Development of a subglacial lake monitored with radio-echo sounding: case study from the eastern Skaftá cauldron in the Vatnajökull ice cap, Iceland

We present repeated radio-echo sounding (RES, 5 MHz) on a profile grid over the eastern Skaftá cauldron (ESC) in Vatnajökull ice cap, Iceland. The ESC is a ∼ 3 km wide and 50–150 m deep ice cauldron created and maintained by subglacial geothermal activity of ∼ 1 GW. Beneath the cauldron and 200–400 m thick ice, water accumulates in a subglacial lake and is released semi-regularly in jökulhlaups. The RES record consists of annual surveys conducted at the beginning of every summer during the period 2014–2020. Comparison of the RES surveys reveals variable lake area (0.5–4.1 km2) and enables traced reflections from the lake roof to be distinguished from bedrock reflections. This allows construction of a digital elevation model (DEM) of the bedrock in the area, further constrained by two borehole measurements at the cauldron centre. It also allows creation of lake thickness maps and an estimate of lake volume at the time of each survey, which we compare with lowering patterns and released water volumes obtained from pre- and post-jökulhlaup surface DEMs. The estimated lake volume was 250 GL (gigalitres = 106 m3) in June 2015, but 320 ± 20 GL drained from the ESC in October 2015. In June 2018, RES profiles revealed a lake volume of 185 GL, while 220 ± 30 GL were released in a jökulhlaup in August 2018. Considering the water accumulation over the periods between RES surveys and jökulhlaups, this indicates 10 %–20 % uncertainty in the RES-derived volumes at times when significant jökulhlaups may be expected.

AN AUTOMATIC EXTRACTION METHOD FOR ANTARCTIC SUBGLACIAL LAKE BASED ON RADIO ECHO SOUNDING DATA

Detection of subglacial lakes and interpretation their hydrological connectivity is of great importance to understanding the mass balance of Antarctic ice sheet. Over the past five decades, a large number of Radio Echo Sounding (RES) data has been collected in Antarctica. However, the identification of subglacial lakes based on RES data mainly relies on visual interpretation due to the lack of quantitative indicators for subglacial lakes distinguishing. To solve this problem, an automatic subglacial lakes detection method based on the reflective characteristics of B-scan echogram is proposed in this paper. Firstly, the echo strength of the original radar echogram is corrected for the dielectric attenuation and geometric spreading in the ice. Secondly, the original radar echogram is binarized to preserve the bright subglacial lake region, and the thickness of bright pixels is measured along the direction of A-scan. Thirdly, the thickness, the variance of the thickness in the neighbourhood and the corrected echo strength are used to obtain the response value for the subglacial lake. Finally, a threshold for the response value is determined to detect subglacial lakes. It is found that the proposed method can determine the location of the subglacial lake in Antarctica's Gamburtsev Province (AGAP) region with high accuracy.

Development of a subglacial lake monitored with radio-echo sounding: Case study from the Eastern Skaftá Cauldron in the Vatnajökull ice cap, Iceland

We present repeated radio-echo sounding (RES, 5 MHz) on a profile grid over the Eastern Skaftá Cauldron (ESC) in Vatnajökull ice cap, Iceland. The ESC is ~3 km wide and 50–150 m deep ice cauldron created and maintained by subglacial geothermal activity of ~1 GW. Beneath the cauldron and 200–400 m thick ice, water accumulates in a lake and is released semi-regularly in jökulhlaups. The RES record consists of annual surveys with 200–400 m between profiles in early summers of 2014–2020. Comparison of the RES surveys (2D migrated profiles) reveals variable lake area (0.5–4.1 km2) and enables traced reflections from the lake roof to be distinguished from bedrock reflections. This allows construction of a digital elevation model (DEM) of the bedrock in the area, further constrained by two borehole measurements at the cauldron centre. It also allows creation of lake thickness maps and an estimate of lake volume at the time of each survey, which we compare with lowering patterns and released water volumes obtained from surface DEMs obtained before and after jökulhlaups. The estimated lake volume is 250 × 106 m3 in June 2015 but 320 ± 20 × 106 m3 drained from the cauldron in October 2015. In June 2018, RES profiles reveal a lake volume of 185 × 106 m3 while 220 ± 30 × 106 m3 was released in a jökulhlaup in August 2018. Considering the water accumulation over the periods between RES surveys and jökulhlaups, this indicates 10–20 % uncertainty in the RES-derived volumes at times when significant jökulhlaups may be expected.

