Vibrocorer for undisturbed sampling of firn aquifer layers in Greenland: general concept

Pavel TALALAY,XIAN Tao,FAN Xiaopeng,LI Bing,LI Yazhou & GONG Da*

1 Polar Research Center,Jilin University,Changchun 130000,China;

2China University of Geosciences (Beijing),Beijing 100083,China

Abstract The firn aquifer beneath the Greenland Ice Sheet may play a significant role in rising sea level.Both traditional mechanical drilling and electric thermal drilling are poorly adapted for effective,low-disturbance sampling in firn aquifers.We propose using a vibrocoring technique for the undisturbed sampling of dry firn and firn aquifer layers.A remote-controlled vibrocorer is designed to obtain 1-m-long cores with a diameter of 100 mm.The depth capacity of the system is approximately 50 m.The total weight of the vibrocoring system with the surface auxiliary equipment is approximately 110 kg,and corer assembly itself weighs～60 kg.

Keywords Greenland Ice Sheet,undisturbed sampling,vibrocoring,firn aquifer

1 Introduction

Since the 1990s,the mass loss of the Greenland Ice Sheet has been accelerating (Rignot et al.,2011),and in recent years,approximately 85% of the increase in its mass loss rate has been due to increased surface melt and subsequent runoff (Enderlin et al.,2014).More than likely,the Greenland Ice Sheet will be a dominant contributor to sea level change in the upcoming decades (Fettweis et al.,2013).In 2011,a perennial storage site of liquid water,the so-called firn aquifer,was discovered in the southeastern Greenland Ice Sheet via radar measurements and ice coring (Forster et al.,2014;Koenig et al.,2014).Aquifers are formed in both high-accumulation and high-melt regions by meltwater percolating downward into the firn and saturating the pore space above the ice-firn transition.The water stored within a firn aquifer is ultimately either frozen into ice or drained out of the system into the ocean and,therefore,may play a significant role in rising sea levels.

A seismic approach combined with radar sounding in the firn showed that the base of the aquifer lies on average 27.7 ± 2.9 m beneath the surface,with an average thickness of 11.5 ± 5.5 m (Montgomery et al.,2017).The average water content was found to be 16% ± 8%,with considerable variation in the water storage capacity.Direct observation by drilling is still considered the only valid method to verify the results of remote sensing observations;however,drilling an aquifer is challenging.

Traditionally,shallow boreholes in firn and ice are drilled with various hand-and power-driven portable auger or electromechanical drills (Talalay,2016).Drilling through an aquifer is not possible with such drills because,when the drill penetrates firn saturated with water,refrozen ice begins to build up on the drill head and the core barrel;the performance of the drill then rapidly deteriorates to the point where penetration stops and the drill itself may become stuck.

In 2011-2016,just over 10 boreholes were drilled to study the firn aquifer in the southeastern Greenland Ice Sheet with the deepest borehole having a depth of 65 m (Miège et al.,2016).In all cases,a mechanical drill was used in the seasonally dry firn above the water table (upper～10-15 m) and an electrothermal drill was used to drill through the saturated firn.However,this thermal drilling through the firn aquifer presented serious shortcomings.First,meltwater produced during drilling was filtrated by the core,influencing measurements of the water content.Second,extracting cores from the core barrel was a complicated procedure.Sometimes,the cores could not be extracted from the barrel and had to be melted out with ethanol.In addition,during core extraction,the water drained from the core,influencing the accuracy of the measurements.In addition,on several occasions,the thermal drill became stuck in the borehole.

The three following key technical issues need to be considered when selecting a sampling method in this environment: (1) Penetration ability—the core sampler should preferably penetrate both the dry and water-saturated firn down to the targeted depth;(2) Penetration disturbance—the core sampler should retain the original stratigraphic characteristics of the firn layers;and (3) In-situ core parameters—the recovered core should retain the original physical characteristics of the stratum (for example,the water content).

In our opinion,vibrocoring is a viable sampling method for both dry and water-saturated firn.In this method,the core barrel of the corer is driven into the firn by the force of gravity,enhanced by the vibration energy.The vibrations cause a thin “l(fā)ubricating” layer to mobilize along the inner and outer walls of the core barrel,reducing the friction and easing penetration,in the same way as this method is traditionally used in aquatic sediments (Smith,1998;Gong et al.,2016).The formation in the annular space directly below the cutter is pushed away to fill the pores or compress the surrounding layer.

