Cavity detection using microgravity in a highly urbanized setting: A case study from Reims, France
Résumé
In this study, we present a microgravity survey carried out in a highly urbanized city centre setting in Reims, France, with the aim of detecting undocumented cavities below ground. The occurrence of two ground collapses affecting a busy street, linked to ill-filled cellars, motivated this survey. In this context, surrounding buildings and underground structures, such as existing cellars or parking lots, constitute respectively excess mass above, and deficit mass below the gravimeter, which result in negative gravity anomalies. These anomalies may have magnitudes comparable to or larger than the target signal, hence obscuring its detection.We present a correction scheme for buildings and underground structures, which is effective, as evidenced by the mitigation of important border anomalies present in the uncorrected Bouguer anomaly and no longer visible in the corrected Bouguer anomaly. In effect, these corrections make the Bouguer anomaly more straightforward to interpret, which facilitates the determination of a regional anomaly, and ultimately leads to a more meaningful residual anomaly. We also present an error budget estimate, in which errors in the anthropogenic corrections are approximated. This allows us to establish a significance threshold, which is used to delineate significant anomalies.A number of significant anomalies were found, and drillholes were recommended to control the anomalies and check for cavities. A two-level cellar was found in the middle of City Hall Plaza, within a major gravity anomaly. Other drill holes found un-compacted fill material.
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