Bore-Hole Survey at Dye 3, South Greenland

Abstract

In 1983 three d irec ti o nal su rveys were m ade in th e bore hole from whi ch a dee p ice core was o bta ined in the sum mers of 1979-8 1. The in clin a tion and azim ut h of th e bo re ho le were measured o n three surveys, tem perature was in cluded on two surveys, nuid press ure a nd ho le diameter on o ne of th e surveys. Fl uid-pressu re measu remen ts show th at th e iceoverbu rden pressure was undercom pensated in the uppe r few hundred meters a nd overcompensated at the bo tt om of th e ho le. Dia meter measure mf' nts sh ow cl osure in th e uppe r porti o n and ex pa nsio n nea r th e bo tt om begin ning at the trans ition from th e Ho locene lO Wisconsin ice at 1784 m. Th e hole expansion and in crease in in clin ati on correla te wi th d ust and sil t content in the \\'isconsin ice. C ha nges in azimuth are d ue to Row oflhe ice an d a re co nsis tent wi th the direc ti on of Aow at the surface. Temperatu re measure m e nts show that th e ho le is a t or nea r eq uilib rium. Th e g radient of 0,012 K /m below 1400 In is less th a n the 0.018 K/m a t Ca m p Century.T here is a sl ig ht reduction in grad ient near the bottom from internal fricti on in the silt y ice. (Rand, 1980), and the hole was drilled from 8l.7 m to the bottom with an electromechani cal drill (Gundestrup and others, in pres s). The drill, using neither melting nor solve nt s , produ c es a hole with a uniform diameter. The hol e i s filled with a liquid in order to minimize hole closure from the overburden pre s sure of the ice. R ESU ME. Relevis dll trOll deJorage a DyeA fter the dri 11 i ng s t opped in 1980 at 901 m, a temperature profile "was meas ured. Shortly before the end of the 1981 s eason, an inclination profile (Gundestrup and others, in press) was obtained using the inclinometers in the drill. The ne xt day the drill was trapped at the bottom while drilling in silty ice. Fo ur hundred liters of per c hlorethylene (pCEl were added to the hole, and the drill was left with tension in the cable. In 1982, the drill had become loose. After recovery of the drill, a temperature profile wa s measured. Soni c logging wa s performed (Tay lor, unpubli s hed) and a 5 m by 0.08 m probe was used as radar target in an attempt to measure the azi muth of the hole inclination s , and the velocity of radar wave s in ice (Jeze k and Roeloffs, 1984). In 1983 three indepe ndent bore-hole surveys were made usin g different tools to measure inclina- tion and azimuth. The hol e diameter, fl uid temperature, and pressure were also measured.While drilling, depth was measured as accumulated core length. The hole length changes with time due to the ice movement. In this work all slant hole depths measured on the logging winch in 1983 were scaled to comply with the 198 1 core length which we believe to be the most accurate meas urement of the lengt h of the hole. This corresponds to a true vert i cal depth (TVD) of 203 3. 2 m as the difference between s lant hole length and TVD was 4.4 m in 198 1. In 1983, the difference was 7.4 m.At the end of ...

Effects of uncertainties in the geothermal heat flux distribution on the Greenland Ice Sheet: An assessment of existing heat flow models

“…In this region, where we have the largest misfit with observations, all GHF models provide relatively high GHFs between 58 and 82 mW/m 2 (tectonic model: 72 to 82 mW/m 2 , seismic model: 70 to 80 mW/m 2 , magnetic model: 58 to 75 mW/m 2 , and spatially uniform model: 63 mW/m 2 ). Such high GHF values contradict the findings of previous studies that suggest low GHFs (20 to 45 mW/m 2 ) in this region [ Dahl‐Jensen et al , 1998; Huybrechts , 1996; Greve , 2005; Gundestrup and Hansen , 1984; Lhomme et al , 2005; Tarasov and Peltier , 2003]. The results of the above mentioned studies are also supported by information about low GHF values obtained from the southern boreholes, Ivigtut and Ilimaussaq [ Sass et al , 1972].…”

“…These sensitivity experiments also reveal that the failures of all simulations in the southern part of the GIS can be attributed to both the overestimated GHF values in this region and inaccuracies in the past surface temperature forcing, in particular during the Holocene period. The results of previous studies [ Dahl‐Jensen et al , 1998; Huybrechts , 1996; Greve , 2005; Gundestrup and Hansen , 1984; Tarasov and Peltier , 2003] suggest that GHF values should be two to three times smaller than the values proposed by the addressed GHF models in southern Greenland. Although a significant decrease in Wisconsin and Holocene surface temperatures could possibly result in colder present‐day basal conditions in this region, the temperature forcing required to fit the observed basal temperatures is unrealistically cold and not supported by past climate reconstructions.…”

Strain-rate enhancement at Dye 3, Greenland

“…The temperature profile (Gundestrup and Hansen, 1984) shows that most o[the change in temperature happens in the lowermost 400 m. This results in an enhancement (through the temperature dependence o[ the flow laws) of a factor of 2 between 400 m and the bed. The data used for strain rate, stress and temperature are shown in Figure 3.…”

“…"a giv"Cll 1983, 1985 and 1986. Temperature, diameter and liquid pressure were measured in addition to inclination and azimuth (Gundestrup and Hansen, 1984; DahlJcnsen and Gundestrup, 1987). The surface temperature at Dye 3 is about -20 u C, and the near-bedrock temperature is -]30e.…”

A synthesis of the basal thermal state of the Greenland Ice Sheet

“…Multiple borehole‐temperature studies within the GrIS interior (>100 km from its margin) have generally reported a frozen ice sheet bed [ Weertman , ; Gundestrup and Hansen , ; Cuffey et al ., ; Dahl‐Jensen et al ., ] (Figure and Table ). However, these boreholes are often located at or near ice divides, where a frozen bed was predicted, so that a longer ice core record (in time) could be recovered.…”