Project: Sinkhole Detection

A sinkhole of unknown origin was initially detected at the exit of a parking lot adjacent to an office building. The sinkhole was filled and the parking lot resurfaced. Two months later the surface of the parking lot collapsed again. The second sinkhole was larger than the first, measuring 5 to 6 meters in depth. The parking garage has two levels and there was an immediate safety concern. There had never been any problems in the twenty year history at this business location. An immediate investigation to locate the origin of the sinkhole was conducted.

Process

The goal of the field investigation was to discover the cause of the subsidence in the research area using the TISA 2D geophysical measuring technique. It was suspected that either an old underground water basin or sewerage leak was the cause. Reflection measurements were made from boreholes A, B, and C, as well as three zero-offset cross-hole measurements between AB, AC and BC at a maximum depth of 15 meters below the surface.

Conclusion

The wave velocity in the medium was determined with a Tx-Rx variable direct wave measurement inside borehole C and was found to be 11.6 cm/ns. The two images below show data acquired in reflection and transillumination modes. In the reflection data (left graphic) an obstacle in 4-5 m depth was identified, which could be related to a 5 m deep pit dug close to the survey area. The transillumination data set (right graphic) shows two sections where the direct wave has a weaker amplitude. The higher one is a result of the same pit which is located between the two measuring boreholes and therefore reduces the amount of recorded energy in this depth range. The second reduction of energy is found in a depth interval of 9.5 to 11.8 meter below the surface. This reduction of amplitude is associated with a later arrival time of the direct wave. Such a later arrival time indicates the presence of a low velocity medium like a water filled void between boreholes A and B. This could explain the subsidence of the surface encountered at this location.

Sinkhole Data

Left: reflection data, borehole C
Right: transillumination data, boreholes A-B