AFT3 Results

Photo of subglacial tillite cored at Minna Bluff.

We completed a successful third round of Antarctic Field Trials (AFT3) with RAID at Minna Bluff during the 2019-20 austral season. Major goals identified for AFT3 were achieved, despite several both inherent and unexpected challenges. Details of the equipment configuration, drilling procedures, borehole creation, and subglacial coring are given in an article published in Annals of Glaciology [see Publications]. A summary and highlights are included below.

HIGHLIGHTS

Major outcomes of AFT3 include the following:

• fast augering of three firn boreholes to accommodate a packer at depths of 85-107 m, now established as a reliable method for making holes in firn
• successful testing of a new borehole ‘bailer’, designed and built by IDP to remove firn cuttings and ready the borehole for deployment of a packer
• demonstrated first use of a conventional ice-core drill with modified cutters to make a microcrack-free packer seal, and to recover several meter-long ice cores for density verification
• found via testing that 350 psi is a safe and effective pressure for packer inflation to seal off the permeable firn with casing
• collection of core samples from firn-ice transition for H₂ measurement
• rapid deep ice drilling using a non-coring rotary ice-cutting bit with fluid circulation (penetration up to about 680 m, a new Antarctic record)
• first successful use of a new pressure relief valve to prevent accidental hydrofracture of ice by fluid overpressure
• measurement of temperature (-10 °C) at the bottom of a borehole reaching the glacier bed, demonstrating a cold-based glacier and validating heat model prediction of -10.3 ˚C
• successful penetration into a mixed zone of glacial ice, mud, and glacial debris, all while maintaining fluid circulation
• cored 1 m of near-basal ice for gas, age, and isotopic analysis using wireline coring inserts
• cored basal glacial till and 3 m of bedrock from just below ice-rock transition
• obtained borehole logs using the ‘slim’ Bay-type laser dust logger made for RAID boreholes
• obtained borehole video logs using an optical televiewer
• development of a new method to evacuate drilling fluid from boreholes
  down-rigging, packing, moving, and re-rigging the RAID system between drill sites in less than 1.5 days
• demonstrated ability to drill ice quickly, equivalent to 2000 ft in 8 hours, in 3rd hole
• discovered that hydrofracture risk does not increase with depth, as predicted
• blog posts during the field season, providing a real-time picture of life in the field and RAID drilling progress to the public

RAID drilling modules at Minna Bluff site for AFT3.

FIELD OPERATIONS

The RAID team was deployed between November 7, 2019 and February 10, 2020. The field site at Minna Bluff was occupied by the RAID team between December 2, 2019 and January 31, 2020. We completed 4 boreholes at three sites, as listed below and shown schematically in the following figure.

Minna Bluff borehole summary.

CHALLENGES
As with earlier field trials, AFT3 was an exploratory, experimental season. Probing the unknown carries with it inherent challenges, including the following:

• a new drilling team that is highly skilled at rock drilling and operation of the RAID drilling rig, but with no prior experience in drilling firn or ice (known/expected)
• warm surface temperatures that risked melting and refreezing (expected), and that required careful, continuous temperature control and fluid management
• high snow accumulation rate and thick firn (expected)
• dynamic glacial ice in stressed condition with unknown hydrofracture strength (expected)
• ice fracture upon hitting a rock 1 m above bed, with normal fluid pressure (unexpected)
• uncertain precise depth to bedrock (expected)
• unknown bed materials, bed properties, and width of ice-rock transition (expected)
• inoperable or inadequate air conditioning units (unknown/unexpected)
• learning through experimentation with new equipment (expected), for example a newly designed pressure relief valve applied to multiphase fluids (Estisol, ice chips, and water)
• limited supply of Estisol drilling fluid, in part due to some fluid losses in firn and hydrofractured ice (not unexpected)
• lack of sufficient chilled Estisol to maintain fast ice drilling for >30 min at a time (unexpected, but this will not be a problem on the Polar Plateau)

RAID boreholes completed at Minna Bluff. Note non-uniform depth scale and scale breaks below 83 m.

BOREHOLE COMPLETIONS

Brief summaries of the 4 boreholes made at three sites are given below. Detailed daily drilling logs, weekly summaries of activities, and individual notes from the field trials documented sequences of events, drilling parameters, and drilling outcomes.

Hole #1a was started by augering to a depth of about 67 m when the drill torque climbed as the cutting bit started packing off. At a depth of about 70 m part of the cutting head broke off at the bottom of the hole, so a decision was made to abandon the hole and pull out the auger string. The conditions were warm enough that melting may have occurred at the cutting head, and a slow penetration rate produced cuttings that were too fine so that they caked together and were poorly transported on the auger flights.

