downhole survey traces with first-break picks

Uphole Surveys Examples

Z (ft) T (ms) Z (ft) T (ms) Z (ft) T (ms)

15 3.8 165 24.3 315 34.8

30 7.3 180 25.4 330 35.7

45 13.1 195 26.3 345 37

60 15.3 210 27.9 360 38.2

75 17.3 225 28.8 375 38.9

90 18.5 240 29.8 390 39.5

105 20.1 255 30.9 405 41

120 21.1 270 31.8 420 42

135 22.1 285 33.1 435 42.9

150 23.4 300 33.5

The following is a T-Z table of an actual uphole

survey after corrections.

Uphole Surveys Example

The following is an interpretation of this uphole

survey:

V1 = 3,785 ft/s

V2 = 14,261 ft/s

H1 = 65 ft

y = 0.0703x + 12.65

R2 = 0.9985

y = 0.2642x

R2 = 0.9737

0

5

10

15

20

25

30

35

40

45

50

0 50 100 150 200 250 300 350 400 450 500

Z (ft)

T (

ms) Weathering-layer

thickness = 65 ft

It involves the recording of first arrivals along a well

that penetrates fairly deep target layers.


subsurface layers.

It is performed using receivers that are placed in the

borehole at known depths and a source that is placed

near the well head.

It is similar to a downhole survey but using a deeper

well and larger receiver spacing.

Check Shot Surveys

S

R1

R2

R3

Z1 Z2 Z3

Check Shot Surveys

Z

Interval Velocity

Interpretation of a check-shot survey data includes

the following steps:

1. Picking the first arrivals from each depth level

2. Applying any necessary corrections to these times

3. Calculating the interval velocity between each successive

receivers

4. Computing the RMS velocity profile

Check Shot Surveys Interpretation

Correction from slant to vertical times may be

neglected because depths are large compared to

shot offset.

The interval velocity between two successive

receivers (Ri, Ri+1) is calculated as:

DZ: receiver spacing

DTvi: difference in vertical time from datum

to receivers (Ri, Ri+1)


vi

iT

ZV

D

D

The RMS velocity to the bottom of the Nth layer is

calculated as:

Vi: interval velocity within the ith interval

DTvi: vertical time within the ith interval

This RMS profile is comparable to the RMS profile

found by velocity analysis of surface seismic data.


D

D

N

i

vi

N

i

vii

RMS

T

TV

VN

1

1

2


Calculate interval velocity Z

Interval Velocity

Z

RMS Velocity

Calculate RMS velocity

Check Shot Surveys Examples

Raw traces of a check-shot survey

Check Shot Surveys Examples

The following is a T-Z plot of an actual check-shot

survey after necessary corrections.

0

3000

6000

9000

12000

15000

0 250 500 750 1000 1250

T (ms)

Z (

ft)

Check Shot Surveys Example

The following is a plot of Vi-Z:

0

3000

6000

9000

12000

15000

5000 10000 15000 20000 25000 30000

Vi (ft/s)

Z (

ft)

Check Shot Surveys Example

The following is a plot of VRMS-Z:

0

3000

6000

9000

12000

15000

5000 7000 9000 11000 13000 15000

VRMS (ft/s)

Z (

ft)

Check Shot

Surveys Example

The following is a plot of

VRMS-TWTT:

0.00

0.25

0.50

0.75

1.00

1.25

1.50

1.75

2.00

2.25

50

00

70

00

90

00

11

00

0

13

00

0

15

00

0VRMS (ft/s)

TW

TT

(s)

It involves the recording and analysis of several

arrivals along a well that penetrates target layers.


subsurface layers.

It is performed using receivers that are placed in the

borehole at known depths and sources that are placed

on the ground surface.

Vertical Seismic Profile (VSP)

S

Layer 1

R

Z H

Vertical Seismic Profile Geometry

Layer 2

D

V

Downgoing

wave

Upgoing wave

T

T=T0 =2H/V

TZ/V

TT0-Z/V

Z

T=D/V

TH/V

Z=H

Vertical Seismic Profile The Direct Wave

The direct wave arrives at the receiver as a

downgoing wave with the following T-Z curve:

This is the equation of a hyperbola that approximates

a line (TZ/V) as D2/Z2 << 1, which is typical in

exploration VSP surveys.

V

ZDTd

22

Vertical Seismic Profile The Reflected Wave

The reflected wave arrives at the receiver as an

upgoing wave with the following T-Z curve:

This is the equation of a hyperbola that approximates

a line (T2H/V-Z/V) as D2/Z2 << 1, which is typical

in exploration VSP surveys.

The depth at which the T-Z curves of the direct and

reflected waves intersect is the layer thickness.

V

ZHDTr

22 )2(

Vertical Seismic Profile Processing

Processing steps specific to VSP include:

1. Correction for tool rotation and well deviation

2. Deconvolution of upgoing wave by downgoing wave

3. Separation of the downgoing and upgoing waves

4. Moveout correction of the primary upgoing event by:

1. Adding the downgoing wave time to the same depth if the source

offset is small compared to the depth

2. Using ray tracing if the source offset is comparable to the depth

The product will be a seismic section that is readily

comparable with stacked surface seismic sections.


