1. Acoustic Techniques (review) Sound is a pressure wave: n ear vibrates in response to changing pressure n hole in eardrum = equal pressure = no hearing n sonic boom = passing of pressure wave in a solid body (like the earth or sediments or rocks) there are 2 types of waves: n 1) compressional = P waves n pass through any phase n 2) shear = shear waves n cannot pass through a liquid n also surface waves = n Love (horizontal) and n Rayleigh (vertical)) travel along interfaces -

2. Each type of wave travels at a specific speed V p = sqrt [(  + 2* µ) /  ] V p = sqrt [(  + 2* µ) /  ] V s = sqrt (µ /  ) V s = sqrt (µ /  )  Lamé's coefficient= E *   Lamé's coefficient= E *  (1+  )(1-2  ) (1+  )(1-2  ) n E = Young's modulus (strain per unit stress)  = Poisson's Ratio (Relates elongation to thinning)  = Poisson's Ratio (Relates elongation to thinning) n µ = modulus of rigidity= E______ 2(1+  ) 2(1+  )  = bulk density

3. V p = sqrt [(  + 2* µ) /  ] V p = sqrt [(  + 2* µ) /  ] V s = sqrt (µ /  ) V s = sqrt (µ /  ) Note: Note: Higher rigidity = higher speed Higher rigidity = higher speed Higher density = lower speed Higher density = lower speed Compressional waves will always be faster (about 2X) Compressional waves will always be faster (about 2X)

4. Example Acoustic Velocities

5. Earth’s Structure (revisited) n Here’s how we know: n S waves: n travel only through solids n are shadowed by core n P waves travel through anything n pass through core n are refracted by outer core n results in shadow zones n Add transmission times to shadow zones and you have our current model

6. Echo sounders work by sending a pulse of sound (a ping) into the water column and timing its return. Echo sounders work by sending a pulse of sound (a ping) into the water column and timing its return. n Sound travels at 1500m/sec so in 1500m of water it takes 2 seconds to return. n We’ll go through this quickly to be sure we agree on vocabulary

7. “Typical” Echosounder Record n note: smoothing, depth-varying resolution, varying gain, scattering layer, sub-surface penetration, surface multiple.

8. “Little” Complications n 1) Multiples: there are several paths sound can take from the source to the receiver:

9. “Little” Complications n Multiples: n parallel direct return n fainter n double slope

10. Hits here first “Little” Complications n 2) “hyperbolae” n If the cone angle is >0 (and it always is), sound can be reflected from objects which are off nadir (not directly beneath the ship):

11. Both returns are recorded “Little” Complications n This causes a false bottom to appear on the record

12. “Little” Complications n Here’s what you end up with Real Bottom Resulting record

13. “Little” Complications n You can see them on most “raw” sections: n This is actually actually a flat a flat valley valley

14. “Little” Complications n Sometimes even zooplankton, or more likely nekton, can produce them

15. “Little” Complications n To compensate, the hyperbolae can be “migrated” back to a point. n This traditionally required massive amounts of labor

16. “Little” Complications n But when done correctly, accurate results are obtained n (okay, these were done digitally) Unmigrated Migrated

17. Sound transmission & “Inhomogeneities” an inhomogeneity can be a: an inhomogeneity can be a: n change in impedance contrast -- reflection n change in velocity -- refraction (bending) impedance contrast = a change in impedance across an interface between layers impedance contrast = a change in impedance across an interface between layers Impedance =  *c where Impedance =  *c where  = density  = density n c = compressional velocity (same as V p before) n At layers in the sediment or rock where impedance changes, sound is reflected back n We call these layers/interfaces “reflectors”* *duh!

18. Reflections from within the sediment Each “reflector” will show up on the record Each “reflector” will show up on the record Stronger reflectors show up better Stronger reflectors show up better The interface between water and sediment is usually the strongest The interface between water and sediment is usually the strongest We’ll talk about these terms/gadgets next

19. Reflection of Acoustic Waves Reflectivity coefficient (Rayleigh) = Reflectivity coefficient (Rayleigh) = V 1  1 - V 2  2 V 1  1 - V 2  2 V 1  1 + V 2  2 V 1  1 + V 2  2  = density  = density n V = compressional velocity n more change = more sound reflected n An example: layer 1  1 = 1.92 gm/cc*, V 1 = 2.0 km/sec layer 1  1 = 1.92 gm/cc*, V 1 = 2.0 km/sec layer 2  2 = 2.39 gm/cc, V 2 = 4.0 km/sec layer 2  2 = 2.39 gm/cc, V 2 = 4.0 km/sec reflection coefficient = 0.43 reflection coefficient = 0.43 *gcm -3 for you purists

