Measurement of Ice Flow Velocities from GPS Positions Logged by Short-Period Seismographs in East Antarctica Lei FU1,2,3, Jingxue GUO4 & Xiaofei CHEN1,2,3*

SCIENCE CHINA Earth Sciences

•RESEARCH PAPER• August 2021 Vol.64 No.8: 1381–1389 https://doi.org/10.1007/s11430-021-9765-6

Measurement of ice flow velocities from GPS positions logged by short-period seismographs in East Antarctica Lei FU1,2,3, Jingxue GUO4 & Xiaofei CHEN1,2,3*

1 Shenzhen Key Laboratory of Deep Offshore Oil and Gas Exploration Technology, Southern University of Science and Technology, Shenzhen 518055, China; 2 Department of Earth and Space Sciences, Southern University of Science and Technology, Shenzhen 518055, China; 3 Southern Marine Science and Engineering Guangdong Laboratory (Guangzhou), Guangzhou 511458, China; 4 Key Laboratory for Polar Science, MNR, Polar Research Institute of China, Shanghai 200136, China

Received January 21, 2021; revised March 8, 2021; accepted March 30, 2021; published online July 8, 2021

Abstract The ice flow velocity is a basic feature of glaciers and ice sheets. Measuring ice flow velocities is very important for estimating the mass balance of ice sheets in the Arctic and Antarctic. Traditional methods for measuring ice flow velocity include the use of stakes, snow pits and on-site geodetic GPS and remote sensing measurement methods. Geodetic GPS measurements have high accuracy, but geodetic GPS monitoring points only sparsely cover the Antarctic ice sheets. Moreover, the resolution and accuracy of ice flow velocities based on remote sensing measurements are low. Although the accuracy of the location data recorded by the navigation-grade GPS receivers embedded in short-period seismographs is not as good as that of geodetic GPS, the ice flow velocity can be accurately measured by these navigation-grade GPS data collected over a sufficiently long period. In this paper, navigation-grade GPS location data obtained by passive seismic observations during the 36th Chinese National Antarctic Research Expedition were used to accurately track the movement characteristics of the ice sheet in the Larsemann Hills of East Antarctica and the Taishan Station area. The results showed that the ice sheet in the two study areas is basically moving northwestward with an average ice flow velocity of approximately 1 m mon−1. The results in the Taishan Station area are basically consistent with the geodetic GPS results, indicating that it is feasible to use the embedded GPS location data from short- period seismographs to track the movement characteristics of ice sheets. The ice flow characteristics in the Larsemann Hills are more complex. The measured ice flow velocities in the Larsemann Hills with a resolution of 200 m help to understand its characteristics. In summary, the ice flow velocities derived from GPS location data are of great significance for studying ice sheet dynamics and glacier mass balance and for evaluating the systematic errors caused by ice sheet movements in seismic imaging. Keywords Short-period seismograph, Antarctic ice sheet, Ice flow velocity, GPS

Citation: Fu L, Guo J, Chen X. 2021. Measurement of ice flow velocities from GPS positions logged by short-period seismographs in East Antarctica. Science China Earth Sciences, 64(8): 1381–1389, https://doi.org/10.1007/s11430-021-9765-6

1. Introduction ocean (Fettweis et al., 2017) have increased significantly, and the cloud cover around Greenland (Hofer et al., 2017) Global warming is accelerating the mass loss of the Arctic decreases in summer. These changes have led to surface and Antarctic ice sheets (Oppenheimer, 1998; Chylek et al., runoff (Trusel et al., 2018), the formation and drainage of 2004; Hanna et al., 2005, 2008). In recent decades, the lakes on glaciers (Leeson et al., 2015; Palmer et al., 2015), temperatures of the air (Straneo and Heimbach, 2013) and iceberg collapse (Nick et al., 2012), and glacier retreat (Joughin et al., 2008). Holland et al. (2008) studied the atmospheric circulation over the North Atlantic and the * Corresponding author: (email: [email protected]).

