Alaska Transportable Array (ATA)

The work began with the only method which would work within the  initial weight requirements , using a modified Mobil ultralight rig running bare continuous flight auger in an uncased hole.  As auger drilling requires significant downpressure,  torque,  and relatively soft ground, we had limited success with this method.  

(Left)  Drilling frozen sand and gravel at Point Barrow AK (approx 70 north).  Open hole drilling requires stable ground conditions.  After removal of auger, a 6" sched 40  PVC casing is installed and grouted into borehole.

In order to drill rock, The rig was modified to run an 7" diameter core barrel, similiar to those used in concrete coring.  This method  also had limited success, as fractured rock caused loss of water circulation, and retrieval of drilled core was very difficult.  Additionally, coring takes a lot of water, which of course had to be transported in by helicopter.

(Left)  Injecting a foaming agent to reduce water consumption.

After working  with the above methods, we proposed  developing a  more comprehensive system which could install a cased borehole in any of the ground types encountered so far.  This system would incorporate a small percussion hammer encased in a shroud and running an oversized overburden pilot and ring bit, and would install a thin wall 6 5/8" od steel casing to the required depth using a relatively small air compressor (see percussion overburden methods).  Drilling with a casing (rather than an open hole) eliminates the loss of uphole velocity, enabling the cuttings to be ejected from the hole, and keeping the hole from caving in softer materials.  After the hole is drilled, the hammer is pulled out, leaving the casing in the hole.  

After completion of borehole and extraction of drill tooling, an aluminum grout plug fitted with a check valve and a sealing ring is inserted and locked into the casing shoe at the bottom of the hole.  Grout is pumped through this disc, filling the annulus between casing and the surrounding soil or rock.  This grout bond couples the bottom of the casing to the surrounding formation, creating an ideal environment for the seismometer.