20. Reflection of Acoustic Waves What does a reflection coefficient = 0.43 mean? What does a reflection coefficient = 0.43 mean? This is the amplitude ratio: This is the amplitude ratio: n the amplitude of the reflected wave is 43% of incident But energy is proportional to the square of amplitude (E= A 2 ), But energy is proportional to the square of amplitude (E= A 2 ), n energy is this value squared = 0.18, n 18% of the energy will be reflected and 82% will be transmitted. n A lot of energy will be lost at these discontinuities. n The more there are, the less sound penetrates

21. Other losses a. Geometric spreading (spherical spreading) a. Geometric spreading (spherical spreading) n E is proportional to 1/(r 2 ) n sound (or light) energy dissipates as it spreads n has to cover more area b. Absorption = conversion to heat b. Absorption = conversion to heat n A r =A o e -ar n where a = absorption coefficient. n this equation accounts for both effects n The amount of loss is thus a factor of: n the distance traveled n a (attenuation coefficient) n but what determines a?

22. Other losses A r =A o e -ar A r =A o e -ar a=pf/Qv where : a=pf/Qv where : f is frequency f is frequency v is velocity v is velocity Q is something called the log decrement of amplitude Q is something called the log decrement of amplitude means that louder sounds attenuate more quickly means that louder sounds attenuate more quickly (ask your acoustics professor if you want more detail!) (ask your acoustics professor if you want more detail!) Attenuation is proportional to the first power of frequency. Attenuation is proportional to the first power of frequency. Therefore, low frequency will go further. Therefore, low frequency will go further. That’s why we hear the “thumps” from low-riders

23. Frequency and Echosounding n If we want to do some serious echosounding, what frequency should we use? n If we only want to find out how deep the water is, it hardly matters n 12 kHz is pretty typical n in shallow water, we might even use 200 kHz n But if we want to see into and beyond the sediment, we better use low frequency to get the penetration we want. n But there’s a tradeoff……..

24. Frequency and Echosounding Low frequency sound penetrates well, but is low resolution: Low frequency sound penetrates well, but is low resolution: We can resolve about  / 2 We can resolve about  / 2  is wavelength  is wavelength For example: For example: n 15,000 Hz / 1,500 m/sec = 10 cycles/m, =0.1m n 150 Hz /1,500m/sec = 0.1 cycle/m, = 10m We pick compromise frequencies: We pick compromise frequencies: n short range navigational sources use several hundred kilohertz (transducer), n ordinary echo sounders use 12 kHz, n to go really deep, we need deep (low frequency) sound

25. n To penetrate the sediment, we use special techniques and have a special name: “Reflection Profiling” n Reflection sources use 30-100 Hz. n air guns, n electric sparkers, n gas exploders (propane/oxygen mix), n explosives. n 3.5 kHz is a compromise n moderate penetration n but excellent resolution n look at recent sediments only.

26. n Reflection profiling used to use dynamite and hydrophones n Now we use “air guns” and “eels” n air guns safely produce loud, low frequency pulses n these travel through the water and sediment and back to the surface n numerous hydrophones in the “eel” detect the echo EelAirgun

27. n This boat was towing 12 (twelve) separate lines n 6 airguns n 6 streamers, each of which was 3+ miles long

28. Earth’s Structure (Cont.) n These studies reveal all kinds of useful information: n salt domes n hydrate deposits n oil n faults n bedrock features n requires lots of ship time and computer processing

29. n This information is used to develop and interpret the sediments, including the history of events

32. These are also sediment, but badly deformed This sediment has accumulated recently

33. n Conformity: reflector which is continuous over space n unconformity: place where a reflector or layer is discontinuousp n onlap: rising sealevel moves deposition shoreward n toplap: normal deposition on top of existing sediments

34. Seismic methods elucidate sea level changes rising stillstand falling

35. n This looks complicated but look how much information is contained in this section.

36. Multibeam n sound can be directed through the water by “beam forming” n where waves are in phase, sound is amplified n where out of phase, sound is cancelled

37. Multibeam n beam forming: n using this principal, sound can be “aimed” in the water by carefully timing outgoing pings n this works for receiving too

38. Multibeam n transducers mounted on the keel produce a “slice” of sound which paints a strip of the seafloor

39. Multibeam n transducers mounted “athwartship” listen for arrivals from specific angles

40. Multibeam n this combination allows depth values from several small patches of seafloor to be determined simultaneously

41. Sidescan n Uses formed beams on each side to “illuminate” the seafloor at an oblique angle n Highlights irregularities n No inherent topographical information n Sediment texture from echo characteristics