Observations of sound reflection by temperature microstructure

Acoustic scattering layers, often reported in lakes and in the ocean, may be due to a variety of causes. An experiment tested whether turbulence, artificially generated by towing objects at constant depth, could be detected by a 102‐kHz narrow‐bean fisheries research transceiver sonar. Echoes were absent (or rapidly decaying) in layers of uniform temperature, a result consistent with predictions of sound scattering from velocity microstructure alone. In stratified regions however, the wakes were always acoustically visible close to the object and, for wake Reynolds number of about 1.6 × 10 5 and Froude numbers of 100, remained visible for times, t, after generation such that Nt ≈ 10, where N was the local buoyancy frequency. At the frequency of the sonar, the scattering is from the temperature microstructure, predominantly from wavenumbers in the viscous diffusive subrange described by the Batchelor spectrum. This scattering may be reduced by using a higher frequency sonar. The observed scattering was of similar intensity to that from natural scatterers and appears to be higher than that which can be reasonably accounted for if the thermal microstructure at the scattering wavenumber is isotropic. Assuming an anisotropic thermal microstructure, the scattering cross‐section per unit volume, estimated from microstructure measurements of naturally occurring temperature fluctuations, exceeds 3 × 10 −6 · The observations indicate that naturally occurring temperature microstructure is sometimes a source of acoustic scattering of sufficient magnitude to be detected by sonar.