Electronic geologist eye looks in the depth

Borehole measurements allow a look at the climate of the past

ANDRILL drilling site in the best weather. © Dr. Andreas Läufer / BGR Hannover
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How did the Antarctic ice sheet form? What are the consequences of the current climate change and what are the consequences for humanity? Answers to these pressing questions will be provided by the ANDRILL Antarctic Drilling Program. German geophysicists will also be taking part, who use borehole measurements to look into the sediments under the ice and thus into the climate of the past.

The aim of the international, US $ 30 million ANDRILL Antarctic drilling program is to reconstruct the climate of the last 50 million years, and in particular the extent of the ice shelf in the Ross Sea during the respective cold and warm periods. "We participate through elaborate geophysical measurements in the borehole and their interpretation and will move to almost a ton of equipment" Thomas Wonik explains the role of the involved Institute for Geoscientific Community Tasks (GGA) in Hannover.

How do you miss a borehole geophysically?

"Without drilling you do not get away with research, exploration and production of hydrocarbons and mineral resources, engineering and hydrogeology and geothermics, " says Wonik. Due to the recovery of the core, extensive scientific investigations in the laboratory are possible after a drilling, which would be too time-consuming on site. "In all these 'expensive' pinpricks into the top layers of the earth, we try to elicit as much information as possible about the past from the pierced mountains, " says Wonik.

The disadvantage: The extraction of cores to investigate the substrate is extremely complex and expensive. A fast and cost-effective alternative, however, offers the use of borehole measurements. Although these require a hole, but the core does not need to be recovered intact from the earth - it is rather sufficient to flush the shattered rocks from the depths upwards.

Borehole probe on the way into the depth. Thomas Grelle / GGA

Next, highly engineered borehole probes are lowered into the hole on a special cable this is as if a geological eye could look directly into the pierced interior of the Earth, explains Wonik smirking. Because when slowly pulling up the probes register the physical properties of the direct borehole environment in decimeter steps and transfer the data through the cable upwards. "Unfortunately, this eye is not perfect because the natural properties of the mountain have already been disturbed by the drilling process. Nevertheless, the probes provide important direct information from the depth." Display

Which physical parameters are measurable?

Borehole probes after transport and shortly before use. Ralf Gelfort (GGA)

Since the first borehole measurements about 70 years ago, a large number of probes has been created that can measure almost every physical parameter of the mountain. The most important are: natural gamma radiation, density, porosity, speed of sound, electrical conductivity, magnetic susceptibility, and the temperature and salinity of the wellbore fluid. In addition, there are methods for detecting the geometry of the borehole, such as its respective diameter and its position in space finally, there is no hole that runs exactly vertically.

Above all, the physical properties of the subsoil can be continuously and objectively determined from the "deep fever curves" obtained in this way. In addition, the scientists can combine geophysical measurements on the earth's surface with subterranean geology and additionally calculate geological, physical and chemical quantities.

Sediments store climate information

In the ANDRILL project, we can also answer questions that interest even average consumers - such as the effects of climate change, Wonik explains. E Usually, ice cores are the focus of interest when it comes to the Antarctic climate. However, these tell us only the climate history of the last one million years. On the other hand, the physical properties of the sediment layers of the seafloor measured with our methods allow us a far reaching insight into the climatic and environmental conditions of the last millions of years, "Wonik shows the advantages of the borehole measurements.

"I am very much looking forward to returning to the Antarctic in October 2007, even if I can not spend Christmas at home" concludes Wonik. "The challenge for me, together with over 50 colleagues in an international research project, is to get the most out of a 1, 000-meter-deep well."

(Thomas Wonik / Institute for Geoscientific Community Tasks (GGA-Institute) / Geozentrum Hannover, 15.06.2007 - AHE)