Coherent methods of sea bottom sounding
A standard modern marine seismic approach to the problem of profiling the bottom structure that contains multiple reflecting layers is to use incoherent impulsive impact-action sources (usually air guns and spark dischargers, or sparkers) and extended receiving antenna systems (seismic streamers). High-power air guns provide a high level of signal in a low-frequency range of up to 100 Hz, which, with allowance for the characteristics of the extended directional receiving antenna, is sufficient to achieve the required probing depths of several kilometers. The only way of increasing the spatial resolution when employing these impact-action sources is to reduce the signal duration that is bounded below by the design features and is no less than 10 ms. This duration provides a resolution of tens of meters for characteristic values of the sound wave velocity in the bottom rocks.
An alternative approach to the construction of seabed seismic systems, based on coherent probing signals, is now under development at the IAP RAS. Typical choice of these signals implies signals with linear frequency modulation (LFM) and phase-shift keyed, or pseudorandom, signals. If rather broadband radiators controlled by a programmable way are used, the choice of various regimes of signal formation is not a technical problem. Such sources, a successful development of which at the IAP RAS is associated with problems of remote ocean sensing, can be considered as a promising tool of marine seismoacoustics. These sources can be used to implement signal detection filtering and convolution with a reference signal, a long accumulation of pulse sequences, synthesis of an extended aperture by a single receiver (space-domain signal accumulation), and provide appropriate conditions for the beam shaping.
To experimentally test the possibilities of the coherent methods of marine seismoacoustics, the scientists of the IAP RAS and P. P. Shirshov Institute of Oceanology, RAS made joint experiments employing a towed receiving-emitting complex in the Caspian Sea area (A. I. Khilko, A. I. Malekhanov, L. A. Rybenkov, and A. A. Stromkov). There were used underwater sound projectors that generated synchronized sequences of pulsed LFM signals in various frequency bands (~50–100 Hz) in a wide range from 100 to 1000 Hz (the highest radiation power of ~130 W corresponded to the 180–230 Hz band). A standard seismic streamer consisting of 25 in-phase hydrophones (IO RAS equipment) was used as a receiving system.
The reconstruction of the bottom structure was upgraded using an original technique of layered path accumulation of pulses with allowance for the inclinations of individual reflective layers, which not only increased the number of pulses in a coherent sequence (up to 100), but also permitted one to adaptively estimate those inclinations. In this case, most of the received signals had a low noise immunity (no more than 0–5 dB at the input), but the resulting gain of the output signal/noise ratio reached 30 dB. This allowed reconstructing the structure of the bottom layers at depths down to 1000 m, the layers at a depth of over 500 m being completely masked by noise if the proposed processing procedures were not used. Processing of the signals obtained in the presence of a pulsed incoherent source (sparker), whose spectrum occupied the low frequency bandwidth of up to 100 Hz and had a power comparable with that of the coherent source, did not virtually ensure the resolution of individual layers at the same depths.
The investigations have shown that seismoacoustic sensing of the sea bottom structure with a high spatial resolution at depths of up to 1000 m can be carried out by using relatively low-power (of ~100 W) coherent underwater sound projectors in the range of a few hundred hertz. This easily meets the requirements of minimizing the detrimental effect on the marine ecosystem.