No single instrument captures subsidence well. Extensometers reveal the mechanism at a point; InSAR maps the extent; GPS and leveling tie it to a vertical datum. The strongest monitoring programs combine them — InSAR for where, extensometers for why, water levels for the driver.
A pipe anchored at depth measures the change in distance to the surface — i.e., compaction of the sediment column above the anchor. Sub-millimeter precision, continuous. Nested extensometers resolve which depth intervals are compacting. The only direct measure of the mechanism (Riley 1969).
Best for: mechanism, parameter estimation (page 07), calibration.
Interferometric synthetic aperture radar differences satellite radar phase over time to map surface deformation across whole basins at ~tens-of-meters pixels and cm-to-mm precision. Revolutionized subsidence mapping (Galloway et al. 1998); the backbone of California's statewide SGMA monitoring.
Best for: spatial extent, change detection, fissure-risk screening.
Permanent stations give continuous, absolute vertical position at a point to better than a centimeter. Anchors InSAR to a reference frame and validates it. CGPS networks (e.g., PBO/Network of the Americas) provide free regional control.
Best for: absolute reference, InSAR calibration, trends.
Repeated precise leveling of benchmark networks is the historical record (Poland's San Joaquin lines) and still defines long baselines. Labor-intensive and episodic, but unmatched for multi-decade continuity.
Paired with extensometers, multiple-depth piezometers give the head record at the same place compaction is measured — exactly the pairing Riley's stress–strain analysis needs (page 07).
Persistent-scatterer InSAR sharpens precision over stable reflectors (urban areas, infrastructure); distributed fiber-optic strain sensing in boreholes is an emerging depth-resolved complement to extensometers.
| Method | What it measures | Coverage | Vertical precision | Depth-resolved? | Temporal |
|---|---|---|---|---|---|
| Borehole extensometer | Compaction of column above anchor | Point | ~0.1 mm | Yes (if nested) | Continuous |
| InSAR | Surface deformation | Basin-wide | ~5–10 mm | No (surface) | Days–weeks (revisit) |
| Continuous GPS | Absolute 3-D position | Point | ~3–10 mm | No (surface) | Continuous |
| Spirit/GPS leveling | Benchmark elevation change | Lines/network | ~1–5 mm | No (surface) | Episodic (years) |
| Multi-depth piezometers | Head (→ effective stress) | Point | n/a (head) | Yes | Continuous |
InSAR finds where subsidence is happening and how fast. Extensometers and nested piezometers explain why at key sites and supply the parameters (page 07). GPS and leveling tie everything to an absolute vertical datum. Together they let a GSA map the indicator, attribute it to specific aquifers, and calibrate a predictive model.