2nd Pan-American/Iberian Meeting on Acoustics

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Simulations of Sound Travel in the Ocean with Oil Present

 

 

Michael Vera - michael.vera@usm.edu

James Stephens

University of Southern Mississippi

Department of Physics and Astronomy

118 College Drive, #5046

Hattiesburg, MS 39406

 

Popular version of paper 3aUWa8

Presented Wednesday morning, November 17, 2010

2nd Pan-American/Iberian Meeting on Acoustics, Cancun, Mexico

 

 

This study uses computer simulations to investigate the possibility of monitoring oil released in the ocean with sound.  These simulations occur in an environment similar to the Gulf of Mexico.  Oil is placed in this model environment and the travel of a sound pulse through a region of the ocean interior is computed.  If there are clear impacts on the structure of the sound from having moved through the oil, then the method may be useful in monitoring actual events.  Different approaches are under consideration by other researchers, such as searching for sound reflections from bodies of oil back toward the original sound source.

 

Sound is often used as a way to transmit a signal through the ocean interior.  One reason for this is that electromagnetic waves (such as radio and radar) are strongly damped in ocean waters and cannot travel for substantial distances before they become undetectable.  The manner in which sound travels through the water is influenced by the behavior of the speed of sound.  A typical value for the speed of sound in the ocean is about 1500 m/s (compared to about 340 m/s through the air) but, because of the nature of the marine environment, the value at a specific location depends on the depth.  This variability in the speed of sound leads to an effect called refraction in which the paths followed by sound are curved, analogous to the way that light is curved by lenses.  The resulting sound structure at a final range (100 km is used in this study) is affected by this refraction.  Even for a single source pulse, sound can reach a specific depth at the ending range along several different curved paths, each with different travel times.  This is called multipath propagation; it is somewhat similar to an echo, in that sound can reach a listener along multiple routes with arrivals separated in time.  Figure 1 shows the sound intensity as a function of depth and travel time at 100 km using sound speeds derived from measurements associated with the Littoral Acoustic Demonstration Center.  The depth of the water is taken to be 1500 m.

 

vera01.jpg

 

Figure 1.  The received sound at 100 km is depicted as a function of depth and time.  There is no oil present in this simulation.

 

The speed of sound in seawater is expressed as a function of depth, but that variability occurs because of changes in pressure and temperature.  Since the way the sound travels depends on the speed of sound and the speed of sound depends on temperature, temperature can be measured with acoustic methods.  Ocean-acoustic thermometry is a well established technique.  For this study, changes in the speed of sound due to the introduction of oil are considered.  The way that the speed of sound in oil varies with temperature and pressure is significantly different from the way that it does in seawater.  Therefore, the presence of oil will lead to variability in the speed of sound with a somewhat different character.  If the changes in the sound speeds at various locations have a sufficient effect on the sound traveling through the region, then it would be possible to extract information about the distribution of the oil from the received sound.  Figure 2 shows the impact on the sound of a particular assumed oil configuration.  This case was an attempt to model a scenario in which a patch of the seafloor at half the final range is regarded as a source of oil.  The oil is regarded as moving upward in a cone with the concentration of oil decreasing with distance from the patch.

 

vera02.jpg

 

Figure 2.  The received sound is depicted as a function of depth and time at a final range of 100 km.  This simulation has oil distributed inside a cone with the highest level at a patch on the seafloor at half the final range.  The fraction of oil decreases with distance away from this patch.  Note that differences in the sound pattern can be observed (at, for example, a travel time of about 68.15 s).

 

This assumed distribution of oil has a clear effect on the sound as shown in Figure 2.  Progress in this research will involve the incorporation of more sophisticated configurations of concentration and a more thorough accounting of the changes that occur in oil in such an extreme environment.  It may also be useful to investigate the acoustic impact of other related substances, such as methane.

 


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