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.
Uphole Surveys Interpretation
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.
Uphole Surveys Interpretation
22 XZ
ZTT S
V
S
R
Z
X S
Uphole Surveys Interpretation
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.
Objective is estimating the velocity and thickness of
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)
Check Shot Surveys Interpretation
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.
Check Shot Surveys Interpretation
D
D
N
i
vi
N
i
vii
RMS
T
TV
VN
1
1
2
Check Shot Surveys Interpretation
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.
Objective is estimating the velocity and thickness of
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.
Vertical Seismic Profile Processing
Transformation to surface-seismic two-way time
Vertical Seismic Profile Processing
Separation of upgoing and downgoing waves and
transformation of upgoing wave from VSP to
surface-seismic display
Vertical Seismic Profile Processing
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
Vertical Seismic Profile Examples
A typical seismic section of a walkaway VSP
Vertical Seismic Profile Examples
3-C VSP survey
Vertical Seismic Profile Examples
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.
Seismic While Drilling
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.
Seismic While Drilling
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.
Seismic While Drilling
Geometry
Seismic While Drilling
Example of pilot and geophone data
Seismic While Drilling
Geometry of reflections in the pilot section
Seismic While Drilling
Example of reflections in the pilot section
Seismic While Drilling
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