The radiation properties of a downhole drill-bit seismic source are related to the amplitude and frequency of the forces exerted by the working bit. The main vibration modes of roller-cone and polycrystalline diamond compact (PDC) bits are investigated under different drilling conditions. The analysis includes vibrations produced by teeth indention, multilobed patterns, bouncing with periodic and random effects, single-cutter forces, stick-slip and whirling effects, mud-pressure modulation forces, and bit wear. Drill-bit radiation properties are calculated using the results obtained in part 1 of this paper and are numerically compared to the radiation of conventional vertical seismic profiling (VSP) sources.

Energy balance of a drill-bit seismic source, part 2: Drill-bit versus conventional seismic sources

In 1979, we began a program of retrieving ice cores from glaciers located at high elevations in low-latitude locations for use in past climate research. The high-resolution records of past climates obtained from these cores provide a unique link to past climates outside the Polar Regions. Since 1986, coring has been successful at various locations in South America and across the Tibetan Plateau using a combination of electromechanical and thermal drills to reach depths of up to 309 m and working altitudes of up to 7200 m. The wide variety of environmental and ice conditions including 'brittle ice' has resulted in the development of a drilling system that can use a variety of power sources and drill ice that varies from -20 ° C to near 0 ° C. Because of the brittle ice phenomenon, we have experimented with ways to improve core quality in the brittle ice zone. We also developed shelters, core processing equipment, and shipping containers capable of returning ice core to the lab in a frozen state. Lessons leaned from these excursions are discussed. 1. Drill development The current wisdom in 1976 was that a helicopter was the transport vehicle of choice because it was considered impossible to carry a drill across mountainous terrain, reassemble it, and drill a core. However, our first attempt to transport a standard electromechanical drilling system to the 5600 m Quelccaya Ice Cap in Peru ended in failure because the helicopter could not safely fly above 5500 m. The drill was a standard electromechanical drill having a heavy steel armored cable and required a heavy generator to power it. There was no choice but to postpone the project until a more portable drill could be developed. In the period from 19801983, we established that it was possible to build a drill that could be assembled from parts having a maximum weight of about 30 kg and that solar panels would deliver power at a level of 25% greater than at sea level. We returned to the Quelccaya Ice Cap in 1983 with a newly designed 2-kW solar-array-powered electrothermal drill developed at the University of Minnesota, which featured a Kevlar electromechanical cable. Despite having to walk to the summit every day, we were able to drill two cores to bedrock, one to a depth of l 63 m and the other to a depth of l 54 m, at a rate of l 4 m per day. The two continuous cores were melted and returned as water samples for analysis. They were the first deep ice cores ever obtained from the tropics.

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A review of high-altitude drilling

As an attempt to clarify the working process of drag bit blades, a plane stress analysis on a static model has been made by photoelasticity and interferometry. Induced stress concentration, lines tangent to critical directions of supposed pre-existing cracks, and fracture initiation zones are described in the vicinity of the contact area. High tensil stresses appear around the cutting edge. Their magnitude increases as the cutting angle increases and does not depend on the clearance angle. High-compression stress concentrations are induced under the tooth for clearance angles other than zero. Their magnitude increases with rake angles. In the grinding mechanism, a double compression stress state is perceived in the vicinity of contact. Tensile stress appears behind the tooth, and high-compression stresses are propagated in front of it. From the Griffith-Walsh failure initiation theory and the discussions of the fracture initiation patterns, in the cutting case, tooth efficiency is determined according to horizontal force. The bit is more efficient, when the rake angle increases.

H. Cholet

Papers

Stress concentration induced in elastic rocks under a diamond bit tooth