The Bottom Pressure Recording (BPR) detects greater water pressure when a passing tsunami increases the height of water above it. The surface buoy receives transmitted information from the BPR via an acoustic link and then transmits data through a satellite link to central stations.
From historical studies, it is clear that all earthquakes in tsunamigenic source regions can not trigger tsunamis. In order to confirm whether the earthquake has actually triggered a Tsunami or not, it is essential to measure the change in water level in the open ocean with high accuracy in real time. Bottom pressure recorders are used to detect the sea level changes near to tsunamigenic source regions and consequent propagation of Tsunami waves in the Open Ocean.
As part of the Indian Tsunami Early Warning System, a real time network of Deep Ocean Assessment and Reporting System has been established by National Institute of Ocean Technology (NIOT). The network is designed to detect, measure and monitor tsunamis. The network comprises of 12 BPRs transmitting real time data through satellite communication to NIOT at Chennai and INCOIS at Hyderabad simultaneously for processing and interpretation. Each BPR is strategically placed at 30 minute and 60 minute tsunami wave arrival times (from hypothetical tsunami sources), so that they offer sufficient warning time and redundancy. At the same time, they are far enough from the earthquake zone so that the tsunami wave signal can be clearly distinguished from the seismic Rayleigh wave. In addition to Indian BPR network, INCOIS is also receiving real time data from internationally coordinated networks like DART (Deep Ocean Assessment and Reporting Tsunamis) in Indian Ocean via Internet. Currently, NIOT has deployed four BPRs out in Bay of Bengal and two BPRs in the Arabian Sea.
Each BPR system consists of an anchored sea floor Bottom Pressure Recorder with acoustic link to a companion Moored Surface Buoy for real time communications and is designed to detect and report tsunamis if the pressure fluctuations are above a preset threshold. The BPR operates in one of two data reporting modes: a low power, scheduled transmission mode Normal mode, samples for every 15 min and transmits for every 1 hour and a triggered event mode called as Tsunami Response Mode, samples for every 15 seconds and transmits for every 5 minutes. The BPR uses a piezoelectric Pressure transducer to make 15 seconds-averaged measurements of the pressure exerted on it by the overlying water column. The Tsunami detection algorithm running in the BPR generates predicted water height values with in the tsunami frequency band and compares all new observed samples with these predicted values. If two 15-second water level values exceed the predicted values greater than the threshold (30 mm), the system will go into the Tsunami Response Mode. An acoustic link transmits data from the BPR on the sea floor to the surface buoy. The data are then relayed via a satellite (e.g. INSAT) communication finally to the Tsunami Warning Centre. Each BPR system has two-way communication link and thus able to send and receive data from Tsunami Warning Centre. The data centre at INCOIS is equipped with state-of-the-art computing hardware for data reception, INSAT two-way communication hub, data processing & visualization and dissemination facilities. The BPR can be accessed from warning centre remotely at any point of time and perform the functions like trigger the BPR into tsunami response mode, inspecting the state of health, requesting the tsunami response mode data from BPR etc.
The real time data received at INCOIS is processed by the open ocean tide prediction software for the residual data by removing the predicted data from real time observed data with some datum correction. The computed residual data is continuously monitored for significant water level changes above 30 mm.