Since the early 20thcentury, the southern Mediterranean sandy coasts of North Africa (i.e., Tunisia, Libya, and Egypt) have experienced an increasing trend of rare extreme rainstorms, stressing the spatial variability of hydroclimatic conditions across the Mediterranean basin. Notably, Storm Daniel struck the eastern coast of Libya on September 10, 2023, resulting in deadly flash floods, principally after the collapse of two dams upstream of the city of Derna. Therefore, using Sentinel-1A C-band SAR images and the Coherent Change Detection method, we characterize and map the erosion and sediment load dynamics resulting from these flash floods over a 115 km × 40 km area, including Derna. Our findings highlight an increased vulnerability of coastal cities located at watershed outlets that need nature-based solutions to mitigate these risks.

Understanding the impacts of sparse rainstorms on surface erosion and soil moisture changes in hyper-arid deserts offers a unique insight into their dynamical evolution under intensifying hydroclimatic extremes. However, the spatiotemporal distribution of these changes remains poorly identified due to the low spatial resolution of orbital radiometers (a few kilometers) and the scarce temporal acquisition of orbital SAR observations. To mitigate these limitations, we propose an approach integrating three processing techniques, forming a decision-tree combining Sentinel-1 C-band radar interferometric coherence with Sentinel-2 multi-spectral soil moisture indices.

This study leverages the YOLOv8m computer vision algorithm with Ground Penetrating Radar (GPR) for enhanced aquifer detection in simulated Qatari terrains. Using image processing techniques like the Canny operator, a detection accuracy of 72% was achieved. Different radar resolutions were tested, highlighting the future potential of Transfer Learning for improvement. While simulations show promise, real-world deployment remains an essential next step.

Shallow aquifers are the primary water source to mitigate rising hydroclimatic fluctuations in arid areas, notably in North Africa and the Arabian Peninsula. The occurrence and dynamics of these expansive water bodies remain poorly characterized due to the reliance on sporadic monitoring wells. To address this deficiency, several studies are exploring the potential of low Earth orbit sounding radars as a large-scale mapping tool that can provide unique insights into the delineation and dynamics of these aquifers. Herein, we analyze the detectability of shallow aquifers (<10 m deep) using the radiometric analysis of surface reflections from a 45-MHz orbital sounder with an 8-MHz bandwidth. We use the ray tracing method to simulate the radar return from two realistic geoelectrical and topographic models of shallow aquifers in North African Sahara desert for omnidirectional and distributed array configurations. Our results suggest that the dielectric change induced by shallow aquifers that are up to 10-m deep can increase the 45-MHz radar surface return of the desiccated desert surface by 5 dB in areas with very low surface roughness of rms height <0.35 m. These preliminary results suggest a constrained potential for a monostatic VHF reflectometry to probe large sedimentary basins, which a distributed architecture can improve.

Large-scale characterization of water table depth in shallow aquifers in hyper-arid areas provides crucial insights into groundwater dynamics under increasing anthropogenic discharge and climatic fluctuations. Due to their penetration capabilities into arid soils, airborne VHF sounding radars can achieve this objective under specific system design, topographic and geophysical constraints, superseding sporadic well logs and ground-based surveys that provide compromised assessments of the distribution and depth of these water bodies. One of the least constrained ambiguities limiting the design of such systems, however, is the maximum penetration depth in desiccated sandy soils, which cover a sizeable fraction of desert landscapes. To constrain the latter, we perform a ground survey using 50-and-80-MHz GPRs with effective dynamic ranges of ~80-dB at the surface to probe the unconfined aquifer under desiccated linear dunes in the Wahiba Sands in Oman. Our survey resolves the water table down to at least 69-m depth, the deepest achieved at VHF frequencies in hyper-arid terrains. We observe the average two-way plane-wave subsurface radar attenuation, accounting for both dielectric and scattering losses, to range from 0.1-to-1.4-dB/m through these sandy formations. Dielectric and scattering losses can be of equal magnitude depending on the sounding frequency and stratigraphic setting of the subsurface. Penetration depths to the water table are validated with TDEM measurements and well-log data. Additionally, we identify shallow paleochannels from L-band SAR observations that suggest modern meteoritic recharge of the probed aquifer, creating shallow localized anomalous losses in the radar signal in the first few meters. We conclude that the minimum requirements for an airborne VHF sounding radar to probe shallow aquifers at depths of tens of meters in sandy formations in hyper-arid areas are an SNR of 55-dB at the surface, a bandwidth of 10-MHz, and a surfacehrmsnot exceeding 2 m.