© Science China Press and Springer-Verlag GmbH Germany, part of Springer Nature 2021 earth.scichina.com link.springer.com 1382 Fu L, et al. Sci China Earth Sci August (2021) Vol.64 No.8 changes in the Jakobshavn Glacier in Greenland and showed During the International Trans-Antarctic Scientific Expedi- that the acceleration of its mass loss was triggered by an tion (ITASE), China conducted 7 ground GPS resurveys increase in the temperature of the subsurface ocean on the from Zhongshan Station to the Dome-A section and carried west coast. Greenland, the Antarctic Peninsula, and parts of out high-precision GPS measurements by the observation West Antarctica are experiencing a moderate mass loss data of the third resurvey phase. Static positioning revealed (approximately 1 mm yr−1 equivalent of sea level rise) that the GPS points along the survey line are moving toward (Hanna et al., 2013); among them, Greenland’s ice loss ac- the northwest (the edge of the ice sheet) at a velocity of counts for the main contributors to global sea level rise. 8–24 m yr−1. Moreover, the closer the points are to the edge Model predictions indicate that in the general trend of cli- of the ice sheet, the faster the velocity, with the maximum mate warming, the mass loss in Greenland will continue reaching 100 m yr−1; at the same time, the flow of the ice (Pattyn et al., 2018). Shepherd et al. (2018) combined sa- sheet is causing a vertical subsidence rate of 0.2 to 1 m yr−1 tellite observations of volume, flow, and gravity changes (Wang et al., 2001). Zhang et al. (2008) used 19 repeated with an ice sheet surface mass balance model and showed GPS observations from Zhongshan Station to the Dome-A that the Antarctic ice sheet lost 27,200±13,900 billion tons section from 1996 to 2006 and calculated that the ice flow between 1992 and 2017, which is equivalent to an average velocity on the section gradually increases from inland to the increase of 7.6±3.9 mm in sea level. coast. The ice flow velocity in Dome-A is less than The ice flow velocity is a basic feature of glaciers and ice 10 m yr−1, while the ice flow velocity in the ice plateau is sheets: it characterizes the rate of ice transport from inland to 8–24 m yr−1, and in some local areas, the ice flow velocity coastal areas, reveals the locations of ice transport channels, reaches 98.2 m yr−1. Furthermore, the direction of ice flow is and reveals how ice blocks evolve over time (Rignot et al., roughly perpendicular to the contour of the ice sheet surface 2011). Measuring ice flow velocity is very important for elevation, mainly directed towards the Lambert Glacier Ba- estimating the mass balance of ice sheets in the Arctic and sin. Antarctic. Traditional ice flow velocity determination However, ground stake and GPS measurements can obtain methods include field in situ and remote sensing measure- only discrete observations in a local area and cannot obtain ments. Field in situ measurements usually utilize stakes, large-area glacier velocity fields; hence, remote sensing snow pits and geodetic Global Positioning System (GPS) measurement methods based on optical or microwave remote data. Dorrer et al. (1969) studied