Transformation to surface-seismic two-way time


Separation of upgoing and downgoing waves and

transformation of upgoing wave from VSP to

surface-seismic display


Deconvolution of upgoing wave after

transformation from VSP to surface-seismic display

Vertical Seismic Profile Advantages and Disadvantages

The main advantages of VSP are:

1. High resolution (usable frequency up to 250 Hz)

2. Better control on multiples

3. Better control on attenuation effects

4. Ability to study converted waves

5. Ability to study areas closely below and above an

interface

The main disadvantages of VSP are:

1. Need a fairly deep borehole

2. Samples only rocks near the borehole (10-100 m)

Vertical Seismic Profile Zero-offset VSP

Vertical Seismic Profile VSP types

Vertical Seismic Profile More VSP types

Vertical Seismic Profile Examples

A typical seismic section of a zero-offset VSP


A typical seismic section of a walkaway VSP


3-C VSP survey


VSP section

dominated by tube

wave

Tube wave is a wave

that travels along the

borehole axis with a

speed that is lower

than the P-wave in

rocks surrounding

the borehole.

Acoustic Well Logging

It involves recording the acoustic characteristics of

subsurface formations.

This is done by measuring the time required for a

sound wave to travel a specific distance through the

formation.

The travel time of the wave in a formation depends

on the following properties of the formation:

– Porosity

– Composition

– Fluid content

Acoustic Well Logging Operation

The main instrument used is called the sonde.

A basic sonde consists of a source and two

receivers one-foot apart.

The sonde is lowered down the borehole and waves

are generated and recorded continuously.

The sonde is usually positioned in the borehole

center using centralizing springs.

Frequencies used are in the range of 2 - 35 kHz.

Typical investigation radius is 0.2 – 1.2 m.

Acoustic Well Logging Sonde types

Acoustic Well Logging Transit time

Acoustic Well Logging Interpretation

The output log is a plot of transit time (Dt) versus tool depth.

The reciprocal of Dt gives the formation velocity (Vr) at that depth.

The total formation (sonic) porosity () is calculated as:

– Vr: formation velocity

– Vf: pore-fluid velocity (tabulated)

– Vs: rock-matrix velocity (tabulated)

sf

sr

VV

VV

/1/1

/1/1

Acoustic Well

Logging

Typical logs

Acoustic Well

Logging Example

Seismic While Drilling

The noise from the drill bit is used as a source, while geophones on the surface are recording.

Drill-bit waves are reflected off deeper interfaces and recorded at the surface.

These reflections are used to predict the rock structure ahead of the drill bit.

It is possibly the only seismic method that can be used for real-time decisions about the drilling operation.


Operation

The noise from the drill bit is used as a source of a continuous random signal.

A geophone at the top end of the drill string recording the drill-bit signal is used as a pilot.

Several surface geophones around the well head are recording the drill-bit signal.

The surface geophone signal is auto-correlated with the pilot signal in order to compress the drill-bit signal into a zero-phase wavelet.


Interpretation

After auto-correlation, the direct arrival from the drill-bit-rock interface will have zero delay time.

A reflection from a deeper rock boundary will have a delay equal to the two-way travel time between the drill bit and the boundary.

Knowing the formation velocity, the distance from the drill bit to the boundary can be estimated.

As the drill bit approaches the rock boundary, the delay time decreases and the reflection becomes a linear event.


Geometry


Example of pilot and geophone data


Geometry of reflections in the pilot section


Example of reflections in the pilot section


Comparing VSP and SWD sections

Crosshole Survey

It involves two nearby wells (< 1 km), one is used for sources and the other for receivers.

It gives a detailed model of the rocks between the wells, especially if tomography is used.

Advantages include:

– It bypasses the problems of the near surface because the source and receivers are below the weathering layer.

– It delivers high –frequency data (~ kHz) leading to very high spatial resolution of the derived model.

– Availability of shear and converted waves.

Crosshole Survey

Operation

The source is fired at the lowest depth and the receivers are allowed to record.

The process is repeated after lowering the source while holding the receivers depths constant.

The receivers depths may be changed in order to have better subsurface coverage.

Crosshole Survey

Processing and Interpretation

Data processing of reflected P-waves involves:

– Removal of direct waves

– Separation of downgoing and upgoing reflected P-wave

– CDP mapping using ray tracing

Interpretation of crosshole data involves:

– Construction of detailed S- and P-wave velocity structure between wells using all arrival types and/or tomography

– Construction of depth-offset seismic section from all shot records

Crosshole Survey

Velocity computation

Crosshole Survey

Application

Crosshole Survey

Example

Crosshole Survey Processing – raw receiver

gather (at depth 1392 m)

Crosshole Survey Processing – after direct-

waves removal

Crosshole Survey Processing – Separation to downgoing and upgoing

reflected P-wave sections

Upgoing Downgoing

Crosshole

Survey Processing – CDP

mapping

Crosshole Survey Interpretation – construction of depth-offset

seismic section

Crosshole Survey Interpretation – end result of crosshole

tomography

geop 501 chapter 2 borehole seismology -...

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