Page 27 - Acquisition and Processing of Marine Seismic Data

18 1. INTRODUCTION

geology and the power of the system used, it is • Single frequency (pinger) systems
generally limited to 60 m. Table 1.5 shows the • Chirp systems
general specifications of subbottom profiler sys- • Boomer
tems. A typical system consists of a transmitter/ • Parametric systems
receiver (together termed a transceiver) unit, a Single frequency subbottom profilers emit sin-
transducer array and a recording unit. They
gle frequency signals as sinuous wave trains into
are very similar to single-beam echosounders,
the water column and are known as linear sys-
but their operating frequency is much lower
tems. The signal frequency depends on the reso-
and the output power is much higher than the
nance frequency of the transducer array and is
echosounders.
generally 3.5 or 5 kHz. Penetration depths of
Transducer arrays of subbottom profilers are
the single frequency subbottom profilers are rel-
either mounted to the hull or over-the-side of the
atively low and change from 2 to 30 m. The main
survey vessel, although there are also deep-tow
advantages of these systems are their ease of use
subbottom profilers mounted on a tow-fish. The
and maintenance, fast ping rates and portability,
signal transmitted from transducers penetrates
whereas their shortcomings are their narrow fre-
into the shallow subsurface sediments and is
quency bands, too long pulse lengths consisting
reflected back from the interfaces. The reflected
of several oscillations resulting in low-resolution
signal is received by the same transducers,
data, and their relatively low output power
amplified and digitally recorded into the disks.
preventing higher subsurface penetration.
Some side-scan sonars also have subbottom pro-
Chirp (acronym for Compressed High Inten-
filer transducers mounted on the tow-fish to col-
sity Radar Pulse) systems use sweep signals
lect subbottom profiler seismic data along the
very similar to those used in land VibroSeis sys-
tow-fish tracklines along with the sonar data,
known as a combined system. When properly tems. They use frequency-modulated (FM)
calibrated, a subbottom profiler system is also sweep signals generated by the computers using
predetermined signal parameters such as the
used to obtain bathymetric profiles along the amplitude, frequency band, etc. They operate
survey route. at frequencies between 1 and 10 kHz (generally
Depending on the signal shape and the equip-
ment used to generate the signal, subbottom 2–7 kHz) and their penetration depth is com-
monly limited to 60 m. The theoretical vertical
profilers can be classified into four categories:
resolution of a Chirp signal of 2–7 kHz for
1500 m/s water velocity is approximately
12.5 cm. Since it is a controlled-source signal,
TABLE 1.5 Specifications of Subbottom Profiler its source signature generated by computers is
Systems Used to Obtain Shallow Stratigraphy of the well known. Reflection signals are commonly
Uppermost Sediments
cross-correlated with the known source signa-
ture before recording the data onto the disks to
System Frequency Penetration Applications
obtain Klauder wavelets of relatively higher res-
(kHz) Depth (m)
Subbottom 1–10 10–60 • Very shallow olution and S/N ratio. Therefore, among the
profiler stratigraphy other high-resolution seismic equipment, Chirp
• Gas bubbles subbottom profilers are the systems that have
in 1D profile
• Gassy the highest possibility of recovering the signal
sediments from the noise. Fig. 1.12 shows the transducer
• Pipeline array of a 3.5 kHz Chirp system and an example
inspection of Chirp data.