the changes in the ice sheet sensing provide a new way to obtain the entire Antarctic ice flow velocity field by using 103 stakes on a 910-km survey sheet flow velocity field. Remote sensing measurements line across the Ross Ice Shelf in Antarctica. The results mainly use image feature tracking and correlation calculation showed that the rate of separation between the McMurdo Ice principles to calculate the flow velocities of glaciers by Shelf and Ross Ice Shelf is increasing, and almost parallel feature matching (Scambos et al., 1992; Frezzotti et al., movement occurs in the middle of the ice shelf. During the 1998; Chen, 2016). Heid and Kääb (2012) compared and second (1985/1986) and third (1986/1987) Chinese Antarctic analyzed six different feature matching algorithms for scientific expeditions, Xu et al. (1988) set up six glacier Landsat optical images of five glaciers around the world and movement monitoring points using stakes on the Nelson considered that the cross-correlation (CCF-O) and phase Island Glacier in the Antarctic Peninsula and, combined with correlation (COSI-CORR) are two of the most effective optical theodolites, measured the geodetic position and ele- matching methods for global glacier velocity monitoring. vation of each point; they concluded that the glacier is Nevertheless, optical remote sensing is susceptible to the flowing toward the sea at a velocity of 14.6 m yr−1. Mea- constraints of polar night, solar radiation, clouds and fog, surements with stakes have the advantages of simple op- image oversaturation, and matching algorithms; moreover, eration and a low measurement failure rate. However, the its accuracy is relatively low. In contrast, synthetic aperture time interval between repeating measurements is long, and radar (SAR) adopts an active microwave imaging mode that this approach requires considerable logistical support. In can realize all-weather ground observations. Consequently, contrast, GPS measurement technology has the advantages SAR is important for extracting glacier flow velocities and is of acquiring omnidirectional data under all weather condi- the main method employed to estimate glacier velocities at tions with low cost, high efficiency, high precision, etc. present. Goldstein et al. (1993) first estimated the glacier Hence, GPS techniques can be very convenient for the rapid surface velocity of the Antarctic ice sheet based on the co- measurement and real-time monitoring of glacier surface herence of interferometric SAR (InSAR) data. Pattyn and velocities in the Antarctic (Chen, 2016). Manson et al. Derauw (2002) calculated the surface velocity field of the (2000) collected 73 geodetic GPS points along the 2500-m Shirase Glacier by matching the small image cores of two contour line of the Lambert Glacier Basin from 1988 to 1995 complex SAR images; the glacier in the downstream part of and showed that the ice flow velocity at the exit of the glacier the Shirase Basin is close to equilibrium, showing a slight along the GPS point route varies from 0.5 to 63 m yr−1. negative imbalance. Rignot (2008) used Phased Array type Fu L, et al. Sci China Earth Sci August (2021) Vol.64 No.8 1383

