C-DRONE GUIDE · 3 SEPTEMBER 2026

Sinkholes and underground gypsum quarries: drone surface monitoring for local authorities under a cavity risk plan, price

Beneath a good part of northern Île-de-France — Seine-Saint-Denis, Val-d'Oise and the Yvelines in particular — run kilometres of galleries dug from the 18th to the mid-20th century to extract gypsum for plaster of Paris. Many of these abandoned quarries, poorly mapped at the time, now expose close to 8,000 hectares to "fontis" risk: a localised ground collapse that occurs when gypsum, a highly soluble mineral, dissolves and weakens the pillars and walls of the galleries. Affected local authorities must map these cavities and work within a risk prevention plan (PPR) that constrains development and requires vigilance over existing buildings. For a municipal or inter-municipal technical department that must monitor a wide area with limited staff, aerial drone monitoring is a useful addition to this system — without ever replacing the geotechnical study that alone can rule on a cavity's condition. Here is exactly where that contribution fits, and the prices charged in 2026.

Published on 3 September 2026, reviewed on 11 September 2026 — regulations in force as of September 2026.

The sinkhole: how an abandoned gypsum quarry eventually gives way at the surface

Gypsum is one of the most soluble minerals in water: prolonged rainwater infiltration or a rising water table can gradually dissolve the pillars quarry operators left in place to support the gallery roofs, or hollow out voids within the rock mass itself. When the vault can no longer hold, it gives way suddenly, usually with little visible warning at the surface: the ground subsides locally, forming a crater that can reach several metres in diameter and depth within hours. This is what French authorities call a fontis (sinkhole), distinct from the widespread, gradual subsidence seen over other types of subsoil.

The Paris-region subsoil bears the mark of several centuries of gypsum extraction for plaster of Paris, from the 18th century to the mid-20th: the most heavily worked gypsum formations are concentrated north of Paris, in Seine-Saint-Denis, the Val-d'Oise and the Yvelines, home to one of France's largest underground gypsum quarries, beneath the Montmorency forest massif. In total, around 8,000 hectares are exposed to sinkhole risk across Île-de-France — a figure that gives a sense of the area affected local authorities must monitor, well beyond the parcels where a cavity is already known with certainty.

What the law requires of local authorities, and what the BRGM inventory already covers

Article L.563-6 of the French Environmental Code requires local authorities, or their competent inter-municipal bodies, to map the sites where underground cavities and marl pits liable to cause ground collapse are located. This obligation draws on BDCavité, the national inventory of abandoned underground cavities available on georisques.gouv.fr, part of national natural-risk prevention policy since 1981 and managed and developed by the BRGM geological survey since 2001. This inventory, built mainly from mining archives, reported incidents and ground-based reconnaissance campaigns, then feeds into hazard maps and ground-movement risk prevention plans (PPR) which, once approved, constrain local planning documents and building permits.

A recent example illustrates the process: the ground-movement PPRN for the municipality of Follainville-Dennemont (Yvelines), prescribed by prefectural decree on 15 June 2021, underwent a supplementary public inquiry from 14 June to 15 July 2024 after an additional cavity and rock face were identified, before being approved by prefectural decree on 28 November 2024. This timeline — several years between prescription and approval, with zoning adjustments along the way — shows how much knowledge of the subsoil keeps evolving: a PPR is built and corrected as discoveries are made, not settled once and for all.

What the drone adds: documenting the surface, not replacing the subsoil study

A drone cannot see underground: it detects neither a gallery nor a weakened pillar, and it in no way replaces the geotechnical and geophysical methods (boreholes, micro-gravimetry, ground-penetrating radar) that alone can rule on a cavity's real condition. What it adds sits entirely at the surface, precisely where a municipal technical department struggles most to cover a wide area with a small team: a photogrammetric survey repeated over time produces a precise, georeferenced digital terrain model, letting successive flights be compared to spot subtle subsidence or emerging depressions before they worsen — roads, green spaces, gardens within the hazard zone. A study by Francesco Gentili and Sergio Madonna, published in 2024 in the journal Geographies, pairs drone photogrammetry with low-cost lidar precisely for sinkhole detection and monitoring in urban areas; it finds this combination can quickly document hard-to-reach cavity entrances and produce reliable comparative monitoring, complementing — not replacing — traditional reconnaissance techniques (see the study on Google Scholar).

In practice, this contribution breaks down into three uses for a local authority under a cavity risk plan: a dated baseline map of the whole at-risk perimeter, useful for establishing the starting condition before any significant rainfall episode; periodic monitoring (typically annual, or tightened after heavy rain recognised as a triggering factor) comparing successive surveys to prioritise sectors for ground inspection; and emergency documentation after an actual collapse — the crater's extent and depth, the safety perimeter to establish — without exposing staff near a void whose stability is, by definition, uncertain.

Who this monitoring is for, and how it fits with the risk plan

Three profiles are chiefly concerned. The local authority or inter-municipal body under a ground-movement risk plan, which must both document the condition of its public assets (roads, green spaces, facilities) within the regulated perimeter and hold a dated record to present in the event of an incident. The social landlord owning housing stock in a hazard zone, for whom regular monitoring of the grounds around its buildings fits with the building-diagnostic obligations already described in our guide to a social landlord's housing-stock audit. And the developer or chartered surveyor commissioned ahead of a construction project in a risk-plan zone, for whom a precise topographic record of the ground complements — without replacing — the geotechnical study required by the plan's regulations.

In every case, the aerial data feeds into a process still steered by government agencies and the geotechnical engineering firm: it serves to prioritise areas that warrant deeper investigation, not to legally qualify a risk. It is the same complementary logic — aerial data points the way, the subsoil expert decides — that we detail for monitoring multi-year works plans in condominiums, where a dated building record likewise conditions access to funding.

Flight method and 2026 prices

A cavity-risk-plan perimeter typically mixes roads, green spaces, private gardens and building façades: mission planning first identifies the sectors classified as high hazard, where image overlap is tightened to maximise the digital terrain model's precision, and the lower-hazard sectors, covered at standard resolution. A dated orthophoto and a digital surface model are delivered with each campaign; the value of the system lies mainly in its repeatability — a single flight says nothing about change, only comparing two successive passes can reveal subsidence.

Ranges observed in France in 2026, excl. VAT:

These amounts cover the flight, photogrammetric processing and delivery of georeferenced data — they do not include the geotechnical study or the backfilling or reinforcement works. Our guide to how much a drone service costs details the parameters that make these amounts vary. The regulatory information on this page reflects the rules in force in September 2026; a risk plan's exact zoning should always be checked with the prefecture or the reviewing authority. For an affected municipality, inter-municipal body or social landlord, request a quote stating the perimeter, the area and any history of campaigns already carried out.

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Put it into practice

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• Security & surveillance in Niort Nouvelle-Aquitaine
• Security & surveillance in Vannes Bretagne

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