Seismic penetrator technology for use in shallow water seismoacoustics

In shallow water areas at very low frequencies (VLF), the sediments forming the upper part of the seafloor become an important part of the acoustic propagation path from source to receiver. It is difficult to fully understand the physics of propagation in this “extended wave guide” when we are limited to placing sensors only in the water. At the Naval Research Laboratory, two technologies for inexpensive deployment of seismic sensors below the seafloor are being developed. The ocean bottom seismic penetrator (OBSP) uses a modified sediment coring technique. A large weight stand (~2200 kg) with an 8 m pipe is suspended from a deep-sea winch wire and pushes a sensor head into the sediments. The weight stand and pipe are then decoupled from the sensor and seafloor recorder and removed from the seafloor. At the conclusion of the seismic measurement period, the recording package is released from the seafloor and returns to the sea-surface for recovery, leaving the buried sensor. A second system is designed for deeper penetration (up to 100 m). In this system, the sensor package contains a small drill motor driven by seawater hydraulics. The sensor/drill package is suspended from a seafloor unit by an electrical/hydraulic umbilical cable and is lowered from the seafloor unit as it drills into the sediments