L-band Synthetic Aperture Radar (PALSAR) data to calcu- single-mode GPS position data. late the velocities of glaciers near the Amundsen Sea in West During the 36th Chinese National Antarctic Research Antarctica. The results showed that between 1996 and 2007, Expedition (CHINARE36) in 2019/2020, we deployed 100 the velocities of Pine Island Glacier and Smith Glacier ac- short-period three-component seismometers with a corner celerated by 42% and 83%, respectively, while that of the frequency of 0.2 Hz on top of the ice sheet in the Larsemann Thwaites Glacier did not accelerate (although the glacier Hills, East Antarctica, for the first time. In addition, four expanded over time and the velocity of its eastern ice shelf short-period seismometers were deployed at Taishan Station. doubled). Zhou et al. (2014) used European Remote Sensing The recording length was approximately one month, and Satellite (ERS)-1/2 tandem data to extract and explore the each station recorded more than 4000 GPS location ob- movements of the Polar Record and Dalker Glacier Glaciers servations while recording seismic data. This paper uses the near Zhongshan Station. The results showed that the seasonal byproduct of these short-period seismographs (GPS position and interannual changes in the ice flow velocity in inland data) to estimate the ice flow velocity vector field in the areas are not very significant. The ice flow velocity at the seismic survey line area. The results are beneficial for re- monitoring point extracted by differential InSAR (DInSAR) search on ice sheet dynamics and glacier mass balance and was highly consistent with the measured value. Zhou et al. for systematic error assessments in seismic imaging due to (2015) used Environmental Satellite (Envisat) Active Mi- ice sheet flow. crowave Instrument (AMI) SAR (ASAR) data at 35-day intervals to extract ice flow movement information in the Grove Mountain area using DInSAR and offset tracking 2. Data acquisition methods. Unfortunately, the accuracy of the ice flow velocity based During the 36th Chinese National Antarctic Research Ex- on remote sensing measurements is not as accurate as that pedition (CHINARE36, 2019/2020), we deployed 100 short- based on geodetic GPS observations. For example, an In- period seismometers to establish a seismic survey line on the SAR-based ice velocity map has a nominal error range of ice sheet of the Larsemann Hills in East Antarctica for the 1–17 m yr−1 (Rignot et al., 2011), while the horizontal error first time. Figure 1a shows a contour map of the East Ant- of single-point geodetic GPS is less than 0.1 m yr−1 (Zhang et arctic ice sheet, the elevation of which gradually increases al., 2008). For inland ice sheets with flat terrain, due to the 4 km from the coastal Zhongshan Station to the inland lack of obvious surface features and the average annual Dome-A section. Zhongshan Station (69.3734°S, 76.3779° change in the ice flow velocity is less than 10 m yr−1, remote E) is a permanent station located on the outcropped bedrock sensing measurements based on InSAR may be invalid (An of the Larsemann Hills; the summer Taishan Station et al., 2015). Therefore, ground geodetic GPS measurements (73.8658°S, 76.9829°E) with an elevation of 2621 m is ap- of inland ice sheets are essential for research on the velocities proximately 520 km from Zhongshan Station. The summer of Antarctic continental ice sheets. Kunlun Station (80.4200°S, 77.1130°E) is located in Dome- Seismic instruments generally have a built-in navigation- A, and its ice elevation is 4087 m. The black arrows in Figure grade single-mode GPS receiver. During the operation of the 1a represent the direction and magnitude of the ice flow station, while recording seismic data, the equipment also velocity at each measurement point obtained by geodetic records the location of the station at regular intervals. GPS from 1997 to 2005. The annual ice sheet movement in Broadband seismic instruments generally record the loca- the Dome-A area is close to 1 m yr−1, and the ice flow ve- tions of stations every hour (or every day), while short-per- locity is close to 100 m yr−1 near the coast. According to iod seismometers often record the locations of stations at previous research, the ice flow velocity in most areas of the shorter intervals (for example, every ten minutes). Although ice sheet is 8–24 m yr−1, and the ice sheet mainly moves the positioning accuracy of these single-mode GPS receivers toward the northwest (Zhang et al., 2008). The red triangles is not as good as that of geodetic GPS, it is still possible to shown in Figure 1b represent seismic stations with a se- measure the ice flow velocity by single-mode GPS data paration of 200 m; the first station is approximately 8 km collected over a sufficiently long period (An et al., 2015). from Zhongshan Station, and its ice elevation is approxi- Den Ouden et al. (2010) used a single-mode GPS receiver to mately 200 m; the distance between the last station and study the ice sheet flow velocity of Svalbard. An et al. (2015) Zhongshan Station is approximately 25 km, and the eleva- used the GPS locations recorded by broadband stations in tion of the ice surface is approximately 650 m. The Dalker West Antarctica and the trans-section from Zhongshan Sta- Glacier is on the east side of the survey line, and the end of its tion to Dome-A to track the ice flow velocity. This research ice tongue flows into Prydz Bay at a rate of 100 m yr−1. After revealed that when the average annual velocity of the ice the observations of the first survey line, we placed seismic sheet is greater than 1 m yr−1, reliable ice sheet flow velo- station 251 (the yellow triangle in Figure 1b) on the bedrock cities can be obtained by several months of navigation-level in the Panda harbor of Zhongshan Station for one month of 1384 Fu L, et al. Sci China Earth Sci August (2021) Vol.64 No.8

