1. What Does a Self-Buoyant Seismometer Do After It Is Dropped From a Ship?
This type of seismometer is called "self-buoyant." The instrument is housed in a container that withstands water pressure, with a frame and weights attached. When lowered from a work ship into the sea, its weight causes it to sink to the bottom [1].
Once it reaches the seafloor, observation begins automatically. No people accompany it, and it is not connected by cable to ships or land. The data collected is stored in a recorder inside the container [1].
In other words, this machine is designed so that "once dropped, it measures on its own and remembers on its own." If you simply leave it at the drop site, there is no way to retrieve the records. This makes the mechanism for the return journey crucial.
So, how does it get back to the surface?
2. How Does a Seafloor Seismometer Rise to the Surface When a Ship Sends a Sound Signal?
During recovery, specific sound waves are sent into the water from the ship. When the seismometer receives this signal, it detaches the weights from the frame. Without the weights, the machine rises to the surface due to buoyancy [1].
The key mechanism here is "hearing the signal and releasing the weight." Even without ropes or lines reaching the surface, the machine activates itself just by receiving a signal from the ship.
After rising, the machine drifts on the surface. To prevent it from being lost in ocean currents, it emits radio waves to indicate its position and flashes a light. Helicopters and ships search for and retrieve it using these lights and radio signals [1].
It is called by sound and found by light and radio. The ship's sound signal acts as the "return switch" for the seismometer on the seafloor.
3. What Were the Difficulties of the Older Method of Mooring Seismometers With Ropes to Surface Buoys?
According to an explanation by the University of Tokyo, a mooring method was previously used, where seismometers were connected to surface buoys by ropes. This method required significant effort for recovery, and buoys were often lost because they drifted away [2].
In contrast, with the current self-buoyant method, when a ship sends an acoustic signal from above the drop location, the seismometer rises on its own [2]. Since there is no need to keep buoys floating on the surface constantly, the worry of losing them decreases, making it easier to handle.
You might find it easy to understand if you think of this as "one less tool." The object that just waited on the surface is gone; now, the ship only needs to send a signal when necessary.
4. How Long Can Seafloor Seismometers Record, and How Is Their Clock Time Corrected?
The observation period was initially about one month with early timer-based models. Since around 2000, more batteries could be carried, allowing for continuous observation for one year [2]. The glossary of the Headquarters for Earthquake Research Promotion also explains that records can now be collected for more than a year [1].
Longer observations help because earthquakes occur unpredictably. However, stored records are useless if you do not know exactly when the shaking happened. Therefore, the clock becomes another important component.
The seafloor clock is synchronized with GPS time before installation and after recovery. There is a method to correct the record times by assuming that drift increased at a constant rate during that interval. This example was introduced in overseas research [3]. It is noted that deep-sea water temperatures are stable, so the speed of the quartz oscillator used in the clock is less likely to change [3].
5. How Are Seafloor Seismometers Used Differently From Cable-Connected Seismometer Networks?
The source regions of trench-type earthquakes are at sea. To capture earthquake characteristics that land-based networks alone cannot easily detect, seismometers are placed directly above the source region. The Earthquake Research Institute at the University of Tokyo conducted observations using long-term self-buoyant seafloor seismometers in the source region of the Great East Japan Earthquake [4].
On the other hand, cable-connected networks send data in real time. The National Research Institute for Earth Science and Disaster Resilience's S-net (Japan Trench Offshore Seismometer and Tsunami Observation Network) connects 150 observation instruments with submarine optical cables, totaling about 5,500 km (about 3,417 miles). Approximately 25 observation points are arranged in a grid pattern at intervals of about 30 km (about 18.6 miles) [5]. In areas shallower than 1,500 m (about 4,921 feet), where fishing occurs, trenches about 1 m (about 3.3 feet) deep are dug to bury the cables and instruments [5].
For DONET, deployed off Kumano Sea and Kii Channel in the Nankai Trough, DONET1 continues operation with 51 observation points. The main purpose is monitoring the "rupture initiation zone" where giant earthquakes are a concern, and data is sent immediately to the Japan Meteorological Agency and research institutions via land stations [6].
Cables require long lengths and large amounts of power. Self-buoyant types are used for limited-period observations when real-time data is not needed [1]. The roles differ between "networks that notify immediately" and "machines that record in detail over a set period."
6. How Can You Try the Floating Mechanism of a Seismometer at Home With a Bottle and Stones?
Try it in water: Prepare a washbasin of water and an empty plastic bottle. Use rubber bands to attach small stones to the bottle, adjusting the number of stones so it sinks. Then, remove the rubber band, and it will float. You can visually confirm the basic mechanism that releasing weights causes floating. Because you are using water, do this where an adult is present and in a place where spills are okay.
Explain the difference: Write down power, communication, and observation period on paper to compare the buoyant method and the cable method. Explain which situation each is suited for in your own words.
Next question: How are records from a year-long wait on the seafloor interpreted after recovery? That is a continuing question not covered in this article.