Exploration Handbook | 33
Receiver Receiver
Source
*
x
CONVERSIONS AND PROCESSING OF SEISMIC
DATA
Receivers
x x
x
There are three main steps in processing seismic data (Ashcroft,
2011):
1. Velocity Analysis. An analysis of the seismic velocities,
used to integrate well and seismic data, can be achieved in
a number of ways. The simplest is from check shot data in
wells (Sutcliffe, 2019).
b)
*
0
c)
CMP
x
0
2600 TWT
(ms)
X (m)
0
Offset X
*
1
x km
1
800
2000 m/s
1000
Reflected
energy
1200
1
4000 m/s
Decrease
in energy
transmitted
Transmitted
energy
2
km
1400
1600
CMP
*
1800
x
Source
Reciever
d)
3 km approx
x
1
*
Shot point
2
3
4
5
6
7
8
48
x
2. Common mid-point gathering. Common mid-point (CMP)
gathering helps enhance signal to noise ratios (Ashcroft,
2011). A point on a reflector is used to gather seismic waves
collected from a range of shot points and receivers (figures
2C and D). Away from a shot point, there is hyperbolic
increase in two-way-time (TWT) that reflects the average
seismic velocity of the layer (Figure 2C). At a point,
modifying the waves by this factor flattens their arrival
times and allows the gathering of traces at one location
to enhance the true signal of the wave. CMP gathering
assumes flat reflectors (figures 2A and B). When a dipping
bed occurs, the point of reflection will not coincide with the
CMP (e.g. McQuillan et al., 1984) (figures 2A and 3A), so
reflectors in seismograms are misplaced (figure 3A and B).
“The collation and
interpretation of seismic
data is computationally
complex. The assumptions
used in processing can
result in uncertainties
in the final subsurface
model.”
3. Migration. The migration of seismic data corrects the
misplacement of reflectors produced by CMP gathering
by modeling all possible locations where a single source-
receiver pair could produce the observed result. Combining
all possible traces across the survey allows zones of
constructive interference to locate the reflector (Figure 3B).
DISPLAY OF SEISMIC DATA
Seismic data are displayed as seismograms made up of individual
“wiggle traces” (Figure 4). Each wiggle trace shows the variations
in seismic amplitude under each receiver. Positive amplitudes are
conventionally shaded black, but the size of amplitudes can also be
used to assign a range of shades or colors (Figure 4).
CDP 1 2 3 4 5 6 7 8
Distance along ship’s course
S 2
t2
S 4
S 5
1
S 7
t71
1
S 8
1 8
1 8
1
1 9
1 9
2 0
2 0
2
2 1
2 1
3
3
3
3
x
3
4
4
4
4
4
4
x
4
5
5
5
4
5
5
5
x
5
6
6
5
6
6
x
7
6
6
6
7
7
7
8
8
8
8
9
9
9
9
9
9
9
8
8
8
8
x
7
7
7
7
x
6
1 0
1 0
1 0
1 0
1 0
1 1
1 1
1 1
1 2
Hydrophone aections numbered 1 to 48
2 2
2 2
2
3
2
Shot Point
t47
2
1
S 6
t6
2
x
3
2 3
2 3
2 4
2 4
2 5
2 5
2 6
2 6
2 7
2 7
2 8
2 8
2 9
2 9
3 0
3 0
3 1
3 1
3 2
4 8
t5
2
1
2
t4
t48
1
S 3
t3
x
1
t1
t8
*
*
*
*
*
*
*
*
S 1
a)
Common Midpoint
Figure 2> A) Schematic diagram showing the pathways for seismic waves during acquisition; B) Model showing layers of differing seismic
velocities. Note the refraction and reflection of waves. ‘CMP’ stands for common mid-point; C) The collation of traces derived from the
modeled reflectors in (B). Note the hyperbolic increase in two-way-time (TWT) with offset distance; and D) Schematic of a multichannel
hydrophone streamer in an aquatic seismic survey. This shows wave paths that can be gathered at common mid-points from shots made at
different times. Images after Gluyas and Swarbrick (2004), Ashcroft (2011), and McQuillin et al., (1984).
Seismic data are initially displayed in TWT, but since seismic
velocity changes with depth, structural geometries are distorted.
More accurate representations of geological structures are realized
when seismic data are converted into depth. This conversion is
enabled by the construction of velocity models that describe lateral
and vertical changes in seismic velocity. The veracity of a velocity
model is tested by data from wells. Maps of depth-converted
horizons, or the thickness of rock between horizons, are then used
as inputs to burial models or in play fairway evaluation.
RESOLUTION OF SEISMIC DATA
Most seismic data have a resolution of tens of meters vertically, and
hundreds of meters laterally (Bertram and Milton, 1996) (Figure 5).
A term commonly encountered in seismic interpretation is “seismic
“A robust tie between well
and seismic data allows
accurate chronostratigraphic
and depositional models
to be populated into the
seismic data.”
32 | Halliburton Landmark