Introduction to Seismic Reflection Imaging
References:
J.M. Reynolds, An Introduction to Applied and Environmental Geophysics, 1997. John
Wiley & Sons, pp. 215-233 (NB missing from Killam Lib)
O. Yilmaz, Seismic Data Processing, 1987. Soc. Exploration Geophys., Tulsa, OK.
Basic data processing flowchart (after
Yilmaz, 1987)
FIELD GEOMETRY
FOR
MULTICHANNEL SEISMIC (MCS) PROFILES
Layouts for
(A) a comon-shot gather;
(B) a cmmon mid-point gather; and
(C) a common receiver gather.
Sequence of survey layouts to acquire
a 6-fold coverage; S indicates the
source and G a geophone (or
hydrophone).
Descriptions of a pre-stack
seismic data set in:
(A) shot-receiver coordinate
systems (a surface
diagram);
(B) Midpoint-offset
coordinate systems (a
sub-surface diagram or
stacking diagram)
A.
B.
Composite shallow reflection record made up of five separate 12-channel
records, each of which was recorded with a different time delay between
the shot instant and the start of recording.
Corresponding time-distance graph identifying the major seismic events
on the record. The optimum window is that range of source-receiver
separations that allows the target reflection to be observed without
interference from other events.
Example of a common midpoint gather
Types of filters: (A) bandpass; (B) low-cut (high-pass); (C) high-cut
(low-pass); (D) notch.
The application of a time-varying gain function to a waveform exhibiting spherical divergence, in
order to recover signal amplitudes at later travel times. Gain functions are applied in discrete
windows(labelled 1 to 5 as shown).
Filtering out a reverberant signal: At each relevant time sample (ie T = 1, 2, …) the signal amplitude S is
multiplied by the corresponding segment of the filter F (S * F). Taking this a stage at a time, we have:
Stage A: At T=0, the first element of the filter (1) is multiplied by the corresponding sample of the signal (1),
hence the output = 1
Stage B: At T=1, the first element of the filter (1) is multiplied by the corresponding sample of the signal (-1)
giving a value of -1. This is added to the product of the second element of the filter (1) and its corresponding
sample of the signal (1) to give a value of 1. The overall output is then, -1 + 1 = 0.
Stage C: As for Stage B but shifted by one time sample (T=2)
Stage D: As for Stage C but shifted by one time sample (T=3)
convolution
deconvolution
The principle of convolution (deonvolution is similar but in opposite direction):
Step 1: Convolve source wavelet and reflectivity series by multiplying the first sample of the source wavelet (1)
by the first component of the reflectivity series (1) to give the first constituent of the output response (= 1)
Step 2: Move the source wavelet array over one sample and multiply the samples for each component; hence
(1x0.5) + (1 x -0.5) = 0
Step 3: Move the source wavelet array over one sample and repeat as for Step 2; hence (0.5x1)+(0.5x-0.5)+(1x0.5)
= 0.5+(-0.25)+0.5 = 0.75
Step 4: (0.5x(-0.5)+(0.5x0.5) = 0
Step 5: (0.5x0.5) = 0.25
VELOCITY ANALYSIS
Depth section of individual seismic velocity layers (Vn) and thicknesses (Zn). For this example,
the total number of layers n = 6, so (n-1) = 5.
Acoustic Impedance
At each interface, a reflection occurs at
normal incidence with amplitude (Ar) that
depends on the change of acoustic
impedance (Z) at the interface between
layers I and i+1,where
Zi = vi*ri ; Zi+1 = vi+1*ri+1 and
Ar = Zi+1–Zi / Zi+1+Zi
Definition of velocity analysis terms and
acoustic impedance and reflection amplitude for
a flat layered Earth structure with n layers.
Selected raypaths and corresponding seismic traces
illustrating the effect of normal moveout (NMO). For
distances x << 2h, DT = x2/(2v2to) where t0 = 2h/v. Thus
in order to make this correction, we need to know v.
Given the source – receiver layout and correspnding
raypaths for a common midpoint gather, the resulting
seismic traces are illustrated uncorrected on the right
and corrected in the middle, after NMO aligns the
reflection events. The final stacked trace is shown on
the far left.
A constant velocity gather for seismic data at one shotpoint. The same seismic dta are shown in each
panel, the only difference being the RMS velocity applied to the data (labelled at the top of each panel).
Three events have been circled (A, B,and C). The two-way travel time (in seconds) and RMS velocity (in
ft/s) for the three events are:
Event A, 0.8s and 6400 ft/s; Event B, 1.7 s and 9600 ft/s; Event C, 2.3 s and 11000 ft/s.
Left panel: CMP gather with 4 prominent reflectors
identified by number.
Centre panel: Amplitude spectrum calculated by
summing the NMO corrected gather for a spectrum of
assumed velocities and travel times.
Right panel: The t2 – x2 velocity analysis applied to the
CMP gather for the major reflectors 1-4. The resulting
velocity values, derived from the slopes of the lines, are
plotted as triangles on the velocity spectrum (centre
panel).
Constant velocity profile along a seismic line. Velocity analyses have been carried out at
the shot points indicated and the velocities are labelled in units of ft/s.
POST-STACK MIGRATION
A stacked seismic section before migration (left) and the same section after time migration (right). Note the narrowing
of the salt dome and the clarification of the collapse structures on its top.
The principle of migration.
(A) Dipping reflector C-D on a stacked
section is migrated to its correct
geometry C’-D’
(B) The migration process moves an event
(E) by a lateral distance dx and vertically
by dt. The gradient of the event increases
from qt to <qt>.
Calculation of vertical and horizontal displacements
through migration depend on knowing the velocity of
the layer.
The principle of the diffraction stack or
Kirchhoff migration
(A) A diffractor lies at a depth h
vertically below a receiver
(B) This gives rise to a diffraction
event (hyperbola) on a seismic
reflection record because the
event is wrongly assumed to
come from a reflector vertically
beneath the receiver.
(C) A diffraction stack hyperbola for a
known velocity is matched against
the observed diffraction.
(D) Given a correct match along the
hyperbola, all the events are
summed and the observed
diffraction is shrunk back to a
point.
Diffraction hyperbolae calculated using
increasing velocities with depth. A slow
velocity results in a “tight” hyperbola
(uppermost curve) while that associated
with the fastest velocity has both the
broadest “window” and the flattest of the
curves.
(A) Migration panel showing a ‘smile’ caused by
a low velocity estimate in layer 1 (arrows)
(B) Migration panel showing a ‘frown’ created by
a high velocity estimate in layer 1,
(C) Migration panel showing a flat event in the
case of a correct velocity. The depth of the
reflected event is not dependent upon offset.
(A) Stack of a seismic reflection record in the Gulf of Mexico. Three events are indicated by
numbers 1-3.
(B) The same section after migration. The difference in position of event 3 (see arrows)
between the two sections is very marked, with the event being restored to the salt
overhang on the migrated section.