Hydrostatic pressure and fluid density profile in deep ice bore-holes

The drilling of deep bore-holes in ice requires that the hole is filled by a liquid to compensate the ice-overburden pressure. Moreover, the hydrostatic pressure of the fluid should be exactly known in order to estimate the hole closure. The estimation of the hydrostatic pressure in the bore-hole can be made in two different ways. The first is the in situ measurements using pressure sensor, and the second is calculation of the pressure using the sampling of drilling fluid from different depths. The second method can be used also for prognosis of the hydrostatic pressure when the necessary density of the hole liquid is chosen. The paper includes the necessary equations for the calculation of hydrostatic pressure for one and two-compound liquids based on the pressure and temperature in the bore-hole. The measured and calculated densities are compared for the GISP2 bore-hole at Summit, Greenland, and it shows a high correlation. The difference between measured pressure and calculated pressure along most of the hole length doesn\u27t exceed 0.12%

Attenuation of drill cables

The frequency characteristics of a drill cable determines both the type of communication that can be used between a drill or logger and the surface, as well as the maximum rate of information that can be transferred. In order to revise the design, the attenuation of the cable was measured. The result was far from the characteristics of a coaxial cable. At medium audio frequencies, the attenuation was quite low. However at both low and high frequencies, the attenuation increased significantly, and the high frequency cut off could be approximated by a 3-order low pass filter with a cut off frequency of 8kHz. For comparison, the attenuation of a similar size coaxial type cable will be in order of 1dB/300m at 150kHz increasing to 2dB/300m at 1MHz. If a bandwidth higher than 10kHz is required for a 4km long oceanographic type electromechanical steel armoured cable, this has to be of coaxial construction with a shield separated from the armour

Locating the Hans Tausen Drill Site

The Hans Tausen drill site was requested to be on a local dome, as high as possible, with a simple flow pattern and with an undisturbed flow pattern even close to bedrock. The first mapping of the surface elevation was performed as part of a general mapping of the elevation and gravity over Greenland. Later, NASA performed a few overflights of the ice cap. Combining the passes, it was possible to get the main features: A mountainous northern part with complicated ice flow, and two more flat southern domes. The eastern dome was higher, and thus more interesting. In 1993, a detailed airborne measurement of the surface elevation and ice thickness was performed. Based on this, the southeastern dome was selected as the best drill site. In 1994, the position of the dome, and additional ice thickness measurements were performed from the surface. This located the exact position of the dome, and allowed staging of drilling material the same year. Finally in 1995, additional measurements of surface elevations over the northern mountains and selected parts of the southeast Dome completed the surface mapping. The measurement showed that the bedrock was very complicated with a southnorth ravine separating the two southern domes. Below the southeast dome however the bedrock was relatively flat with an ice thickness of 345 metres and bedrock changes less than 25 metres. This dome was consequently selected as the site for the drilling to bedrock in 1995