Basic Geophysics Borehole Seismics

9,101 meters deep the drill hole of the German Continental Deep Drilling Program - 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 Windischeschenbach in the Upper Palatinate in Germany. This is about 90 kilometres northeast of Nuremberg and is located on a geological boundary between the older Variscan 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 rocks some 50-250 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 familiarised 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 the KTB. In addition to Vibroseis 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 depths 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 depths 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 a 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 cross-hole methods. The term cross-hole comes from the transverse ray paths 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 cross-hole measurements, the area between two boreholes can therefore be investigated. Typically, tomographic methods are used here to evaluate the traveltimes between the sources and receivers so that the two-dimensional distribution of the propagation velocity can be determined. For both applications: VSP and cross-hole 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 fulfil 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 kilometres in the borehole. The measurements were therefore among the deepest ever carried out. At this depth, temperatures of 260°C and pressures of 140 megapascals 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 four kilometres 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 Variscan basement rock on the right in the profile from the sedimentary rocks to the left. This fault zone lies at a depth of seven kilometres, this means it is about one kilometre 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 cross-hole 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 one kilometre, a measurement which has not been possible with near-surface seismics.

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