## 2 Remote Sensing Technologies for the Monitoring of Coastal and River Delta Regions
### Ground Deformation Analyses  
In coastal and river delta regions, ground deformations due to subsidence are poorly understood in many cases. Ground subsidence of deltas can occur naturally due to sediment compaction, peat oxidation, and tectonics [94], but can be highly exacerbated by anthropogenic activities, such as reduced sedimentation due to embankments, over the withdrawal of sub-surface fluid [181], underground construction [182], and land reclamation [183; 184; 185; 186; 187]. Coastal subsidence can cause an increase in relative sea level rise (RSLR), increased flooding frequency and areal extent, permanent loss of land elevation, subsequent coastal line retreat, failure of coastal defenses, wetland loss, and amplified groundwater salinization. In some cases, the amplitude of ground subsidence may be much larger than absolute SLR, which may amplify local RSLR. Precise measurements of the Earth's surface deformations are routinely recovered by exploiting the Interferometric synthetic aperture radar (InSAR) technology. It relies on extracting the phase difference between complex-valued SAR images collected at separate times and from slightly different illumination angles [188; 189; 190]. Many advanced interferometric SAR methodologies [116; 118; 191; 192] have been developed to process heterogeneous sets of SAR acquisitions collected from different orbiting SAR constellation sensors during the last two decades. These research techniques have greatly advanced from the investigation of single deformation events [193; 194] to follow the time evolution of the ground deformations through multi-temporal differential InSAR (MT-InSAR) techniques [110; 111; 113; 116; 118; 119; 117; 116; 119; 118; 119; 120; 121; 122; 123; 124; 125; 126; 127; 128; 129; 130; 131; 132; 133; 134; 135; 136; 137; 138; 139; 140; 141; 142; 143; 144; 145; 146; 147; 148; 149; 150; 151; 152; 153; 154; 155; 156; 157; 158; 159; 160; 161; 162; 163; 164; 165; 166; 167; 168; 169; 170; 171; 172; 173; 174; 175; 176; 177; 178; 179; 180; 181; 182; 183; 184; 185; 186; 187; 188; 189; 190]. Historically, pioneering works developed for the analysis of ground deformations of coherent, pointwise targets (i.e., permanent scatters (PS)) were made by [PERSON] et al. [197; 198]. Many other PS-related works have been developed [112; 114; 199; 190; 191; 192; 193; 194; 195; 196; 197; 198; 199; 200; 201; 202], which increased the attainable measurement points' density. Other PS-like approaches have been used, e.g., [203; 204; 205]. Among the various techniques, the Small Baseline Subset (SBAS) algorithm [110] was developed to produce a ground displacement time series of distributed scatterers (DS) on the ground by processing multi-look interferograms. Many alternative methods for the analysis of DS targets [111; 115; 195; 206; 207; 208; 209; 210; 211; 212; 213], and the generation of 2D (3D) displacement maps from multi-satellite/orbit data [214; 215] have been designed. In the work of [216; 217], a methodology was also proposed to employ the phase values of intermittent coherent DS targets. The recent work of [119] proposed an extension of the canonical SBAS approach to better operate in mid-to-low coherent scenarios, based on Weighted-SVD (WSVD), which exacerbates the concept of intermittent coherent targets and produces reliable displacement time series with variable sample length. Recently, another class of InSAR methods that explore the temporal relationships of a set of interferograms has also been proposed to enhance the InSAR performance [109; 218; 219; 220; 221; 222; 223]. InSAR products are primarily used to monitor ground stability and perform analyses of the potential risk in zones subjected to natural or anthropogenic causes of ground displacements. Interested readers can find an in-depth description of such methods and a series of applications in heterogeneous scenarios in [76; 190; 224; 225; 226; 227; 228; 229; 230; 231; 232; 233; 234; 235; 236; 237; 238; 239; 240; 241; 242; 243; 244; 245].