Figure 1 (a) A contour map of the East Antarctic ice sheet. The black arrows in the figure represent the direction and annual magnitude of the ice flow velocity at the geodetic GPS points along the section from Zhongshan Station to Kunlun Station (quoted from Zhang et al., 2008). During the 36th Chinese National Antarctic scientific expedition (2019/2020), the short-period seismographs (red triangles) were deployed in (b) the Larsemann Hill ice sheet near Zhongshan Station; to the northeast of the seismic survey line is the Dalker Glacier, and the Polar Record Glacier is located to the west. The northern end of the survey line is Prydz Bay. The red boxes are the Russian Progress Station, Chinese Zhongshan Station, and Indian Bharati Station. The observation time of station 250 is 2019/12/06–2019/12/27, and the observation time of station 251 is 2020/01/01–2020/01/30. (c) Ice sheet area around Taishan Station, where station 288 is located. Purple areas represent exposed bedrock, and the black arrows represent the direction and magnitude of the ice flow velocity. observation. Figure 1c shows the seismic station deployed at data are divided into K segments at a certain time interval Taishan Station, whose elevation is relatively flat. Station (for example, 5 days), and the longitude and latitude of each

288 is located at Taishan Station, while three other seism- segment are Λk(t) and ϕk(t), respectively. In theory, each ometers are equally spaced from Taishan Station along a segment of longitude and latitude satisfies a normal dis- circle with a radius of 1.67 km. The short-period seismo- tribution; the longitude and latitude that deviate from the graph used in this experiment is a Smartsolo-BD3C-5 with a mean values by 2.5 times the standard deviation do not meet built-in Ublox-7Q single-mode GPS receiver with an accu- the normal distribution and should be eliminated. For ex- racy of 2 m. While the seismograph collects seismic wave- ample, the GPS longitude and latitude data recorded by form data, it simultaneously records the GPS location of the station 251 on bedrock are shown in Figure 2a and 2b, re- station every 10 min. spectively. The measurements inside the black dashed ellipses are abnormal points that do not satisfy the normal distribution, and thus, they are eliminated. The red histogram 3. Data processing represents the GPS location data from January 1, 2020, to January 05, 2020; the green histogram represents the GPS In the process of seismic data analysis, it is often assumed location data from January 26, 2020, to January 30, 2020. that the location of the seismic station is fixed; if the When the seismic station is located on bedrock, for both movement of the ice sheet (or glacier) is not considered, longitude and latitude, the distribution characteristics do not systematic errors may be encountered during seismic ima- change over time. Figure 2c and 2d show the GPS location ging, so it is necessary to use the GPS location recorded by data from seismic station 250 located on the Larsemann Hills the seismograph to track the movement of the ice sheet. GPS ice sheet: The red histogram represents the data during the location data are continuously recorded during seismic data period from December 06, 2019, to December 10, 2019; the acquisition, and the longitude and latitude are represented by green histogram represents the data from December 23, Λ(t) and ϕ(t), respectively. First, the original GPS location 2019, to December 27, 2019. For this seismic station located Fu L, et al. Sci China Earth Sci August (2021) Vol.64 No.8 1385

Figure 2 The histograms of the original GPS locations recorded by two seismic stations. The (a) longitude and (b) latitude distributions of station 251 located on an outcrop during the period 2020/01/01–2020/01/30, the black dashed ellipses represent 2.5 standard deviations. The (c) longitude and (d) latitude distributions of station 250 located on the Larsemann Hills ice sheet during 2019/12/06–2019/12/27. The red and green histograms represent the GPS data at the beginning and end of the observation period, respectively. on an ice sheet, its longitude distribution characteristics shift zero; however, the calculated maximum displacement in over time, while the latitude distribution only slightly Figure 3a is approximately 0.2 m. Therefore, the error of changes. This implies that seismic station 250 on the ice tracking the ice sheet flow by position data from the single- sheet moved westward over time. mode GPS receiver embedded in the short-period seismo- To quantify the direction and displacement of the ice sheet, graph is approximately 0.2 m mon−1. Figure 3b shows the the average longitude and latitude during the first time period GPS position coordinates of seismic station 250 located on the Larsemann Hills ice sheet in different time periods. As are recorded as 1 and 1, respectively, and the average longitude and latitude for the kth segment are recorded as time passes, its average position moves approximately 1 m k westward. and k. After a coordinate transformation, the geodetic coordinates corresponding to the first and kth segments are (X1, Y1) and (X2, Y2), respectively. The corresponding flow di- 4. Results and discussion rection and displacement of the ice sheet are expressed as: YY The GPS position data of all stations are divided into several =arctan 2 1 , (1) XX2 1 sections at equal intervals. After deleting the abnormal

