9101 meters deep, the drill hole of the German Continental Deep
Drilling Programme, KTB for short, extends under this drilling rig.
It is therefore one of the deepest boreholes on the planet.
Between 1990 and 1994, this deep borehole was drilled below Windisch
-Eschenbach in the Upper Palatinate in Germany.
This is about 90 km northeast of Nuremberg and is located on a
geological boundary between the older Veriskan Basement Rock in the
northeast and the younger sedimentary rocks in the southwest.
This border is known as the Franconian Line.
This is where the basement rock was pushed over the sedimentary rock
some 50 to 250 million years ago.
The exploration of this fault zone and the methodical development of
the drilling technique at great depth were the goals of the KTB.
For example, scientists wanted to investigate how wide the fault zone
of the Franconian Line is at depth.
For this purpose, boundary conditions for processes and rock
properties of the earth's crust were to be derived from measurements
in the borehole.
Many geophysical procedures have been applied and refined to get the
most complete picture of the subsurface.
These include geoelectric methods, magnetic procedures, and those of
borehole seismics.
Hello and welcome.
This video will provide you with an insight into the field of borehole
seismics.
You will learn about the seismic measuring instruments used in the
borehole and the different methods which are used.
You shall become familiarized with vertical seismic profiling as well
as the use of crosshole methods.
The large-scale continental deep drilling program and the
investigation of the Franconian Line at depth serves as an example of
this application.
Borehole seismics puts specific demands on seismic sources and
receivers.
Unlike land or marine seismics, the instruments must be specially
designed for use in boreholes.
Near the surface, they have a diameter of about 40 cm before they
taper down towards the end of the borehole where they have a diameter
of about 10 cm.
As sources within the borehole itself on the one hand, noise emissions
from the drill head can be used.
These take place anyway during the drilling process, but they do not
feature a constant frequency spectrum.
This is a disadvantage for the subsequent processing.
On the other hand, small explosive charges can also be used.
However, the problem with these is that damage can occur to the
borehole during detonation.
So-called downhole sparkers are used more often.
These are underwater sound sources that can be used in boreholes below
the groundwater table and which work by using electrical discharges.
In this case, a discharge occurs between two electrodes, which leads
to a flashover and the formation of an air bubble.
After a few milliseconds, this bubble collapses.
Both effects, the formation and collapse of the bubble, create a sound
impulse which couples as a P-wave into the surrounding rock.
Instead of such sources within the borehole, the seismic signals can
also be generated conventionally on the earth's surface and recorded
with instruments in the borehole.
This was also the procedure used at a KTB.
In addition to vibrocyst trucks, explosions with dynamite in specially
prepared shallow boreholes were detonated.
This method is called vertical seismic profiling, or VSP for short.
In this process, seismic signals are generated on the earth's surface
and measured using borehole geophones.
The most commonly used variant of VSP is the zero-offset gather, where
the source is located near to the borehole.
The geophones are suspended from chains in the borehole and are moved
up or down on a shot-by-shot basis to record the signals.
Zero-offset VSP measurements are mainly used to create a vertical
velocity profile.
In this way, for example, times can be converted into depth in
subsequent reflection seismics.
Since the seismic wave passes directly from the source to the
receiver, the average propagation velocity along the ray path is
easily calculated.
Another advantage over pure surface measurements is the generally
higher signal amplitude for equal depth of investigation.
With pure surface measurements, the wave normally traverses the
subsurface layers twice on the way down and back up again.
Because of the geometry, the amplitude therefore decreases more than
would be the case with the borehole measurement of a single traverse.
In addition to the direct waves shown here, however, reflections at
deeper layer boundaries are also used in borehole methods so that they
can be imaged near the borehole.
There are other variants of VSP, including walk-away VSP, which is
also referred to as moving source profiling.
Here, the geophone is usually left at a certain depth and seismic
sources then generate signals at different positions.
This makes it possible to map area-wide velocity variations or
reflectors.
The ray paths then, however, no longer propagate vertically.
Almost horizontal ray paths are used in what we call crosshole
methods.
The term crosshole comes from the transverse ray path that arise when
measurements are made between two boreholes.
One borehole contains the seismic sources, while the other contains
the receivers.
The sources are positioned at different depths and gradually
detonated.
This creates a good ray overlap between the drill holes.
With crosshole measurements, the area between two boreholes can
therefore be investigated.
Typically, tomographic methods are used here to evaluate the travel
times between the sources and receivers so that the two-dimensional
distribution of the propagation velocity can be determined.
For both applications, VSP and crosshole, signals must be recorded in
the borehole.
But how does this happen?
To do this, special geophones are needed that can be lowered into the
borehole on long chains.
For this purpose, the instruments must fulfill certain requirements.
Borehole geophones are special sensors that are accommodated within a
cylindrical housing.
They must be designed to withstand the high pressure and temperature
conditions within the borehole.
Such an instrument often contains geophones in all three spatial
directions so that the entire wave motion can be recorded.
In order to establish a good contact with the borehole wall, special
devices are used to clamp the borehole geophone.
Here you can see one.
Permanently installed borehole instruments are sometimes encased in
concrete or sand at the borehole end for optimal coupling.
At the KTB, mobile instruments were used down to a depth of 8.5 km in
the borehole.
The measurements were therefore among the deepest ever carried out.
At this depth, temperatures of 260°C and pressures of 140 MPa prevail.
Under these conditions, it is only possible to work with specially
made instruments that can withstand these conditions.
In order to study the structure of the Franconian line at depth, a
moving source VSP was performed at the KTB.
As you have just seen, seismic sources were distributed over the
surface and measured using borehole geophones.
You can see the results of the VSP measurement here.
The figure shows the depth-migrated section of the P-wave reflections.
The horizontal extent of the profile runs from the borehole to about 4
km to the southwest.
Different structures can be clearly seen, which are highlighted in the
interpretation with drawn lines.
The structure corresponds to the position and angle of incidence of
the Franconian line, which separates the Veriskon basement rock on the
right and the profile from the sedimentary rocks to the left.
This fault zone lies at a depth of 7 km.
This means it is about 1 km wide.
This is much wider than was previously thought possible for such
zones.
VSP measurements allowed this to be shown for the first time.
In this video, you have learned about the basic methods of borehole
seismics.
Sources and receivers can be used in the borehole itself or on the
earth's surface.
In vertical seismic profiling, VSP, borehole geophones record seismic
signals generated on the surface.
Such measurements are useful for creating vertical velocity profiles.
For a crosshole measurement, however, sources and receivers are
positioned in opposite holes to explore the space in between.
Using the KTB continental deep drilling borehole as an example, you
have seen how borehole seismics can be used to precisely explore rock
properties near the borehole.
It allowed the diameter of the Franconian line at 8.5 km depth to be
determined as about 1 km, a measurement which has not been possible
with near-surface seismics.