Re: Terminology of win, sea wave, swell and sea current
I'm abit confuse in definition of sea wave, swell and sea current and their relationship with wind on designing seismic acquisition. Could anyone here can give me a hand, thank you.
Cheers - Junior
"Swell Noise" appears on seismic data as a low-frequency "bursts" in a cone; it usually starts at a single trace and point in time, and then spreads over the rest of the record. It tends to sit in a band that can be up to 15Hz or 20Hz, and interferes with the seismic signals. A common way to avoid swell noise is to tow the cable more deeply; however this impacts on the frequency content. There will usually be a maximum level of swell noise that is acceptable. Processing solutions include filtering and projective filterering.http://seismicreflections.globeclaritas ... noise.html
"Sea Wave" - or wave-height is one way in which the risk of swell noise can be expressed; it can also cause other issues. For example, the variation in water depth will change the pressure around the airguns, which can modify the signature. The "rugged" water surface is also not a classic plane free surface, which can cause issues with the removal of multiples (reflections within the water column) - again, there is usually a maximum wave height that is allowed for the survey.
Currents are different set of issues. The vessel has to move at about 5 knots for their to be enough water flowing over the equipment to keep it in place - the depth controllers, paravanes and other equipment all work like underwater "wings" that need a current over them to work. Towing faster than this becomes a problem - it costs a lot more fuel, increases noise, and there's a risk of damage or loss of equipment as the pressure on cables increases.
5 knots is a speed through the water, not over the seafloor. With no currents it takes about 10 seconds to cover 25m, which is more than enough time to record a "standard" six second seismic record. If, however, you had a 5 knot current running with the boat, you would cover the same 25m over the seafloor in 5 seconds; the speed through the water is still 5 knots, however the water is also moving. You would arrive at the location for the next shot while you were still recording data - a situation identified on Observer's Logs as "shot missed due to vessel speed."
Similarly, shooting into a current (say 2-3 knots) means that progress over the seafloor is very slow - the line takes a long time to shoot, and becomes more expensive.
Shooting with or into the current is preferred to avoid cable "feather" - that is the cable not being directly behind the boat. On 2D data this creates a "swath image" with out of plane energy and requires work to regularise the offsets (as a straight line approximation isn't good enough); with 3D data you need to be careful about the azimuths of data collected within a single "CDP bin" depending on your survey design.
Unpredictable currents can create other issues, especially when shooting around obstacles (like platforms) as there is a risk of a collision.