To the best of our knowledge,currently,there is only one vibrocorer that has been designed and tested by the Arctic and Antarctic Research Institute,USSR,for drilling large-diameter boreholes in firn as an alternative to pit excavation (Morev and Zagorodnov,1992).This vibrocorer included a core barrel with a hardened steel tip with inner and outer diameters of 0.38 m and 0.4 m,respectively,and a 1.5-kW electrical motor vibrator.The length of the drill was 2.2 m,and its weight was 80 kg.The vibrator provided vertical vibrations to the core barrel at a frequency of 50 Hz.An electric winch and cable were used to raise and lower the drill in and out of the borehole.Field tests at Vostok Station,East Antarctica,showed a drill penetration rate of 6-8 m·min-1in a snow-firn layer at a temperature of -55 °C.One complete drilling run took less than 5 min.

2 Design of a new vibrocorer for firn aquifer layer sampling

In this paper,we describe a newly proposed vibrocorer for the undisturbed sampling of dry firn and firn aquifer layers (Figure 1).To lower and raise the vibrocorer in and out of the borehole,a lightweight tripod is designed with a small hand winch fixed on one foot of the tripod.The use of a tripod to assist in lifting the vibrocorer increases the depth capacity to～50 m.The total weight of the system is approximately 110 kg.

Systems for dry and water-saturated firn are differentiated by their core prevention methods.Normally,a core catcher breaks the core and prevents it from dropping out of the barrel as it is being lifted from the borehole (Hodgson et al.,2016).In our case,the core catcher in the water-saturated firn completely closes the core barrel to avoid water leakage from the bottom of the core.Accordingly,the corer for the dry firn is equipped with a unidirectional “orange-peel” spring steel core catcher used in conventional drilling for very soft and loose strata (He et al.,2020).The corer for the water-saturated firn is equipped with a hydraulic controlled core catcher that can completely seal the lower end of the barrel,and the surface equipment includes a small manual pump for activating the core catcher (Gallmetzer et al.,2016).

The vibrocorer for water-saturated firn is also equipped with a pressure-release tube connected at its lower end to the cutter and at its upper end to the top of the corer.This tube can balance the pressure in the space below the corer to avoid the influence of negative pressure on the core quality during lifting.In addition,this corer has a removable check valve installed in the upper section of the core barrel assembly.During drilling,this valve is opened by the air pressure inside the core barrel.While lifting the corer,the valve is automatically closed,sealing the core,water,and even the air inside the core barrel (Makinson,2021).

The vibrocorer itself has length of～2.5 m and weight of～60 kg.It is able to recover a core with a maximum length of 1 m.The inner and outer diameters of the cutter are 100 mm and 120 mm,respectively.The vibrocorer can easily be divided into two sections: the core barrel assembly and the vibration unit (Figure 2a).The core barrel assembly consists of an annular cutter at the lower end,a core catcher,a single core barrel,and a check valve at the upper end.The core barrel is made from a thick-walled transparent polycarbonate tube to enable the core to be observed when the corer is raised to the surface with a sample (Gong et al.,2019).

The vibration unit is rigidly connected to the top of the core barrel assembly and is driven by a 500-W electric motor.The unit converts the original rotational motion of the motor into a mass block-reciprocating linear motion via gears and a belt drive (Figure 2b).The reciprocating motion of the mass block generates a periodic excitation force and drives the corer to the vibration state.This type of vibrator has two main advantages: a compact radial structure and a high transmission efficiency.A pressure chamber is required to protect the vibrator from short circuits and corrosion in water.

Figure 1 Schematics of the vibrocoring systems for dry firn (a)and water-saturated firn(b).

Figure 2 Three-dimensional models of the vibrocorer working prototype (corer body is shown as transparent) (a) and the vibration unit (b).

The motor of the vibration unit is switched on and off by the controller on the surface via separate electrical lines.The controller can also adjust the motor rotation speed and,accordingly,the vibration frequency to match the properties of the formation.The vibration force can be adjusted by changing the weight of the mass block.

The two main parameters of the vibrocorer are the vibration load and the vibration frequency.The combined force of vibration and gravity should be higher than the threshold at which the core barrel unit can advance in both dry firn and firn aquifers,and the vibration frequency should be in a range that enables the effective liquefaction of the firn aquifer around the core barrel.Normally,the adjustable frequency range of commercial motors is approximately from 0 to 200 Hz;this range is likely sufficient for dry firn and firn aquifers,which have much lower bearing capacities than underwater sediments.

The resistance force and vibration liquefaction behavior of firn aquifers need to be calibrated via experiments,for example,using orthogonal tests with varying vibration forces and frequencies.In the future,depending on available funds,we plan to conduct detailed development and engineering work,including laboratory and field testing of the proposed vibrocorer.This vibrocorer is also well suited for other applications,such as stratigraphical coring and the drilling of seismic boreholes in both Antarctica and Greenland.

Conflicts of interestNo conflicts of interest were involved with this work.

AcknowledgmentsThis study was supported the by the National Key R&D Program of China (Grant no.2021YFC2801400).We appreciate two anonymous reviewers and Guest Editor Dr.Tong Zhang,for their constructive comments that further improved this manuscript.