2 2 points, the direction and magnitude of the ice sheet at each s= ( X2 X 1) + ( Y2 Y 1). (2) station’s location are estimated according to eqs. (1) and (2), For seismic station 251 located on bedrock, the GPS lo- respectively. The results are as follows. cation data are divided into five segments at a time interval of 6 days. In Figure 3a, the GPS location coordinates of dif- 4.1 Ice flow velocity in the Taishan Station area ferent time periods are represented by red, green, blue, yellow, and cyan, and the corresponding average value of each From January 1, 2020, to January 30, 2020, we deployed 4 segment is represented by a large circle. Theoretically, when short-period seismic instruments in the Taishan Station area. the seismic station is located on bedrock, the displacement is Station 288 is located at the Taishan Station site. The other 1386 Fu L, et al. Sci China Earth Sci August (2021) Vol.64 No.8

Figure 3 The ice flow direction and displacement during the observation period for (a) seismic station 251 located on the bedrock of Panda harbor of Zhongshan Station (its maximum displacement is approximately 0.2 m) and (b) seismic station 250 located on the Larsemann Hills ice cap (its maximum displacement is approximately 1 m). The average positions of seismic stations in different time periods are represented by red, green, blue, yellow, and cyan circles. three seismographs are equally spaced along a circle with a stations 40–100 exhibit good continuity; its flow direction is radius of 1.67 km. The GPS location data recorded by these approximately 270°, indicating that the overall flow direc- four seismometers were processed, and the calculated ice tion of the ice sheet here is northwestward. However, the ice sheet flow velocities are shown in Table 1. The ice flow azi- sheet flow directions at stations 1–39 vary from 180° to 270°, muth at Taishan Station is basically 273°, and the magnitude and the difference at some stations reaches 90°, indicating of the ice flow velocity during the observation period is ap- that the ice flow of this sheet is relatively complicated. proximately 1 m. The ice flow direction indicates that the ice Figure 4b shows the corresponding ice sheet flow rates. The sheet at Taishan Station is moving towards the Lambert Gla- ice sheet flow rate during the observation period (2019/12/ cier Basin at a rate of 1 m mon−1; the terrain here is flat, and 06–2019/12/27) is basically approximately 0.5 to the direction of ice sheet flow is roughly perpendicular to the 1.5 m mon−1; the rate at some stations (such as stations 17 to contour, which means that its movement is affected mainly by 21) exceeds 1.5 m mon−1, and the corresponding ice sheet the gravity of the ice sheet. Zhang et al. (2008) employed flow direction is basically equal to 180°, indicating that the static GPS observation data for seven years (1998–2004) and corresponding ice sheet flow direction and displacement are reported the average ice sheet flow direction at Taishan Station quite different in some local areas where the ice flow velo- as 277° and the ice flow rate as 17.7 m yr−1 at a location 8 km city is somewhat complicated. from Taishan Station (see Figure 1a). These findings are ba- Figure 5 shows the Larsemann Hills ice flow velocities sically consistent with the ice sheet flow velocity field tracked estimated at a 7-day interval. The direction of the red arrow by the built-in single-mode GPS position data of short-period at each station represents the direction of ice sheet flow, and seismographs in this paper. the length of the arrow represents the ice flow rate of the ice sheet. The inland ice sheet with an elevation greater than 4.2 Ice flow velocity in the Larsemann Hills 450 m, that is, station numbers 40–100, flows northwestward overall, basically perpendicular to the topographic contour. Figure 4 shows the results of the ice sheet flow velocity in the This indicates that the glacier here flows toward the Polar Larsemann Hills. The horizontal axis in Figure 4a is the Record Glacier due to gravity. station number; the vertical axis is the direction of ice flow. In contrast, the elevation of the ice sheet near the coast The first seismograph is located at the northernmost end of (station numbers 1–39) is approximately 200 to 400 m. The the survey line, and the 100th seismograph is located at the surface topography on the east side of the Dalker Glacier is southernmost end of the survey line. Red, green, and black drastically different, and the annual ice flow velocity at the denote the ice flow velocity results with 5-, 6-, and 7-day of ice tongue reaches 100 m yr−1 (Zhou et al., 2014). In this segmentation, respectively. The results with 6- and 7-day of case, the ice sheet flow velocity is not controlled by only segmentation are relatively similar, but there are a few dif- gravity; in other words, the disturbance of the ice sheet ferences from the results with 5-day of segmentation. In movement by external extrusion stress cannot be ignored addition, the flow directions of the ice sheet calculated by (Schoof and Hewitt, 2016). Fu L, et al. Sci China Earth Sci August (2021) Vol.64 No.8 1387

