Page 16 - Acquisition and Processing of Marine Seismic Data

1.1 UNDERWATER ACOUSTICS 7

• Stanford Exploration Project, SEPlib TABLE 1.1 Effects of the Seawater Physical Parameters
(Stanford University) on the Sound Velocity
• Seismic Un*x (SU) (Colorado School
of Mines) Parameter Effect on the Sound Velocity
• FreeUSP (BP America Inc.) Temperature (T) 2.7 (m/s)/°C
Salinity (S) 1.2 (m/s)/ppt
1.1 UNDERWATER ACOUSTICS Pressure or depth (P) 0.017 (m/s)/m

Since we use sound waves to explore the
ocean bottom and subsurface sediments, we mouths, seabed freshwater discharge areas, and
should know the velocity of the sound waves glacial melting zones. In practice, the most
in seawater and the parameters affecting the important agent that affects the sound velocity
velocity and other properties of our acoustic sig- in the oceans is the temperature. Although the
nal. Behavior of the sound waves is the study temperature at the ocean floor is very stable,
area of ocean acoustics. In marine seismic explo- rapid temperature changes in both vertical and
ration, the seismic signal is always produced in horizontal directions can occur in the surficial
seawater, and once it is created, it is no longer waters due to climatic conditions.
under our control. When we apply an impact There are specific types of layers within the
in the water, it creates pressure waves that suc- water column, termed clines, which have differ-
cessively compress and decompress the water ent physical properties from the surrounding
molecules, resulting in the traveling of the water. The physical properties of seawater, such
sound wave in all directions in three dimensions as density, temperature and salinity, may
away from the source through the seawater. change with depth at a particular location,
Sound in the oceans travels as pressure varia- creating well-established specific zones just
tions as compressions and decompressions below the surficial water layer within the water
and can be detected by specific pressure sensors, column. These zones are known as pycnocline,
termed hydrophones. In this section, brief defi- thermocline, and halocline, respectively
nitions of the factors affecting the sound velocity (Fig. 1.5).
in the water column as well as the fundamental Warm water is less dense than cold water,
physical characteristics of the seawater are and therefore it remains along the sea surface
discussed. and gets warmer and warmer because of solar
Sound waves within the frequency band of heating. This situation results in the formation
the seismic signal can travel large distances of a relatively warmer surficial water zone,
due to the relatively low signal attenuation char- termed the surficial water layer or mixed layer.
acteristics of the oceans. This makes the sound The thermocline is a transition zone from the
waves excellent tools for acoustic exploration mixed layer at the surface to the deep water
of the sea. The sound velocity in the water col- layer (Fig. 1.5A). In the thermocline zone, the
umn (approximately 1500 m/s) is determined temperature rapidly decreases from the surficial
by the physical properties of the ocean water, layer temperature to a relatively colder deep
such as salinity, temperature, and density water temperature. The depth and thickness of
(hence the pressure). Table 1.1 shows the influ- the thermocline zone are affected by climatic
ences of these parameters on the sound velocity. variations, latitude and local tide and current
Salinity in a specific region generally does not conditions. The halocline is a layer within the
change significantly, except in the areas of river seawater column, where salinity changes