After moving the Drill and Rod modules a short distance without moving the other modules, Hole #1b was started. We augered to about 90 m in a short time, but the auger string became stuck when trying to pull out 30-ft sections with no rotation. After receiving approval, step-wise application of small amounts of glycol allowed the augers to eventually came free and were removed. The borehole was further deepened and cleared of glycol by using the IDP 4-Inch drill and the new chip ‘bailer’ to a depth of about 107 m. A borehole packer was set at this depth, and fluid-assisted ice drilling was completed to a depth of about 140 m when we lost fluid circulation due to hydrofracture. Fluid observed above the packer indicated that the ice crack crossed the packer, allowing fluid to be lost to the firn. in retrospect, hydrofracture occurred during resetting of the PRV that caused an inadvertent spike in fluid pressure, which was first released by mistakenly opening a valve in a downstream direction rather than upstream. This hole was abandoned and evacuated, and we moved the modules to a second site.

Hole #2 was started after moving about 100 m from the previous site. Augering was completed quickly to a depth of 61 m, followed by deepening of the firn borehole, chip removal, and conditioning of the borehole wall with the chip ‘bailer’. A packer was successfully set at a depth of 85 m. Ice drilling proceeded deliberately at a pace of about 2 ft/min following a schedule that optimized size of cuttings produced, fluid flow rate, and drill penetration rate. Over the course of several days, we experienced a nearly continuous set of problems with blockages in fluid circulation, which required different approaches to overcome. Despite the ongoing problems, we made slow but steady progress toward deepening the borehole without fracturing the ice. Ice drilling thus proceeded effectively and reached the bed depth indicated by an ice-penetrating radar profile (~600 m). Below 600 m, ice drilling proceeded cautiously and we encountered zones of ‘dirty’ ice that brought up discolored cuttings and small bits of dark material. Based on returned cuttings and behavior of the drill, we concluded that we were near glacier bottom and decided to switch to coring mode. By experimenting with different coring bits and drilling parameters, we were able to reach the base of the glacial ice, core across the glacial-subglacial transition, and retrieve intact cores of subglacial till and bedrock. A detailed narrative of the successful drilling and coring process near the glacial bed in Hole #2 follows below.

Hole #3 was started after a quick move from the previous site. Given the success of Hole #2, goals for this hole included sampling air from firn cores obtained with the 4-in drill, testing ability to drill ice at fast penetration rates, and conducting a hydrofracture test in ice by intentionally causing fluid overpressure. Augering was completed quickly to a depth of about 82 m, followed by chip removal and conditioning of the borehole wall with the chip ‘bailer’. A packer was successfully set at a depth of 85 m. Ice drilling proceeded on a new rapid schedule and reached a maximum penetration rate of about 4 ft/min, which was thought to be the maximum rate achievable with the currently configured drilling bit. Drilling proceeded effectively to a depth of 442 m, when the pressure relief valve became iced up and allowed fluid pressure to spike, resulting in fracturing of the ice. Drilling operations were halted on January 25, 2020.

ICE-DRILLING AND SUBGLACIAL CORING IN BOREHOLE #2

Once a packer was set and passed a pressure test, we set up the system for ice-drilling using reverse fluid circulation and commenced ice drilling in Hole #2 on December 29. Over the next two weeks, we were able to drill progressively deeper into ice in reverse fluid-assisted circulation, deeper than any such system has gone before by an order of magnitude. We drilled smoothly and continuously at rates of 2-3 ft/min, with occasional interruptions by freezing and blockages in the hose lines and fittings. We tested different drilling conditions (rotation rate, penetration rate, fluid flow rate, etc.) to optimize good chip production and transport. We reduced the speed of penetration to focus on cutting process and maintaining integrity of the borehole. Following are highlights from the lower section of the borehole.

