Development of an Ultra-Light Multipurpose Drill and Tooling for the Transportable Array in Alaska

Abstract:

Over the next four years the EarthScope Transportable Array (TA) will install approximately 250 to 275 broadband seismic stations in Alaska and Western Canada. The station plans build on recent developments in posthole broadband seismometer design and call for sensors to be installed in boreholes 7 inches diameter, from 1 to 5 meters deep. These boreholes will be lined with PVC or steel casing, grouted in place. The proposed station locations are in a grid-like pattern with a nominal spacing of 85 km. Since most of these locations will only be accessible by helicopter, it was necessary to develop an ultra-light drilling system that could be transported to site in one sling load by a high performance light helicopter (i.e. AS350B2 or Bell 407) and still be able to drill the variety of ground conditions we expect to encounter.

In the past year we have developed a working prototype, gasoline-hydraulic drill rig that can be configured to run auger, diamond core or DTH tools, and weighs <1,300 lbs, including tooling. We have successfully drilled over 30 boreholes with this drill, including 12 for TA installations in Alaska and 13 at the Piñon Flat Observatory for testing sensor performance and placement techniques. Our drilling solution comprises:

- Hydraulic system using a variable flow pump with on-demand load sensing valves to reduce the engine size needed and to cut down on heat build-up;

- Rotation head mounting system on the travelling block to enable quick change of drilling tools;

- Low speed, high torque rotation head for the auger, and an anchoring system that enables us to apply up to 5,000 lbs downforce for augering in permafrost;

- Custom DTH that can run on low air pressure and air flow, yet is still robust enough to drill a 7 inch hole 2.5 meters through solid rock;

- One-trip casing advance drilling with the DTH, steel casing is loaded at the start of drilling and follows the drill bit down;

- Grout-through bottom caps for sealing the borehole casing and cementing it in place.

Our next step is to build a dedicated DTH drilling system that will be light enough to mobilize to sites in one helicopter sling, including an air compressor. This rig is currently on the drawing board and we expect to build it this winter for field testing in the spring.