The uncontrolled development of cavities near the earth's surface may result in a major engineering hazard. Such cavities may be the direct result of natural phenomena or due to the presence of prior underground evacuation such as that associated with mining. Regardless of the cause these cavities can result in localized instabilities such as the collapse of highway or airport runway pavements, foundation subsidence or collapse, and dam leakage. These instabilities increase the risk of physical damage and human injury. Consequently it is necessary to both locate and monitor the stability of such cavities. This paper will describe the results of a recent study conducted at the Capital City Airport, near Harrisburg, Pennsylvania, U.S.A.. The purpose of this study was to investigate the feasibility of using acoustic emission and seismic tomography techniques to study the development of one type of near-surface cavity known as a "sinkhole". In recent years, sink-holes have developed under large sections of the airport runway and taxiway complex and at sometimes two of the three runways have had to be closed while maintenance operations were underway. During the study, instrumentation for carrying out low frequency acoustic emission and near-surface S-wave seismic tomography were installed at selected areas. In all cases the sensors utilized were velocity gauges (geophones) with an intrinsic sensitivity of 0.3 to 0.4 V/cm/sec. Normally the frequency response and gain of the overall monitoring system was 5Hz to 100Hz and 76dB to 104dB, respectively. The studies verified the feasibility of using seismic tomography techniques as a means of locating subsurface sinkholes and monitoring their gross development. Unfortunately sinkhole development during the study period was limited and no major acoustic emission activity was observed. A number of tests using artificial acoustic emission sources were undertaken and it was established that procedures based on zonal and arrivaltime-sequence techniques could be successfully utilized to locate sources of sinkhole instability. For sources located within the monitoring array, source location accuracy was found to be in the range of 1.5m. In most cases this was equal or better than the accuracy obtained using conventional arrival-time-difference techniques.

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