Kriosfera Zemli, 2018, vol. XXII, No. 3, pp. 16–23 http://www.izdatgeo.ru DOI: 10.21782/EC2541-9994-2018-3(16-23) RESULTS OF GROUNDPENETRATING RADAR INVESTIGATIONS ON THE NELLA FJORD SEA ICE (PROGRESS STATION AREA, EAST ANTARCTICA) IN THE 2016/17 AUSTRAL SUMMER FIELD SEASON S.V. Popov1, V.L. Kuznetsov2, S.S. Pryakhin2, M.P. Kashkevich3 1Polar Marine Geosurvey Expedition (PMGE), 24, Pobeda str., Lomonosov, St. Petersburg, 198412, Russia; [email protected] 2Arctic and Antarctic Research Institute, 38, Bering str., St. Petersburg, 199397, Russia 3Saint-Petersburg University, 7-9, Universitetskaya emb., St. Petersburg, 199034, Russia The paper presents the main results of the fi eld investigations using multi-frequency (150 MHz, 270 MHz, 400 MHz and 900 MHz) ground-penetrating radar profi ling and georadar sensing by the common depth point method.The investigations have been carried out on the fast sea ice of Nella Fjord (Eastern Antarctica) during the 2016/17 austral summer fi eld season. The studies have included seawater salinity measurements. The sound- ing of sea ice with thickness of about 1 m thick have proved to be most eff ective at the radar frequencies which correspond to the wavelengths in a meter range. Besides the intensive refl ections from the lower edge of sea ice, the ground-penetrating radar data have revealed a boundary between fresh- and saltwaters whose position has been corroborated by the salinity measurements. The ground-penetrating radar technique in CDP mode has served as a basis for the subsurface velocity model showing that the eff ective dielectric constant for sea ice equals 10, which is accounted for intense melting of sea ice and its saturation with fresh water. The value of eff ective permittivity the fresh water layer is found to be 75, which may have been prompted by roughness of the lower edge surface of sea ice. East Antarctica, Progress Station, ground-penetrating radar sounding, sea ice, common depth point, seawater salinity INTRODUCTION The research focused on Arctic and Antarctic sea to high conductivity of seawater, it creates a shield ice contains both fundamental scientifi c interest and for electromagnetic waves, impeding thereby the ra- specifi cally applied tasks, such as unloading of ships, dar’s penetration to the entire ice thickness. This cir- construction of ice runway for receiving the aircraft cumstance largely constrains the range of the GPR and providing for other types of logistics. While con- applicability within the focus of this discussion. Be- sidering drilling ice cores to be the most reliable sides, the applied interest in sea ice is related primar- method of studying the natural environments and ily to the summer fi eld season, which is coincident their evolution, we should admit its complexity: in with the peak of logistical activities in Antarctica. the context of Antarctica, it is a signifi cant logistical However, on account of the specifi c environmental exercise, and requires much eff ort, time and patience conditions of polar latitudes, this time period is also from the researchers. Besides, this method allows ac- associated with the onset of the intense melting of sea quiring information only from selected points, where- ice, promoting its signifi cant saturation with fresh as geophysical surveys provide more complete infor- water and, as a result, the desalination of the surface mation along the profi le even after drilling. In this layer of sea water. The eff ect of fresh water consists in sense, electromagnetic soundings recognized as high- reducing salinity and therefore conductivity of sea ly effi cient and economic operations, appear more at- ice, and ultimately, the shielding properties of seawa- tractive. ter, making it possible to eff ectively use electromag- The numerous publications devoted to the GPR netic methods. studies of sea ice not only provide results of fi eld mea- The experimental verifi cation of the above con- surements, but also discuss the theoretical aspects of siderations was implemented through the acquisition this method. Among them, the most recent and sig- tests (AT) performed on the sea ice of Nella Fjord near nificant publications are: [Finkelshtein et al., 1977; Russian research station Progress (East Antarctica) Haas et al., 1997, 2009; Lebedev and Sukhorukov, during the austral summer field season of the 62nd 2001; Haas, 2004; Bobrov et al., 2008; Galley et al., Russian Antarctic Expedition (RAE, 2016/17), which 2009; Holt et al., 2009; Panzer et al., 2010, 2013; Lee et encapsulated the research works commenced during al., 2011]. Despite all its advantages of this progres- the 60th RAE season (2014/15) in the area of the sive geophysical method, however, the GPR tech- Molodyozhnaya fi eld base [Popov and Polyakov, 2015] nique has a number of drawbacks. In particular, due and continued in the RAE 61st season (2015/16). Copyright © 2018 S.V. Popov, V.L. Kuznetsov, S.S. Pryakhin, M.P. Kashkevich, All rights reserved. 