Prior studies have shown that models and remote sensing data cannot accurately estimate snow emissivity due to limitations attributed to each of them. Hence, in this study, we merged Common Land Model (CLM) and snow emission (MEMLS) models with multi-frequency passive microwave data within an EnBS scheme to estimate snow emissivity. Its feasibility was tested via a test where passive microwave (at 1.4, 18.7, 36.5, and 87 GHz) measurements at the point scale were individually and simultaneously assimilated to estimate snow emissivity. The contribution of each channel in estimating the true snow emissivity is examined at the local-scale observation site of the National Aeronautics and Space Administration Cold Land Processes Experiment (NASA-CLPX) Field Campaign in Fall 2002-Winter 2003. All of the assimilated passive microwave measurements are found to contain complementary information for retrieving snow emissivity.

The Orbiting Arid Subsurface and Ice Sheet Sounder (OASIS) mission concept is proposed to explore the signatures of climate change beneath the surface of two of the least well-understood arid regions of the Earth: the polar ice sheets and the hyper-arid deserts. With these first-of-a-kind measurements of land ice and shallow aquifers, OASIS has two well-defined science objectives. The first is to determine the thickness, inner structure, and basal boundary conditions of Earth's ice sheets to understand their dynamics and to improve models of current and future ice sheet response to climate change and, hence, to better constrain ice sheet contribution to sea level rise. The second objective is to perform detailed mapping of the spatial distribution of shallow (<100 m deep) aquifers in the most arid regions on Earth to understand groundwater hydrology, enhance groundwater flow models, and provide new insights into available water resources and paleoclimatic conditions. These two mission objectives, which align closely with two NASA Earth Science program objectives on climate and water cycle, are achieved using measurements made by a single, low-cost and proven-heritage instrument: a 45 MHz center frequency radar sounder with 10 MHz bandwidth. The OASIS radar is similar to instruments on two successful Mars missions, Mars Express and Mars Reconnaissance Orbiter, presently probing the Martian subsurface.

In hyper-arid areas, soil moisture controls surface dust emissivity, surface run-offs during flash floods, aquifers recharge, and soil induration, as well as the biological diversity of these extreme environments. Of particular interest is assessing the soil moisture spatial distribution after a rare rainstorm event in fractured karstic environments. Therefore, we use three different remote sensing methods to map the soil moisture change as well as the volumetric water content following a storm event in the hyper-arid, unvegetated, and karstic Qatar Peninsula: (1) C-band SENTINEL1 SAR backscatter intensity difference as well as its interferometric coherence, (2) Principal Component Analysis SENTINEL2-multispectral moisture index, and (3) L-band SMAP Level3 radiometer. Our results suggest that transient soil moisture spatial patterns with volumetric water content higher than 0.12 cm3/cm3can persist longer than 48 hours following a major storm event with 50–100 mm precipitation in depressions. The above is a crucial step for assessing the origins and temporal evolution of soil moisture in Hyper-arid areas.

On 14 March 2019, Tropical Cyclone Idai generated a devastating flood incident in Mozambique with considerable socioeconomic consequences. 1.85 million people were affected, and 602 others were killed. Generally, this region has been prone to flood disasters. Hence, preparing flood susceptibility maps can play a significant role in reducing the negative impacts of floods on lives and properties of people as well as the critical infrastructures. In this study, 10 influential factors on flood occurrence were analyzed using the geographic information system (GIS) coupled with the synthetic-aperture radar (SAR) data and the hybrid gray wolf optimizer-group method of data handling (GWO-GMDH). The results indicated that the hybrid GWO-GMDH model outperformed GMDH in mapping flood susceptibility. According to the GWO-GMDH model, 19.40% and 15.52% of the study area was found to be high and very high susceptible to flooding, respectively. The generated maps are beneficial for decision makers in mitigating flood damages and risk management.

