Sub-bottom profilers

 Sub-bottom profilers are acoustic devices used to visualize sedimentary layers buried several dozens of metres deep.

Data collected by sub-bottom profilers can be used for qualitative studies of geological structures and sedimentation processes, and to estimate geoacoustic parameters (reflection coefficient, absorption, etc.) and, consequently, to characterize the geological environment.

The sub-bottom profilers of the oceanographic vessels Pourquoi pas ?, L’Atalante and Thalassa work generally at low frequencies of between 1.5 kHz and 6.5 kHz to ensure considerable penetration with excellent vertical resolution.

The sub-bottom profiler on the Marion Dufresne exploits higher frequencies of between 2.5 kHz and 7 kHz, compensated by a considerable level of transmission. The acoustic signals it transmits are linear frequency modulation pulses and the signal it receives comprises a time series of echoes reflected off the sedimentary interfaces.

At the frequencies under consideration, the received energy comes mostly from coherent reflections generated by differences in the acoustic impedance of the seabed.

The sub-bottom profilers of the deep-sea vessels Pourquoi pas ? (2005), L’Atalante (2009) and Thalassa (2017) are the result of a low-frequency development undertaken in 1998–99 by the companies Triton-Elics, Eramer and Enertest, in collaboration with IFREMER as part of the modernization of the research vessel Suroît. Currently, the acoustic array and electronics (power, adapting, receiving, switching) of the profiler are marketed by the company Ixblue. These sub-bottom profilers constitute a homogenous equipment pool supported by the acquisition system SUBOP.

The transmitting and receiving acoustic array comprises Tonpilz-type transducers and is built into the hull. It is protected by an acoustically transparent window. The array’s geometry may change depending on the vessel concerned: seven elementary electroacoustic transducers with specific arrangements on the vessels Pourquoi pas ? and L’Atalante, and five transducers for the Thalassa. Enjoying significant levels of transmission (~210 dB ref. 1 µPa at 1 m), these profilers can be deployed in waters with a maximum depth of 4,000–5,000 m, depending on the sediments present. Depth of penetration can attain 100 m in loose sediment and vertical resolution is approximately 25 cm.

Examples of profiles on average and very high water levels

SBP profile at depths of 250–300 m

~120 m penetration 

 

Characteristics 

  • Electronics: Ixblue PWM 4 kVA
  • Water height: 20–5,000 m
  • Penetration: ~100 m in loose sediment
  • Vertical resolution: ~25 cm
  • Transmission level: between 209 and 212 dB ref. 1 µPa at 1 m depending on the geometry of the array
  • Signal: Chirp 1500–6500 Hz from 10 to 100 ms

The sub-bottom profiler of the Marion Dufresne is a Kongsberg Maritime SBP120. The TX transmitter (3 x 32 transducers) and RX receiver (64 transducers) arrays are more than 7 m long. The receiver array is shared with the multibeam echo sounder EM122.

Beam width and transmission level can be modulated according to the chosen number of transducers:

  • 3° x 3° for full TX & RX arrays; 230 dB ref. 1 µPa at 1 m
  • 6° x 6° for half TX & RX arrays; 224 dB ref. 1 µPa at 1 m
  • 12° x 12° for quarter TX & RX arrays; 218 dB ref. 1 µPa at 1 m

The large number of transducers produces a high transmission level and a penetration of approximately 100 m in loose sediment. The transmitted signals are linear modulations at frequencies of between 2.5 and 7 kHz for a maximum duration of 100 ms.

The vertical resolution, approximately 30 cm, is comparable to that of the sub-bottom profilers of the vessels Pourquoi pas ?, L’Atalante and Thalassa.

The general architecture of a sub-bottom profiler suitable for very shallow waters (<20 m) was defined for use on the coastal launch Haliotis.

The receiver array is separate from the transmitter array to ensure operation in very shallow waters while using frequency-modulated signals of sufficient duration. It uses low frequency, provides excellent vertical resolution, and can acquire data at speeds of up to 7 knots.

Transmitter levels are compatible with a penetration of approximately 20 m.

Example for a water height of 10 m

Characteristics

  • Water height: 1–30 m
  • Penetration: ~25 m in loose sediment
  • Vertical resolution: ~25 cm
  • Transmission level: 193 dB ref. 1 µPa at 1 m
  • Electronics: Enertest linear 500 VA
  • Signal: Chirp 1700-5500 Hz from 10 to 50 ms
  • Acquisition speed: up to 7 knots