16 RESULTS OF GROUNDPENETRATING RADAR INVESTIGATIONS ON THE NELLA FJORD SEA ICE MULTIFREQUENCY GPR PROFILING however, if the branches of the hyperboles generated The works were carried out on January 5, 2017 for diff racted waves were extended upwards, these on the AARI multi-year oceanographic profi le in the will be found in the interval between boundaries 1 point with coordinates 69°22.869′ S, 76°21.777′ E and 2, which allows an inference that the fi rst one is (Fig. 1, a). Prior to commencement of the AT works, related to the “ice–water” interface. a traverse line marked point-to-point throughout its Diff racted wave 4 identifi ed on the time section length (20 m), with its center located in the vicinity (Fig. 2, a) with its position corresponding to the pro- of the ice-hole (maina), the remnant of the arrange- fi le center, but slightly lower with respect to bound- ments for oceanographic measurements taken a day ary 2, was probably refl ected by the lower part of the earlier (Fig. 1, b). The GPR profi ling was performed ice-hole. The shift of apex 4 below the assumed ice- at several frequencies: 150, 270, 400 and 900 MHz. water boundary may be due to the GPR track slightly The fi rst and third frequencies were intended for the deviating from the hole, without passing directly “OKO-2” GPR (LLC “Logic systems”, Russia) using above it (Fig. 2, b), which results in an increase of a antennas AB-150 and AB-400M, respectively [Ra- raypath to the reflector [Popov and Kashkevich, diotechnical device…, 2009]. The extensible antenna 2015]. Besides, the refl ection is likely associated not AB-150 can be used for multi-off set GPR sounding. with the hole itself, but with fragments of ice gener- Investigations at the frequencies of 270 and 900 MHz ated during its cutting and localized in the low-velo- was performed using GSSI GPR (Geophysical Sur- city water column. There is no other evidence for the vey Systems Inc., USA) with SIR-3000 system mo- ice-hole on the time section. dule and accordingly 50270S and 3101 antennas Figure 2, b represents by a 270 MHz frequency [GSSI…, 2014]. The tools were carried while walking GPR time section resulted from horizontal fi ltering of at a steady pace along the profi le. 100 tracks, exhibiting numerous subhorizontal refl ec- Figure 2, a shows a 150 MHz frequency GPR tion events (probably including re-refl ections) pro- time-section. Due to the presence of correlated noise duced by the sea ice and, assumingly, by the medium of unknown origin, the fi ltering was performed using (seawater). The previously identifi ed boundaries 1 horizontal window of 100 traces. The observed refl ec- and 2 are present among them. Note that they repre- tions are characterized by a mosaic structure with sent a series of horizontal refl ections within the ~4 ns poorly distinguished two partially fi lter-suppressed interval of penetration. Their middle parts are regis- boundaries 1 and 2 located in the immediate proxim- tered approximately at the same time delays, which is ity to the maina, at 26.6 and 42.3 ns delays, respec- particularly evident after applying the horizontal re- tively (Fig. 2, a). fl ection fi ltering. The bottom part shows a large number of diff rac- In addition to 1 and 2, an intense reflection tion events 3, which may have been caused by diff rac- boundary 5 is distinctly observed in the upper part of tion from the ice blocks at the lower edge of the sea the section, which is likely to be related to the posi- ice. A.L. Novikov, the oceanologist from Mirny sta- tion of the horizon which supplies freshwater via tion (private communication), believes that they can seepage during the intense melting of sea ice cover. be linked with the anchor ice for whatever reason This boundary represents a series of horizontal refl ec- floated up beneath the surface ice. The diffraction tions characterized by about 4 ns penetration interval points are not distinctly discernible in the section, and their middle part position in the hole area at a Fig. 1. Location of the acquisition test works on the Nella Fjord sea ice. The area of wo rks is shown in yellow circle; the red line marks the fragment of survey line in the vicinity of the maina. Photograph by S.V. Popov, Junuary 2017. 17 S.V. POPOV ET AL. Fig. 2. GPR time sections resulting from the radar soundings using antenna frequencies of 150 MHz (а), 270 MHz (b), 400 MHz (c) и 900 MHz (d) during the acquisition works on the Nella Fjord sea ice. 1 – boundary between sea ice and seawater; 2 – boundary between fresh and seawater; 3 – diff ractions from inhomogeneities; 4 – diff ractions formed assumingly by the lower part of the maina; 5 – intra-ice boundary; 6 – noise from hand-held radio-station.