A DART II system, shown inside the dashed lines of Figure 3, consists of two physical
components: a tsunameter on the ocean floor and a surface buoy with satellite
telecommunications capability. The DART II systems have bi-directional communication links
and are thus able to send and receive data from the Tsunami Warning Center and others via the
Internet. The web site for the DART data is supported by the National Data Buoy Center and
can be seen at: http://www.ndbc.noaa.gov/dart.shtml.
TsunameterThe block diagram below
shows how the components of a tsunameter function together. The
computer reads pressure readings, runs a tsunami detection algorithm,
and sends and receives commands and data to and from the buoy via an
1 Pressure Sensor
The DART II pressure sensor is a 0-10,000 psi model 410K Digiquartz® unit manufactured by
Paroscientific, Inc.9 The transducers use a very thin quartz crystal beam, electrically induced to
vibrate at its lowest resonant mode. The oscillator is attached to a Bourdon tube that is open on
one end to the ocean environment10. The pressure sensor outputs two frequency-modulated
square waves, proportional to the ambient pressure and temperature. The temperature data is
used to compensate for the thermal effects on the pressure-sensing element.
2 Reciprocal Counter
The high resolution precision reciprocal counting circuit continuously measures the pressure
and temperature signals simultaneously, integrating them over the entire sampling window,
nominally set to 15 seconds. There is no dead period between the sampling windows. The
circuit has a sub-millimeter pressure and sub-millidegree temperature least-count resolution.
The reference frequency for the reciprocal counter is derived from a low power, very stable,
2.097152 MHz, temperature-compensated crystal oscillator. A real time calendar-clock in the
computer also uses this reference for a time base. At the end of each sampling window, the
computer reads the pressure and temperature data and stores the data in a flash memory card.
A 15-second sampling period generates about 18 megabytes of data per year.
The embedded computer system in both the buoy and the tsunameter was designed around the
32-bit, 3.3 volt Motorola 68332 microcontroller, and was programmed in C. It was built to be
energy efficient for long-term battery powered deployment. The computer has 4 Mb of flash
memory, a 12-bit A/D converter with 8 input channels, two RS232 channels, a hardware
watchdog timer, a real-time clock, and 512 bytes of RAM. The embedded computer implements
and regulates the primary functions of the surface and seafloor units: transmitting data
communications, running the tsunami detection algorithm, storing and retrieving water column
heights, generating checksums, and conducting automatic mode switching.
4 Acoustic Modem and Transducer
A Benthos11 ATM-880 Telesonar acoustic modem with an AT-421LF directional transducer has
a 40 conical beam which is used to transmit data between the tsunameter and the surface
buoy. Modems transmit digital data via MFSK modulated sound signals with options for
redundancy and convolutional coding. Transducers are baffled to minimize ambient noise from
entering the receiver.
5 Tilt Sensor
Each tsunameter has a Geometrics 900-45 tilt sensor mounted in the base of one of the
housings. This is used to determine the orientation of the acoustic transducer when the system
has settled on the seafloor. If the tilt is greater than 10 degrees the tsunameter can be
recovered and redeployed. The watch circle of the surface buoy could carry it out of the acoustic
projection cone from the tsunameter if the angle from the vertical is too great.
The tsunameter computer and pressure measurement system uses an Alkaline D-Cell battery
pack with a capacity of 1560 watt-hours. The acoustic modem in the tsunameter is powered by
similar battery packs that can deliver over 2,000 watt-hours of energy. These batteries are
designed to last for four years on the seafloor; however, this is based on assumptions about the
number of events that may occur and the volume of data request from the shore. Battery
monitoring is required to maximize the life of the system.
7 Tsunami Detection Algorithm
Each DART II tsunameter is designed to detect and report tsunamis autonomously12. The
Tsunami Detection Algorithm works by first estimating the amplitudes of the pressure
fluctuations within the tsunami frequency band, and then testing these amplitudes against a
threshold value. The amplitudes are computed by subtracting predicted pressures from the
observations, in which the predictions closely match the tides and lower frequency fluctuations.
If the amplitudes exceed the threshold, the tsunameter goes into Event Mode to provide detailed
information about the tsunami.
8 Reporting Modes
Tsunameters operate in one of two data reporting modes: A low power, scheduled transmission
mode called “Standard Mode” and a triggered event mode simply called “Event Mode”.
Standard Mode reports once every six hours. Information reported includes the average
water column height, battery voltages, status indicator, and a time stamp. These
continuous measurements provide assurance that the system is working correctly.
Event Mode reports events such as earthquakes and /or tsunamis when a detection
threshold is exceeded. The Tsunami Detection Algorithm triggers when measured and
predicted values differ by more than the threshold value. Waveform data are transmitted
immediately (less than a three-minute delay).
Tsunami waveform data continue to be transmitted every hour until the Tsunami Detection
Algorithm is in a non-triggered status. At this point the system returns to the Standard Mode.
***Information From : http://www.ndbc.noaa.gov/