Large areas behind the historic quay walls and bridges in Amsterdam city center are prone to soil mobilization and cavity (sinkhole) formation due to intensified infrastructure developments and extreme groundwater level fluctuations caused by climate change. We carried out a geophysical survey to investigate a sinkhole formed under the Muntplein (Amsterdam, The Netherlands). The surface trace (hole) of the sinkhole was triggered by a heavy vehicle passing over the street which lies in the vicinity of a quay wall and behind the abutment of the Muntsluis bridge. The application of ground penetrating radar (GPR) and electric resistivity tomography (ERT) provided continuous data of the shallow subsurface which enabled the detection of the backfilled cavity, its southwest (SW) extension, the bridge abutment-to-soil transition, key utility lines and the presence of two potential targets for further investigation. A follow-up geotechnical assessment supported by hydrographic survey in the canal validated our findings and substantiated our first interpretation (i.e., sinkhole in development). The paper demonstrates the applicability of non-invasive electromagnetic (EM) methods to rapidly detect cavities in critical urban areas, and, thus, to de-risk climate-smart infrastructure developments.

We conducted ground penetrating radar (GPR) surveys to detect the presence of simulated clandestine burials at the Amsterdam Research Initiative for Subsurface Taphonomy and Anthropology (ARISTA) test facility. Our aim is to determine the characteristic responses of the simulated clandestine burials in this man-made sandy environment (reclaimed land) and use them to provide recommendations for forensic investigations. We performed GPR surveys over three simulated clandestine burials at ARISTA during four non-consecutive days. The acquired data represent common-offset data to investigate changes to burial detectability depending on central antenna frequency (250 MHz and 500 MHz), different GPR instruments (NOGGIN or pulseEKKO), changes to survey grid orientation relative to burials, and increased soil moisture content in the survey area. In common-offset radargrams the burial anomalies take on many forms, appearing as disruptions to existing features (direct-wave arrivals and soil horizons) and as isolated reflection events (hyperbolic events and burial-length horizontal anomalies). In time slices, the burials are characterized by high- or low-amplitude rectangular anomalies. When used in conjunction, the radargrams and time slices produce characteristic responses consistent with the locations of the burials, regardless of the survey grid orientation. Increased soil moisture at the site improves the detectability of the burials.

The geometric variability of soil layers is a large source of uncertainty in the reliability assessment of dikes. Because direct samples of the subsurface soils are often insufficient to capture the complexity of the subsurface, geophysical methods provide a powerful source of complementary information. A combined approach to estimate the geometry of soil layers is presented. The approach combines local point data, i.e. data obtained from a CPT or a borehole log, and geophysical tomography in a universal cokriging framework. The approach uses the contact points between soil layers obtained from local point data and the orientations of the layers derived from geophysical tomography. To reduce subjectivity in the interpretation of tomographic images, an automated edge detection technique was used. The combined approach was applied to characterise two test sites where the presence of paleochannels locally change the geometry of soil layers. The results show that a combined approach enables the reduction of sampling efforts with an improved estimation of geometric variability.