Kearey et al., 2002

Other seismic techniques (reflection
and/or refraction)
We have focused on 2D vertical sections of seismic structure
à sources and detectors in straight lines
Other ways to collect seismic reflection data …
Vertical Seismic Processing (VSP)
à detectors located at different depths in a borehole, or sources in
a borehole
à  source at wellhead (zero-offset VSP) or at some distance away
(offset VSP)
à  Provides a better constraint on
structures near Borehole, at depth
à  Quieter (less noise)
à  Direct arrival is more sensitive
to depth
(Kearey et al., 2002)
Raypath geometry
Main advantage of VSP: higher resolution
Main drawback: asymmetric raypaths, difficulties in
sorting/stacking and removal of multiples/reverberations
Tabakov and Baranov, 2008
3D Seismic Surveys
Problems with 2D (pseudo 3D)
- features between adjacent 2D lines may not be imaged
- some reflections generated by out-of-plane reflections
Full 3D surveying à arrange detectors so that seismic rays are
recorded from a 3D volume (not a line)
On land: crossed-array
method (dots are mid-points)
(Kearey et al., 2002)
Marine: multiple airgun-streamer surveys
Petroleum Geo-Services
Schematic illustration of a
Ramform vessel towing a large GeoStreamer spread with deep towing depth.
3D seismic ray
(Kearey et al., 2002)
Processing is similar to 2D studies
à note that migration is 3D
(overcomes limitations of 2D assumption)
2D/3D + VSP: Similar to conventional surface seismics,
use downhole/well receivers
Tabakov and Baranov, 2008
Looking at 3D seismic data (data cube)
(Kearey et al., 2002)
Vertical slice = 2D seismic section
Time slice / depth slice = horizontal plane through volume
Seismic data cube over a salt
dome in Gulf of Mexico
Red = positive polarity
Black = negative polarity
Time slice at 3.76 seconds
à shows that layers are
(Kearey et al., 2002)
Three-component (3C) Surveys
Most studies use vertical geophones
à P-waves with nearly vertical incidence angles
But the ground motion is three-dimensional
à P-waves and S-waves
Record using 3-component instruments
à 3 mutually orthogonal geophones
à 3 times as much data
In marine surveys:
Ocean bottom seismometers
Ocean bottom cables
Source of S-waves:
- using special sources
à horizontal or oblique impact (hammer strike)
à horizontal vibrations of a Vibroseis truck
- mode conversion of an oblique P-wave
What do S-waves tell us?
(1) Lithology – Vs to Vp ratio (or Poisson’s ratio σ) can be
indicative of rock type
- shales can overlap with other rock types (non-uniqueness)
(Sheriff and
Geldart, 1995)
Poisson’s ratio:
VP ⎡ 2(1 − σ)⎤
= ⎢
VS ⎣ (1 − 2σ)⎦
What do S-waves tell us?
(2) Nature of pore fluids – P-waves travel through rock matrix and
fluids, S-waves only travel through matrix
à P-waves are sensitive to pore fluid composition
(Sheriff and Geldart, 1995)
Gas is more compressible than oil or water
à P-wave velocities will decrease significantly if there is gas
Gas is more compressible than oil or water
à causes attenuation of P-waves
à can enhance seismic images by using S-waves
Gas escaping from a gas chimney in the North Sea
(Caldwell, 1999)
What do S-waves tell us?
(3) Identification of a reservoir – some reflectors may only be
detected with P-wave or S-wave data
North Sea – sand reservoir
overlain by shale
à no P-wave reflection
P-wave data from
S-wave data from
ocean bottom
(Caldwell, 1999)
Direct hydrocarbon indicators (DHI)
Examine seismic reflections to determine if there is a
hydrocarbon reservoir
Attribute analysis (amplitude)
à need true amplitudes
à care must taken in data collection and processing
à amplitude depends on distance travelled
(spreading, attenuation, scattering)
- deeper reflections have a lower amplitude
- can correct for this
Bright spots and dim spots
•  P-waves are sensitive to pore fluid (gas vs. oil vs. water)
•  gas results in a very low P-wave velocity
•  large impedance contrast at top and bottom of gas-filled layer
à large reflection amplitudes (negative / positive)
(From earlier example)
Seismic record:
Non-unique: bright spots also caused by igneous sills and partial melt
(Kearey et al., 2002)
•  reflection amplitude also depends on the surrounding material
•  if overlying rock has low velocity, there may not be a strong
impedance contrast at top of gas reservoir
•  low amplitude reflection = “dim spot”
(Sheriff and
Geldart, 1995)
Flat spots
Change in pore fluid composition = change in velocity = reflection/
In a reservoir, pore fluids may be layered
à may get a reflection from gas-oil or
gas-water interface
à horizontal
(Kearey et
al., 2002)
LBR FlatSpotsBrightSpots01.odg 11/2011
What happens for a shale overlying a porous sandstone?
(Vp / Vs ) 2 − 2
2((Vp / Vs ) 2 − 1)
(Sheriff and Geldart, 1995)
(figures from Section 4.8)
σ = 0.15-0.35 for shale
Water-filled sandstone à σ is similar to that of shale
Gas-filled sandstone à σ is less than that of shale,
gas slows down P waves dramatically, not so much on S