Subsurface scattering losses associated with very high frequency (VHF) subwavelength heterogeneities are crucial for assessing the total radar signal attenuation in fractured and heterogeneous environments such as carbonate formations, which represent one of the most predominant aquifer types in hyperarid areas. Yet, the resulting signal losses are largely unquantified due to the difficulties in constraining their sources, amplitude, and frequency dependence on the VHF spectrum, hence, compromising the ability to perform large-scale characterization of shallow aquifers using radar probing in these areas. To address this deficiency, we present an experimental model quantifying volumetric scattering losses in heterogeneous carbonate formations accounting for wavelength-sized structural elements and subwavelength-sized heterogeneities ranging from 1 mm to 2 cm for the VHF radar signals. In particular, we use an analytical model that estimates the overall scattering effects produced by the interaction of an electromagnetic plane wave with randomly distributed vugs scatterers. We then compare our analysis with field radar measurements collected in the karst limestone aquiferous formation in the Qatar Peninsula at 80 MHz (10-dB bandwidth (BW): 30-150 MHz). Our results estimate that the total losses in karstic environments range between ~0.6 and 1.4 dB/m, of which 20%-65% is due to volume scattering. Furthermore, we find that despite being usually underestimated, subwavelength volumetric scattering accounts for a considerable portion (~40%) of the overall radar attenuation: between ~0.045 and 0.35 dB/m. The results of our analysis can constrain the degree of karstification associated with vertical artesian movements in fossil aquifer systems in hyperarid environments.

Resolving structural elements in hyper-arid eroded karstic environments is crucial to assess their role as groundwater conduits governing the dynamic of these complex and least characterized hydrogeological systems. Of particular interest is the fine mapping of fractured carbonate sedimentary formations in Qatar Peninsula as it provides crucial insights into characterizing the dynamic of karst systems in fossil aquifers. To achieve this objective, we vectorize 3107 km of transverse tension fractures and joints at a 1:6000 scale, trending WNW-ESE and W-E around the N-S Qatar-Arch anticline in Qatar, using both L-Band SAR orbital scenes and high-resolution aerial photography. We distinguish three highly fractured areas of 15x15 km each that are correlated to areas showing upward leakage and groundwater mounding. Our preliminary results suggest these fractures to act as vertical and horizontal conduits for groundwater, from the different fossil aquifer systems in the Qatar peninsula. Validations using semi-empirical backscattering model derived from SAR and ground penetrating radar acquisitions as well as correlation to potentiometric and Total Dissolved Solids maps will be necessary to further quantify the phenomena.

Assessing the variability of surface roughness on planetary bodies is crucial for understanding the physical mechanisms that have governed their surface evolution. In addition, it provides constraints on surface texture uncertainties associated with landing, anchoring, surface trafficability, and sampling. Herein, we present how orbital bistatic radar (BSR) observations can be successfully conducted during occultations, and thereby used to achieve the above scientific and technical objectives for key planetary bodies that are under investigation by current and future missions. For different planetary bodies without dense atmospheres, we calculate the theoretical differential Doppler shift (δf) between the direct and surface-scattered signals that precede and follow radio occultations during opportunistic BSR experiments. For occultations that last ~10 min, we find that small bodies with diameters ≲100 km will exhibit |δf| ≲5 Hz, and larger bodies with diameters ≳500 km will have |δf| ≳50 Hz. Our results suggest that opportunistic BSR observations of targets ≲100 km in diameter will require an ultrastable oscillator (USO) aboard the spacecraft to retrieve surface echoes and constrain surface roughness.

The identification and characterization of small-scale subsurface heterogeneities is crucial for fully understanding the total radar signal attenuation in complex and heterogeneous environments. In this study, we present a comparative analysis of the volumetric scattering effects of subsurface heterogeneities with sub-wavelength sizes (i.e. 1 mm to 2 cm) on VHF radar signals. In particular, we provide a first approximation of the scattering losses by using an analytical model considering randomly distributed vugs scatterers. We then compare our analysis with field radar measurements collected in the karst limestone aquiferous formation of Qatar at 80 MHz (10-dB Bandwidth: 30-150 MHz). Our results estimate that total losses in karstic environments range between 0.61 and 1.392 dB/m, of which 20 to 65% is due to scattering. Furthermore, we find that despite being usually underestimated, sub-wavelength volumetric scattering accounts for a considerable portion of the overall radar attenuation: between 0.045 to 0.35 dB/m.

