Here you can see a seismic profile of about 13 km in length.
The seismic recordings are shown as stacked zero-offset profiles.
Red and blue components represent both positive and negative
deflections from over 1000 seismograms.
These types of extensive measurements are used for raw material
prospecting.
But what can you see in this profile?
Can we already see deep structures that allow us to conclude the
existence of raw material deposits?
Hello and welcome.
In this video, you will find out more about how seismic profiles
produced by reflection seismics provide us with information on the
reflectors in a subsurface.
In seismics, this process is called migration.
I'll show you what assumptions and prerequisites are needed and how a
special procedure, namely Kirchhoff migration, is conducted.
Using simple examples, you will see the practical impact of migration.
You will then learn how important it is for evaluating complex 3D
seismic measurements.
Seismic profiles are composed of many hundreds or even thousands of
measured seismograms.
The recordings contain information about structures in the earth's
subsurface.
In the introductory example, you see layers that are obviously
horizontal, rising towards the center of the layer.
Nevertheless, this type of seismic profile does not actually represent
an image of the earth.
Initially, these waveforms actually represent the impact of different
background structures on the seismic source signals.
The superposition of all paths from different reflectors in the
subsurface produces the wave field measured on the earth's surface
using geophones.
If the location of the reflectors and the propagation effects on the
way to the surface are known, one can calculate the observed wave
field.
The reverse case is what migration seeks to do, namely to determine
the precise location of the reflector in the subsurface by observing
the wave field at the earth's surface.
Migration represents the depiction of waveforms to their true point of
origin in depth.
In addition, I shall also show you the impact of wave propagation
using a simple example.
A point-shaped scattering body in a medium with constant wave
propagation velocity reflects seismic waves in all directions.
Geophones on the earth's surface record the signals.
The signal first reaches the station directly above the scattering
body.
At the more distant stations, the signal arrives after a delay.
Now you can see the associated seismic profile recorded with a zero
-offset gather.
For the centrally lying station, the travel-time signal is represented
directly at the location of the station.
The signal for the neighbouring station arrives later.
However, in the zero-offset representation, it is also represented
here at the location of the station, although the point-shaped
scattering body is located away from it.
Since the propagation is radially symmetric, the point-shaped
scatterer creates a hyperbola-shaped curve in the seismic profile.
It is also referred to as diffraction hyperbola, while the associated
scattering body is called a diffractor.
Here you can see the entire wave field.
It consists of seismograms that have been synthetically calculated
along the surface.
In addition, you can also see the depth section below, including the
original scattering body.
To better recognize the wave inserts, positive deflections are
coloured black.
This forward calculation should now be reversed by migration.
In other words, the migration depicts the wave field which arises at
the time of reflection.
Officially, a number of migration methods are available.
One widely used procedure is Kirchhoff migration.
The idea behind it is this.
All the points of the seismic profiles that lie along a hyperbola
result from a single point reflector in the subsurface.
If we want to find the origin of this hyperbola in the subsurface, we
add up all the signals along this hyperbola.
This sum is then assigned to the corresponding diffractor in the depth
section.
This point is called the apex.
For each individual point in depth, the corresponding hyperbolic
curves in the seismic profiles are selected and summed.
Extensive reflectors, that means layer boundaries, appear in the
seismic profile as entire swarms of diffraction hyperbola.
By summing, the signals of the individual hyperbola are depicted in
individual depth points and from these points the migrated profile is
then collated.
At the point of the original reflector, there is constructive
interference.
All other points mainly show uncorrelated noise.
I shall show you that here.
If a seismic wave is reflected at a horizontal layer boundary, then
according to Huygens' principle, an infinite number of spherical
elementary waves are produced.
Here I shall draw an example of three of them.
As you have seen, a hyperbola arises from each of them in the seismic
profile.
In zero-offset geometry, they lie exactly at the location of the
points in the subsurface.
The more hyperbola I draw, the clearer the form of the horizontal line
appears.
This line represents the reflector and on this line there is
constructive interference.
Hyperbola are also produced in the profile in the case of an inclined
layer.
Here I shall draw another example of three points in the subsurface.
In this case, however, they are all displaced to the right.
They are located precisely at the location of the reflected ray path.
The reason for this displacement is the zero-offset geometry.
In this case, the seismograms are recorded directly at the location of
the geophones, even though the reflection point here is further to the
left.
The goal of the migration is to reverse precisely this effect of the
distortion.
The structure of the earth is, of course, much more complex than just
a series of horizontal layers.
A typical structure, for example, is that of a syncline or hollow.
I shall now show you the resulting seismic profile.
The travel time curves in the seismic profiles resemble a tie loop or
a bow tie, while the migrated image shows a simple syncline as a layer
boundary in the subsurface.
In this case, several zero-offset rays are arriving at a receiver.
All are located at different positions perpendicular to the reflector.
The rays have different lengths, meaning that the associated
reflection events are recorded at different times.
The example shows how different patterns in the seismic profiles can
be attributed to the structures in the subsurface.
An important prerequisite for migration, however, is that the
underlying velocities in the subsurface should be known.
Only in this way is it possible to calculate the course of the
diffraction curves in the seismic profiles and add them up.
For non-homogeneous media with a complex propagation velocity
structure, the hyperbolae are transformed to irregularly shaped bent
curves.
These must usually be numerically calculated.
In practice, that means before the migration, a velocity model is
calculated using a different method.
For the real example, this type of model is available from the onset.
This is a stacked profile of seismograms that has already been shown.
Obviously, there are no simple layer boundaries in the subsurface
here.
Instead, the picture is dominated by a structure extending far, both
vertically and horizontally.
In order to migrate the profile, the following model for the
propagation velocity structure in the subsurface is appropriate.
For each point on a subsurface, diffraction curves can be calculated
with the help of the velocity model.
One such curve is shown here as an example.
As you can see, it no longer corresponds to a hyperbola, as would be
seen in the homogeneous case.
For the Kirchhoff migration, summation is performed along the curve
and the result is plotted in the migrated profile.
Many such summation curves make up the migrated profile as well as the
concrete image of the structures in the subsurface.
In this case, the typical shape of a salt dome can be seen.
Since the salt layer is relatively homogeneous, no boundaries can be
recognized within it.
Further interpretation in a geological context is then a task for
geoscientists.
In this video, I have introduced to you the concept of migration.
I have shown you how diffraction curves of the seismic profiles can be
translated to true subsurface structures.
You have learned that point reflectors in the subsurface are
represented as hyperbola in the zero-offset gather.
More complex structures produce more complex diffraction curves.
In Kirchhoff migration, the diffraction curves of seismic profiles are
added up.
These summations are projected on the corresponding points in the
subsurface so that they form the migrated image of the sought-after
subsurface structure.