EM-31 thickness

 

Indirect measurements of ice thickness were completed with a Geonics EM-31 electromagnetic induction (EM) device. The instrument determines the apparent conductivity of the underlying medium based on measurements of the secondary electromagnetic field induced by a transmitter coil in the ice and the seawater underneath. Owing to low summertime sea ice conductivities (<50 mS m-1) and high seawater conductivities (>2500 mS m-1), the signal is controlled by eddy currents generated at the ice/seawater interface. The two coplanar transmitter and receiver antenna coils are mounted at a spacing of 3.66 m and operate at 9.8 kHz. The entire instrument was mounted in a polyethylene kayak hull to allow for towing by hand or snowmobile across the ice surface and melt ponds.

To assess the amount and rate of ice ablation, EM data have been obtained along five main profiles (between 60 and 900 m in length) at 5-m spacing and at several additional sites. Apart from a set of measurements completed before the onset of the melt season and after the start of fall freezeup, repeated sampling along each of the profiles at intervals ranging between 1 and 3 weeks provided data on the progression of melt throughout the summer.

Ice thicknesses were derived from apparent conductivity measurements through inversion of an empirical ice-thickness/conductivity relationship based on a large data set collected over Arctic first-year and multiyear summer sea ice (Haas et al. 1997, Haas; unpublished). For further validation, concurrent ice thickness and EM measurements have been obtained at 74 ablation gauge sites. Furthermore, based on ice-core and upper ocean conductivity measurements, a one-dimensional, two-layer conductivity model has been employed to further assess sources of error and the reliability of the data set.

The accuracy of the EM-31 instrument is specified at better than 1 mS m-1. Based on the first derivative of the empirical thickness-conductivity relations, the sensitivity of the method has been derived as 0.015 m for 3-m-thick level ice and 0.09 m for 5-m-thick level ice. Uncertainties in the distance between the instrument and the ice surface and in coil orientation increase the total measurement error to approximately 0.05 m for 2-m-thick ice when compared against profile drillhole ice thickness measurements (at a point spacing and profile length that captures the relevant ice roughness features; Haas et al. 1997). Taking into account errors associated with drill-hole measurements and averaging over repeat profile measurements results in an error estimated at approximately 0.05 m (ice thickness 2 m) for the present ice ablation data set. Because derivation of ice thicknesses in ridged ice sections with keel depths >7.5 m or level ice thicknesses >6 m requires 2-D modelling and further data analysis, ablation measurements have been restricted to ice thicknesses smaller than these critical thresholds.

Thickness data have been obtained repeatedly from the start  to the end of the ablation season along a total of 13 profiles  at seven different locations. Except where noted otherwise, the  co-location error between the same point at different dates is less than 2 m for all SEA, TP, and APL profiles, and less  than 5 m for the ATL and MAI profiles. Three parallel profiles have been  measured perpendicular to the lead's edge. Profile AL1 is at  the center and originates at ablation gauge 300 (originally  about 5 m away from the lead's edge), profile AL2 is 10 m  closer to the ship and profile AL3 is 15 m further from the ship.

The EM-31 sites are:

ATL: Atlanta. The profile starts at the ablation gauge site  and extends along the snow depth profile towards Tuk.

MAI: Main snow depth/ice thickness profile. This profile  starts at post no. 5 near the Seattle ablation gauge sites and  continues along the main snow depth profile across The  Ridge past Pittsburgh towards the Generator Hut.

SEA or SEn, where n is an integer from 1 to 5: Seattle  Lead. Four to five parallel profiles have been measured  perpendicular to the lead's edge. Profile SE1 is near the  center and runs past the ablation gauges 306 to 310 (see Comments for exact locations). The other profiles are  parallel to SE1 spaced from 5 to 20 m apart on either  side.

TP1 or TP2 or Pxy, where x and y are either 1 or 2: Ice  surface topography profiles number 1 and 2. Profile 1 was  located in level second-year ice parallel to the ocean hut line  towards Sediment City. Profile 2 was located in deformed  multiyear ice extending at a right angle from the Pass  (through the active ridge separating the Des Groseilliers  from the l main camp) towards the logistics  huts (for details on these profiles see metadata file on sea ice topography measurements). In the filename EMdddPxy, x indicates the number of the surface  topography profile and y indicates the measurement run  along that profile (which was occasionally repeated running  in the opposite direction). Note that the profiles for day 239  were obtained after melt ponds had started to freeze. Thus, ice thicknesses indicated for all profiles EM239 include the thickness of the meltwater layer in underlying ponds  (i.e., in contrast with convention that applies to all other  data sets).

TUK: Long ice thickness profile originating at the Tuk ablation gauge site, traversing an area of deformed ice, and after crossing a ridge onto a neighbouring floe, continuing along through mostly level second-year ice towards a ridge at the end. Note that, due to losses of markers along  this profile, the co-location error for the four data files may  be >50 m in spots such that no direct comparison of  individual morphological features is warranted. Note that  the profiles for day 243 were obtained after melt ponds had  started to freeze. Thus, ice thicknesses indicated for profile  EM243TUK include the thickness of the meltwater layer in  underlying ponds (i.e., in contrast with convention that  applies to all other data sets).

 

 

Ice thickness profiles along the snow mainline from EM-31 measurements.

 

Ice thickness distribution before and after melt measured along the snow mainline using EM-31.

 

 
 
                                          

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