Table 1 Ice flow velocity results in the Taishan Station area

Station Date Mean position Mean velocity # From To Lon (°E) Lat (°S) Azimuth (°) Rate (m mon–1)

288 2020/01/01 2020/01/30 76.98069 73.86422 271 1.05

247 2020/01/01 2020/01/30 77.01110 73.85274 274 1.16

267 2020/01/01 2020/01/30 77.01451 73.87502 273 1.06

314 2020/01/01 2020/01/30 76.92689 73.86447 274 0.90

Figure 4 The ice sheet (a) flow direction and (b) monthly average velocity for the seismic survey line in the Larsemann Hills, where the red, green and black colors denote the results calculated at 5-, 6- and 7-day intervals, respectively.

Figure 5 The ice velocity vector field in the Larsemann Hills. The direction and length of each red arrow represent the ice flow direction and monthly rate, respectively. The yellow arrow in the figure points to the Polar Record Glacier. 1388 Fu L, et al. Sci China Earth Sci August (2021) Vol.64 No.8

4.3 Influence of station movements on seismic ob- cation data logged by short-period seismic stations. servations During the one-month passive seismic observation period, the ice sheet in the Taishan Station area and the Larsemann During this passive seismic survey, the Larsemann Hills ice Hills flowed approximately 1 m; the impact of this move- sheet moved approximately 1 m in one month (Figure 4b). ment on seismic observations is negligible. However, if the This ice flow may have caused an error of approximately 1 m ice velocity is greater than a few hundred meters or if long- in the positions of stations or possibly an error of approxi- term observations are being recorded, this error may not be mately 1 m in the propagation distance of the seismic waves negligible. At the same time, considering the impact of recorded by each station. Moreover, this uneven ice flow snowfall, it is recommended that marker poles be placed caused a change in the distance between stations, up to a when deploying stations on Antarctic ice sheets to efficiently maximum of approximately 1.86 m (Figure 4, station 18). If and safely recover the stations. the shear-wave velocity of the ice sheet is approximately −1 1.85 km s , the abovementioned error has an effect of ap- Acknowledgements We acknowledge the Polar Expedition Office of proximately 1 ms on the arrival of surface waves, which can the Ministry of Natural Resources, the members of the 36th Chinese Na- be ignored; therefore, the ice sheet flow in the study area will tional Antarctic Research Expedition, and Professor Tong HAO from Tongji not affect the results of future surface wave tomography. In University for their support of this field experiment; we also thank Professor Meijian AN from the Chinese Academy of Geological Sciences and As- the case of ice velocities greater than a few hundred meters sociate Professor Gang QIAO from Tongji University for their valuable per year or with longer seismic observations, the error caused advice on the data processing and discussion of the results. This work was by the ice flow at the station position and the distance be- supported by the National Natural Science Foundation of China (Grant Nos. tween stations may be nearly equal to or greater than the 41974044, U1901602, 41790465, and 41876227) and the Science and close spacing between the stations in this article (200 m); Technology Project of Shenzhen (Grant No. KQTD2017081011725321). accordingly, we will need to carefully evaluate the sys- References tematic errors caused by ice sheet flow in the seismic data processing. 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