Hydrophone

Key Takeaways

• Piezoelectric sensing in hydrophones exploits the fundamental property of crystalline materials such as PZT (lead zirconate titanate) that develop an electrical polarization and surface charge when mechanically stressed, with the charge proportional to the applied stress (and thus the incident acoustic pressure) over the hydrophone's operating frequency range: a typical marine seismic hydrophone element consists of two PZT discs wired in parallel (with one disc wired in reverse polarity) in a configuration that makes the assembly sensitive to hydrostatic (omnidirectional) pressure while canceling the response to the housing acceleration that would cause spurious signals if the hydrophone is mechanically vibrated by streamer motion; the two-disc configuration (the acceleration-canceling or self-noise-canceling design) is essential for towed streamer hydrophones where cable motion in the tow flow induces mechanical vibrations that would otherwise swamp the seismic signal; the electrical output of the piezoelectric element (typically 50 to 150 microvolts per Pascal of applied pressure) is amplified by a preamplifier in the hydrophone housing before transmission to the recording electronics in the streamer telemetry system.
• MEMS (micro-electromechanical systems) digital hydrophones represent a newer generation of marine receivers that integrate the sensing element and analog-to-digital converter within the hydrophone housing, transmitting digital data rather than analog electrical signals through the streamer cable: MEMS-based sensors measure pressure by detecting the deflection of a microfabricated membrane caused by the acoustic pressure wave, with the deflection sensed by a capacitive or piezoresistive bridge circuit; the digital hydrophone architecture eliminates the noise introduced by long analog signal runs through the streamer cable (which are sensitive to cable-induced electromagnetic interference) and allows each hydrophone to be independently calibrated and quality-monitored in real time through the digital telemetry link; digital hydrophone streamers using MEMS or conventional PZT sensors with in-housing digitization have become standard in modern high-density seismic acquisition systems (with hydrophone group intervals of 3.125 meters or even 1.5625 meters in broadband acquisition designs) because the digital architecture scales to high channel counts without the analog signal degradation that limited conventional hydrophone streamers.
• Hydrophone sensitivity, frequency response, and self-noise determine the quality of the seismic data recorded, with different applications demanding different performance specifications: marine seismic hydrophones used for commercial reflection surveys must cover the frequency range of 2 to 200 Hz (the bandwidth of conventional marine seismic energy after sea surface ghost notch filtering) with flat response within 3 dB and self-noise below the sea state 0 ambient noise floor across the full seismic band; high-resolution site survey hydrophones used for shallow geohazard assessment and geotechnical characterization require extended high-frequency response from 50 to 2,000 Hz or beyond, sacrificing low-frequency sensitivity (which is less important for the shallow, short-path targets being imaged); ocean bottom hydrophones used in passive seismic monitoring for induced seismicity or reservoir microseismic monitoring must achieve sensitivity and self-noise performance at very low frequencies (0.1 to 20 Hz) where the seismic energy from small earthquakes is concentrated, requiring larger hydrophone elements or differential pressure sensor designs that reject the hydrostatic ambient pressure variations that dominate the total pressure at the seafloor.
• The ghost problem in hydrophone recordings arises from the sea surface acting as a near-perfect acoustic reflector for the upgoing seismic wavefield, creating a downgoing ghost arrival that reaches the hydrophone with opposite polarity and a time delay equal to twice the hydrophone depth divided by the water velocity: a hydrophone at 10-meter depth in 1,500 m/s water receives the ghost 13.3 milliseconds after the primary upgoing wave, and the interference between primary and ghost creates a spectral notch (zero sensitivity) at 75 Hz (the frequency at which the ghost delay equals half the wave period) that removes high-frequency signal and limits vertical resolution; the ghost notch can be deconvolved (ghost deconvolution) using knowledge of the hydrophone depth from the depth sensor telemetry, or it can be suppressed by combining hydrophone data (which sums primary and ghost) with co-located velocity sensors (geophones or accelerometers, which subtract primary and ghost in different proportions) to separate the upgoing and downgoing wavefields; the multi-sensor streamer (combining hydrophones with MEMS accelerometers at the same position) has become standard in broadband marine seismic acquisition for this reason, enabling ghost-free recordings that extend the usable bandwidth from the conventional 2-80 Hz of single-sensor systems to 2-200 Hz or beyond.
• Ocean bottom hydrophone applications differ from towed streamer applications in that the hydrophone is stationary on the seafloor during recording, enabling much longer recording times, 4D repeatability for time-lapse seismic monitoring, and co-registration with geophone data for converted wave analysis: an OBC or OBN hydrophone records all seismic energy reaching the seafloor regardless of the direction of arrival (including energy from shots at large offsets and azimuths that towed streamers cannot cover), giving access to the full range of offset and azimuth combinations needed for amplitude-versus-offset analysis and azimuthal anisotropy mapping; the seafloor hydrophone also records in a much quieter mechanical environment than a towed streamer (no tow noise, no surface wave noise from nearby ships), allowing it to detect smaller seismic signals and to record useful data at lower frequencies where towed streamers are limited by flow noise; the combination of pressure (hydrophone) and velocity (geophone) at the seafloor allows the upgoing and downgoing wavefields to be mathematically separated (P-Z summation, where Z is the vertical geophone component), removing the seafloor ghost from the data and enabling broadband wavefield imaging without the ghost notch limitations of surface hydrophone recordings.