To assess the detectability of fossil aquifers using sounding radars, we investigate the VHF attenuation characteristics of the most common sedimentary layers overlaying shallow groundwater systems in hyper-arid deserts. In particular, we quantify the dielectric and scattering losses using GPR profiles acquired over mafic dunes and sand sheets in Oman, and on the karstic limestone in Qatar, which both present radar characteristics representative of substantial portions of the upper layer of most desert environments of North Africa and the Arabian Peninsula. Understanding the amplitude and spatial variability of these losses will significantly support the ongoing development of airborne and orbital VHF sounders dedicated to large-scale groundwater mapping in hyper-arid areas [1]. Our results suggest that VHF radars with 80-100dB effective dynamic range at the surface are capable of probing the water table in hyper-arid sedimentary environments up to 70-120 m of depth, hence covering most of the shallow aquifers located in the considered area. We observe that dielectric absorption is the main loss factor in aeolian deposits, while volume scattering, controls ~60% of the overall losses in karstic environments.

Radar sounding from aircraft or ground-coupled radars has long provided scientists with a powerful technique to sound through ice layers to retrieve local depth and layering structure. More recently, it has been used to detect shallow aquifers in warm, dry, and desert regions. At Mars, a long-wavelength radar sounding from low orbit altitudes has produced global maps that reveal the presence of ice layering at all latitudes and glacial deposits on the flanks of volcanoes. Until now, sounding from the earth orbit at wavelengths long enough to penetrate ice sheets and arid sand was thought to be infeasible, because of the electromagnetic properties of the ionosphere. In this paper, we show that a radar sounding at frequencies as low as 45 MHz is, in fact, theoretically possible under viewing conditions that occur often enough to be practical. This conclusion opens up a previously unutilized portion of the electromagnetic spectrum for large-scale, spaceborne remote sensing of subsurface features on the earth.

Monitoring ground deformations arising from groundwater dynamics in dense urban coastal terrains is crucial for the sustainable development of infrastructures in these highly populated areas. The city of Montreal, which is located in the Saint-Laurent plain in eastern Canada, with its fast-growing populations, is a unique case study for other similar cities in coastal terrains. The city undergoes high-level house foundation damages with densities reaching up to 89 repairs/km2resulting from time-dependent ground deformations that are correlated to groundwater dynamics and evapotranspiration. Using Radarsat-2 C-Band synthetic aperture radar interferometry, we observe 3- to 5-mm ground line-of-sight displacement variations temporally outphased by few months relative to the 2-m subartesian aquifer hydraulic head variations. The deformations are observed over a 60-km2area located in the central part of the Montreal Island in Canada, from 2008 to 2010. We observe displacements of ~1 mm/year uplift in the areas covered by 15-m-thick clay layer. These displacements are well correlated to the number of house repairs. We also observe ~2 mm/year subsidence on elevated terrains, associated with evapotranspiration. The amplitudes of the displacements observed during this two-year study are significant when integrated over the average lifetime of urban structures. We conclude that the observed ground deformations are related to the seasonal variation of hydraulic head in most of the areas of Montreal. Moreover, wetter climate forecasts over upcoming decades for this area, will accentuate groundwater level fluctuations; thus, more ground deformations are foreseen, and have to be considered in future infrastructure design standards.

The Orbiting Arid Subsurface and Ice Sheet Sounder (OASIS) mission concept is the first to directly explore the signatures of climate change beneath the surface of two of the least well-understood arid regions of the Earth: the polar ice sheets and the hyper-arid deserts (Figure 1). With these first-of-a-kind measurements of land ice and shallow aquifers, OASIS has two well-defined science objectives. The first is to determine the thickness, inner structure, and basal boundary conditions of Earth's ice sheets to understand their dynamics and to improve models of current and future ice sheet response to climate change and, hence, to better constrain ice sheet contribution to sea level rise. The second objective is to perform detailed mapping of the spatial distribution of shallow (<;100 m deep) aquifers in the most arid regions on Earth to understand groundwater hydrology, enhance groundwater flow models, and provide new insights into available water resources and paleoclimatic conditions. These two mission objectives, which align closely with two NASA Earth Science program objectives on climate and water cycle, are achieved using measurements made by a single, low-cost and proven-heritage instrument: a 45 MHz center frequency radar sounder with 10 MHz bandwidth. The OASIS radar is similar to instruments on two successful Mars missions, Mars Express and Mars Reconnaissance Orbiter, presently probing the Martian subsurface.