Four decades of radar-echo sounding: The past, present, and future of radar applications for understanding subglacial environments

<p>Airborne radar sounding observations have been instrumental in understanding subglacial environments and basal processes of ice sheets. Since the advent of analog radar-echo sounding (RES) system in the early 1970s, there have been tremendous innovations in both RES hardware and signal processing techniques. These technological advancements have provided high-resolution ice thickness measurements, improved detection and characterization of subglacial hydrology, as well as improved understanding of basal thermal conditions, bed roughness and geomorphology, and other processes that govern the basal boundary of the polar ice sheets. In this talk, I will provide an overview of the recent developments in radar processing approaches and system designs and highlight some of the new understanding of ice sheet subglacial processes that emerge from these breakthroughs. I will end by discussing areas where future radar applications and discoveries may be possible, including the utilization of machine learning algorithms, space-borne radar missions, and ground-based passive radar platforms to provide long-term monitoring of ice sheet subglacial environments.</p>

Comparative overview of reservoir siltation assessment techniques depending on the type of sediment

<p>Over many decades it has become evident, that sediment accumulation threatens the fundamental operation of reservoirs by reducing the storage volume, hindering technical functions and deteriorating water quality over time. Most scientists, operators and authorities are aware of this, often “silent” but enduring process. However, not often mitigation measures are applied with foresight and in an appropriate manner according to this global problem. One fundamental reason for the often hesitant implementation of measures is the lack of precise and applicable assessment techniques. The type of reservoir, available historic data and especially the composition of the sediment may allow only for one available method to be applied successfully. In this study we present a workflow to select the best available method to detect the sediment thickness correctly. We compare topographic differencing, dual-frequency echo sounding, sub-bottom echo sounding, free-fall penetrometer measurements and sediment coring. Next to the general applicability, the precision (vertical resolution) and the time requirement for the measurement are relevant factors. A special point of discussion is the presence of free gas inside the sediment, often creating measurement errors, leading to underestimation of the sediment thickness.</p>

Reducing the residual error of multibeam sounding data through empirical mode decomposition based on cubic Bessel interpolation

The systematic residual errors present in multibeam echo-sounding data cause the areas of overlap between adjacent swaths to become distorted. A method is proposed in this paper to reduce the residual error of multibeam sounding data through empirical mode decomposition (EMD) based on cubic Bessel interpolation. Numerical experiments confirm that the discrepancy in the overlap between two swaths is significantly reduced after applying EMD improved by cubic Bessel interpolation compared with both the original water depth data and with data processed using conventional EMD based on cubic spline interpolation. The mean square error of the improved method is decreased by 67% and 29% compared with that of the original and conventional EMD cases, respectively. Therefore, EMD with cubic Bessel interpolation can efficiently reduce the residual error of multibeam echo-sounding data.

Direction-of-Arrival Estimation Methods in Interferometric Echo Sounding

Nowadays, there are two leading sea sounding technologies: the multibeam echo sounder and the multiphase echo sounder (also known as phase-difference side scan sonar or bathymetric side scan sonar). Both solutions have their advantages and disadvantages, and they can be perceived as complementary to each other. The article reviews the development of interferometric echo sounding array configurations and the various methods applied to determine the direction-of-arrival. “Interferometric echo sounder” is a broad term, applied to various devices that primarily utilize phase difference measurements to estimate the direction-of-arrival. The article focuses on modifications to the interferometric sonar array that have led to the state-of-the-art multiphase echo sounder. The main algorithms for classical and modern interferometric echo sounder direction-of-arrival estimation are also outlined. The accuracy of direction-of-arrival estimation methods is dependent on the configuration of the array and external and internal noise sources. The main sources of errors, which influence the accuracy of the phase difference measurements, are also briefly characterized. The article ends with a review of the current research into improvements in the accuracy of interferometric echo sounding and the application of the principle of interferometric in other devices.