• After encountering difficult clay-bearing ice zones near the bed, we prepared tooling for ice and/or rock coring using the ‘Alien’ tool on wireline.
• We then encountered some zones where the ice-cutting head was apparently plugged up, causing loss of circulation and pressurization in the fluid lines. These zones were associated with return of darker-colored ice shavings on the shaker screens, composed of small dark grains (possible dirty basal ice?).
• During one circulation stage, filter socks in the cold room became coated and plugged with dark mud-like material. A sample was collected and dried (possible glacial till material?).
• Samples of ice cuttings were collected at 30 ft intervals for laboratory analysis of water ¹⁸O.
• The drill struck rock on January 10 at 677 m depth below surface, based on drill torque, vibration, and load-on-bit readings (possible sub-glacial material?), and the bed’s temperature was measured to be -10˚C, indicating a frozen bed as expected from thermal modeling. ‘Dirty’ ice shavings from the shaker and inspection of the face-centered ice bit showed knicks and scratches that indicated contact with material harder than ice and steel. We experienced an immediate loss of fluid circulation, indicating that the ice had fractured, despite normal fluid pressures. In retrospect, this fracture probably was caused by a percussive shock to the ice by the drill head. In subsequent drilling, we occasionally recovered fluid circulation, in contrast to the other hydrofracture events where circulation was permanently lost.
• We deployed the ‘Alien’ tool again to recover material from the glacial bed. We experienced periodic loss of fluid circulation and difficulty penetrating, but each time were able to pull up and restore circulation to normal fluid flow rates and pressures. At one stage we decided to pull the NQ drill rods to inspect for packing off with cuttings at bit face and/or in drill rods. The outer ice-cutting bit also showed the marring seen on the center bit, further indicating contact with hard material. We then tripped the NQ rods back in hole.
• We continued to deploy the Alien tool for coring, using different bits with modifications as needed to open up waterways for better transport of chips and cuttings. Recovered 1.5 feet of mostly clean ice core in near-perfect condition, which contained small pebbles.
• On January 16 using a ‘mixed media’ bit, we recovered about 3 feet of ice and glacial till starting at a depth of about 677.5 m (see graphic core log). From top to bottom, we recovered a nearly complete and intact section of dirty ice, banded ice, loose unconsolidated till, and about 30 cm of dry, compacted, hard glacial till (see photos below). This core run effectively sampled across the transition from ice to subglacial till, indicating that the ice-rock boundary lies at a depth of 678.3 m (2,225 ft).
• On the next coring runs, we obtained more glacial bed material, including gritty mud, compacted till, and pebbly debris with rock fragments up to about 1 cm in size. We reached a depth of about 679 m (2,227 ft).
• Still on January 16, we recovered a core of lithified glacigenic tillite (rock) with essentially complete recovery totaling about 7.3 feet. This was the first recovery of actual subglacial bedrock. It is a polymict tillite containing clasts up to 25 cm size of heterogeneous volcanic, altered volcanic, and hypabyssal igneous rock (see lead photo). Hole bottom was at 681 m (2,234 ft). This is an important milestone for RAID and, to our knowledge, is the deepest recovery of subglacial bedrock in Antarctica. Our blog post from January 21, 2020 covers this exciting achievement.
• We continued coring that yielded shorter, less complete runs of subglacial material to a depth of about 2,235 ft.
• On January 17, we decided to stop further rock coring and prepare for borehole logging. This decision was based on consideration of the remaining field season calendar and other objectives for the next hole. Drill rods were pulled and fluid level in the borehole was confirmed to be high enough for logging with the laser dust logger and the optical televiewer. Logging commenced on January 17 and continued afterward.

Graphic core log of Borehole #2.

Cored materials recovered in Borehole #2. (a) Dirty glacial ice below 677 m with flecks of rock debris. (b) Close-up of glacial ice containing air bubbles. (c) Dirty, banded ice from about 678 m. (d) Soft, gritty till containing pebbles. (e) Hard, dry, compacted till (surface is wet from Estisol). (f) Bedrock core of heterolithic tillite from about 679-681 m. Measuring tape in cm.

GENERAL REFLECTIONS

AFT3 demonstrated the high value of field experimentation with a working prototype. As an entirely new approach to deep ice drilling and subglacial access coring, many of the bespoke drilling components designed for RAID were impossible to test in any other setting than in thick Antarctic ice. Looking back over the three Antarctic Field Trials (AFT1 in 2016-17, AFT2 in 2017-18, and AFT3 in 2019-20), it was wise to get RAID into the field in a timely way to quickly learn as many valuable lessons as possible. During development of a new system like RAID, we found that it is often better to take the system into its operating environment without delay to get rapid feedback on what works and what does not. When problems arise, getting information quickly helps avoid following unproductive paths and the time-consuming process of over-engineering to mitigate all possible outcomes. Active field experimentation leads to quick progress on a steep learning curve. Thus, rather than view the earlier field trials as falling short on some goals, we can instead take heart in the tangible accomplishments and lessons learned that led to success at AFT3. Given that the total field time allotted for three drilling trials was limited to 4 months, with only one month each allocated for AFT1 and AFT2, it is not surprising that early challenges arose which prevented early completion of first-order goals. Despite a variety of challenges, we have made enormous progress and achieved many major goals. In short, the Antarctic Field Trials have demonstrated that as a working prototype RAID has the capability for which it was designed.

PROSPECTS FOR PLATEAU DRILLING

In many respects, drilling operations on the high Polar Plateau will be less of a challenge. The more extreme conditions, while harder on people, will actually make ice drilling and bedrock coring more straightforward due to the colder surface temperatures we will encounter. Colder air temperatures will ensure that drilling fluid reservoirs at the surface will be adequately chilled prior to introduction in boreholes, mitigating down-hole warm temperature advection and surface melting as was experienced at Minna Bluff. In addition, the most likely sites for achieving scientific goals on the polar plateau will overlie cold (hence, stronger), low-stress ice, reducing significantly the risk of hydrofracture due to fluid overpressure.

COMPONENT-LEVEL OUTCOMES

During AFT3 the RAID drilling team made innumerable field alterations to various drilling components, but many additional technical modifications were identified for future implementation. These component-level modifications are described in our published report.