We combine ALOS-PALSAR coherence images with airborne LiDAR data, both acquired over the Piton de la Fournaise volcano (Reunion Island, France), to study the main errors affecting repeat-pass InSAR measurements and understand their causes. The high resolution DTM generated using LiDAR data is used to subtract out the topographic contribution from the interferogram and to improve the radar coherence maps. The relationship between LiDAR intensity and radar coherence is then analyzed over several typical volcanic surfaces: it helps to evaluate the coherence loss terms. Additionally, the geometric and physical properties of these surfaces have been measured in situ. Coherence deteriorates over pyroclastic deposits and rough lava flows due to volume and surface scattering. In the presence of vegetation, it is directly related to plant density: the higher the Leaf Area Index (LAI), the lower the coherence. The accuracy of InSAR measurements strongly decreases for LAI higher than 7.

Until recently the coarse resolution of topographic mapping acted as a break on understanding the forces and processes that shape the Earth's surface. However, active surface deformation is an important indicator for the earth crustal dynamics since it is directly linked to earthquakes, volcanic eruptions and landslides. Both airborne laser scanning systems (LiDAR) and spaceborne interferometric synthetic aperture radars (InSAR) have provided valuable information for many case studies requiring highresolution characterization of ground movement in relatively large areas to assess the threat and impact of natural hazards especially for volcanic eruptions. The Piton de la Fournaise volcano (Reunion Island, France) is one of the most active basaltic shield volcanoes in the world. It has reached an anomalous activity level in the past years with a major eruption occurring in April 2007. In this paper, we explore the statistical, spatial and temporal behavior of the L-Band backscattering coefficient at both HH and HV polarizations over different type of terrains in the Fournaise lava field as a function of the LiDAR intensity data. The correlation will be used in setting empirical models to correct for the L-Band phase distortion on ash and rough surfaces in volcanic terrains.

In the frame of the NETLANDER project, we have developed a ground penetrating radar (GPR) aimed at initial observations of the geological features in the deep Martian subsurface and the detection of potential liquid water reservoirs. Initial ground tests at 2, 3 and 4 MHz were recently performed on the Pyla Dune, which is a sand dune nearly 100 meter high along the south-west Atlantic coast in France. The horizontal reflecting layer located at the base of the dune together with the known permittivity value of the sand offer the opportunity to test the performance of the device in a rather well-documented and simple environment. These first experimental results are reported in the paper. We will focus on the precise measurement of the electric antenna characteristics and the analysis of the backscattered signals using both electric and magnetic components of the received field. Comparisons with numerical simulations taking into account the actual environment of the GPR are also presented for comparisons.

We present the capabilities of low-frequency radar systems to sound the subsurface for a site located in south-central Egypt, the Bir Safsaf region. This site was already intensively studied since the SIR-A and SIR-B orbital radars revealed buried paleodrainage channels. Our approach is based on the coupling between two complementary radar techniques: the orbital synthetic aperture radar (SAR) in C and L bands (5.3 and 1.25 GHz) for imaging large-scale subsurface structures, and the ground-penetrating radar (GPR) at 500 and 900 MHz for sounding the soil at a local scale. We show that the total backscattered power computed from L-band SAR and 900-MHz GPR profiles can be correlated, and we combined both data to derive the geological structure of the subsurface. GPR data provide information on the geometry of the buried scatterers and layers, while the analysis of polarimetric SAR data provides information on the distribution of rocks in the sedimentary layers and at the interface between these layers. The analysis of 500-MHz GPR data revealed some deeper structures that should be detected by lower frequency SARs, such as a P-band system.