When a dam is impounded, the massive body of water places new stresses on the rock beneath it. The increased load effect, coupled with increased pore pressures that diffuse in the cracks beneath the dam, can generate fluid-induced seismicity. Regional seismic monitoring systems are commonly employed around the body of water to monitor these potentially large induced seismic events. At least three seismometers (combination of the seismic sensor and data acquisition unit), but mostly 5 to 7 seismometers, are placed around the dam to record and locate the seismic events.
A second seismic monitoring objective is to monitor the dam wall itself for spurious resonance frequencies. The structure would have been designed with a certain resonance frequency, and vibration monitoring is useful to see if this has been achieved and if it changes over time as this may indicate accumulating damage in the structure. Usually at least two seismometers are used to record the response of the dam wall to natural and induced seismicity. At least one seismic sensor would be located within the bedrock at the bottom of the wall, and at least one seismic sensor would be installed on the crest of the wall.
Acoustic emission monitoring can also be added for early crack detection in the dam wall.
The seismic stations communicate with a central computer (seismic server) via cellular modems or WiFi digital radios or alternatively seismic data can be written to a portable USB drive, using standalone recording mode. Use of common cellular modems minimises infrastructural set-up costs. The seismometers are linked to GPS modules to provide accurate time synchronization.
At midnight every day all seismic stations are contacted and the seismic information is downloaded. Seismograms of common events are post-associated for off-line processing and interpretation. Digital radios (WiFi) are used when continuous recording of data (active link) is required for real time processing of seismic data.
Typically low frequency (1Hz) geophones are used to monitor the regional seismicity around dams. High frequency geophones or DC-stable force-balance accelerometers (e.g. 5kHz MEMS accelerometers) are used for structural vibration monitoring of the dam wall. The sensors are installed into small vaults, bolted to a concrete pad. Each sensor is connected to a single nearby netADC. High-frequency (25kHz) piezo-electric accelerometers are used for acoustic emission monitoring.
SEISMIC DATA PROCESSING & INTERPRETATION
For off-line structural vibration monitoring of the dam wall (standalone mode), the seismograms of every seismic event that triggers at least two sensors are post-associated on the central seismic server. Seismograms are then exported to ASCII format for processing with an engineering structural response package.
Real-time continuous recording of seismicity is normally employed for regional monitoring. IMS can remotely download, process and analyse all the seismic data from our office in South Africa. This continuous, real-time seismological processing service comes with routine (daily, weekly or monthly) analysis and reporting.