Sensors employed in undersea applications rely on precise timing to be effective. However, because time from GPS is unavailable underwater, these sensors have generally relied on OCXOs for stable and accurate time stamping within the sensor. Now those applications have a better option: the SA.45s chip scale atomic clocks (CSAC). Compared to OCXOs, the SA.45 CSAC maintains far higher accuracy for far longer periods, uses much less power, and maintains a much more stable frequency in spite of the wide variations in temperature experienced.

One such undersea application is reflection seismology (or seismic). In seismic applications, oil exploration firms place a grid of geophysical sensors on the ocean floor to determine likely spots where oil will be located. The sensors can be dropped over the slide of a ship or laid down by a remotely piloted vehicle. The sensors can be independent or a cable can connect the row of sensors. Each sensor typically includes a hydrophone, a geophone, and a TCXO or OCXO that is used to timestamp the data received by the other two devices. Once the sensors are in place, a powerful air gun or array of air guns launch a sonic pulse from the ship. The ship moves in a pattern that allows the air gun to be fired from many different angles relative to the sensor grid. Some of the pulse’s energy reflects off the ocean floor, travels through the layers of rock, and eventually reflects back to the sensors on the ocean floor where it is detected and time stamped. Once the ship has finished its predetermined pattern, the sensors are retrieved along with the time stamp data.

Because the sonic pulses travel at different speeds through different materials, the time it takes to reflect back to the sensors off the various rock layers is different depending on which materials the pulse traverses. When timing data is post-processed, it creates a picture of the layers of rock and sediment beneath the ocean floor, showing which locations likely hold oil or gas deposits. The more precise the timing, the more accurate the images of where oil and gas actually exist.

Why CSAC: Benefits in this Application

• Improved Accuracy

  During a typical deployment, the sensors can be underwater for several weeks at a time (deploying the sensors, taking measurements, and retrieving sensors takes time, even more so if the weather conditions are not optimal). Through the deployment, the OCXOs are aging, producing a time stamping error that varies as the square of the time under water. The CSAC’s low aging rate, which can be even 1/100th of a good OCXO, greatly reduces the time stamp error as sensors are deployed for longer periods.

• Reduced Power: Lower Battery/Sensor Costs

  Batteries are typically the biggest expense in underwater sensors and the number of sensors in a typical grid is increasing. Because the SA.45s CSAC consumes 1/10th to 1/20th of the power of an OCXO, it requires much less battery power so sensors can be smaller and cost less. Alternatively, sensor manufacturers can choose to retain the existing battery capacity and use the CSAC to create sensors for much longer missions.

• Reduced Effects from Wide Temperature Swings

  Today, most marine geophysical sensors are calibrated to GPS on the deck of the boat before being dropped into ocean. Because the water at the bottom of the ocean is often just a few degrees above freezing temperature, the sensor can see a temperature change of 30 oC or more from its calibration temperature, causing a shift in frequency and a linear error in time. Some sensors use software models to correct for this error, but the best approach is to minimize the error to begin with. With a temperature co-efficient of ±5.0×10-10 over its entire temperature range, the SA.45s CSAC can offer a 10x to 1000x improvement over OCXO and TCXO alternatives.

For more information on how CSAC can help address your applications, please contact us at timing@microsemi.com.

Tags: Chip Scale Atomic Clocks, CSAC, Time Stamping