Hydrophones and pressure sensing

Part 2 — Receivers

Learning objectives

Describe piezoelectric pressure sensing as an omnidirectional scalar measurement
Compare hydrophone (pressure) and geophone (velocity) responses to the same wavefield
Explain why a free-surface ghost flips the pressure signal but preserves vertical velocity
Use the p/Z + v_z combination to cancel ghosts (dual-sensor streamers)

In water, a fluid cannot sustain shear, so the only wavefield component is a scalar pressure disturbance p(x,t). A hydrophone is a piezoelectric sensor that converts pressure to voltage — omnidirectional, small, cheap, and linear. It is the standard marine receiver.

The piezoelectric effect in two sentences

A piezoelectric crystal (lead-zirconate-titanate, PZT) develops a surface voltage proportional to applied mechanical stress. Enclose a small PZT element in a compliant mounting so that only pressure (not shear) reaches it; you have a hydrophone. Typical sensitivity: −200 dB re 1 V/μPa — i.e. a 1 μPa pressure disturbance gives roughly 10 nanovolts of output. Seismic survey pressures are much larger, so the output is easily amplifiable.

Hydrophone vs geophone at a free surface

The sea surface is a free surface: acoustic pressure must be zero there (air above supports no compressive stress to match). Vertical velocity, conversely, is maximum at the free surface (nothing to constrain it). Consequence: a sound wave reflecting off the sea surface comes back with pressure polarity flipped, but vertical velocity polarity preserved (effectively a mirror).

A towed-streamer hydrophone at depth h below the surface records

p(t) = p_0(t) - p_0(t - 2h\cos\theta/V_w)

where the minus sign is the free-surface inversion and the delay is the round trip to the surface and back. The two terms interfere — destructive at f = V_w/(2h cosθ) (the ghost notch) and its multiples. Below the first notch the hydrophone has weak low-frequency response; this is the single biggest reason conventional marine surveys struggle below 10 Hz.

Dual-sensor acquisition cancels the ghost

Co-located hydrophone and vertical geophone (e.g., PGS GeoStreamer, Schlumberger IsoMetrix) record

p \propto p_{\text{up}} - p_{\text{down}}, \qquad v_z \propto (p_{\text{up}} + p_{\text{down}}) \cos\theta / \rho V_w

The sum and difference of these give up-going and down-going wavefields separately — perfect deghosting, in principle. In practice the calibration between the two sensors is the trickiest bit; modern systems do it in real time from the data.

References

Tenghamn, R., Brown, J. (2000). A new dual-sensor towed-streamer technology. SEG Annual Meeting Expanded Abstracts, 1–4.
Sheriff, R. E., Geldart, L. P. (1995). Exploration Seismology (2nd ed.). Cambridge University Press.
Pritchett, W. C. (1990). Acquiring Better Seismic Data. Chapman & Hall.