alternative system using gyroscopes, also with manual implementation of the steering information (Miyazaki, 1993). For the same reasons, the gyroscopic system seems better adapted to the remotely operated microtunneling machines that drive tunnels a couple of feet or so in diameter. A disadvantage of the gyroscopic system is that it must be reset periodically, for example, by removing it from the tunnel so it can "see" a reference satellite.

Directional drilling (e.g., Cooper, 1994) is not new. Since the earliest times, boreholes were made to deviate by placing tapered wedges or "whipstocks" in the borehole to force the bit sideways into a new direction, and it was known that different bottom-hole assemblies (BHAs) had a tendency either to increase or to decrease the inclination of the hole. Most developments in directional drilling started in the petroleum industry and then spread to other fields. Pioneering work was carried out in oil well drilling by John Zublin, H. John Eastman, and others during the 1950s in drilling directional drainholes from existing wells (Stormont, 1953; Eastman, 1954). Significant work was also done during the 1960s in directional drilling from Huntington Beach, California. However, progress was slow until the mid-1980s, when it was realized that significant increases in productivity could be obtained by orienting the borehole correctly in the reservoir (Reiss, 1987). Simultaneously, it was realized that substantial economies could be obtained in offshore operations if, by extending the horizontal reach of the production wells, the area accessed from any platform could be increased (Wilson and Willis, 1986) or the number of platforms needed to exploit a reservoir could be reduced (Tolle and Dellinger, 1986).

Since the mid-1980s, a large amount of work has been done to investigate all aspects of directional drilling. In addition to the control of direction, other important issues include surveying methods; borehole stability and cuttings removal in inclined boreholes; the reservoir engineering aspects of horizontal wells; and the various special problems associated