Kinematics I: Earth’s structure, plate boundaries and plate geometry
SOES6004 acquisition and geometry SOES6004 Data acquisition and geometry What do wave kinematics...
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Transcript of SOES6004 acquisition and geometry SOES6004 Data acquisition and geometry What do wave kinematics...
SOES6004 acquisition and geometry
SOES6004 Data acquisition and geometry
What do wave kinematics mean about the way that we do an
experiment?
SOES6004 acquisition and geometry
Data acquisition
To acquire data we need a source, some means of detecting waves within the Earth, and a way of recording the data.
Marine sources:• Airguns, water guns, boomer and Chirp• Normally detected at sea with a hydrophone although they can
also be detected with seismometers (eg in an ocean bottom cable or on land nearby.)
Land sources: • Large explosive charges (up to 5000kg is not unusual for
seismic refraction), smaller charges (5-10kg for commercial reflection, 1kg for near surface work), shotgun shells, hammer, to vibroseis (vertical or shear).
• Usually recorded using geophones banged into the ground surface.
SOES6004 acquisition and geometry
Airgun• Left end is reservoir, right end
mechanism• Air expelled through annular ports
SOES6004 acquisition and geometry
Experimental geometry
• We want Common Midpoint gathers for processing, but they are inefficient to collect
• By collecting many shots each recorded at many receivers simultaneously we can collect all the traces that would be present in multiple CMPs
• Sort it all out in a computer later!
SOES6004 acquisition and geometry
Stacking
Why do we stack?• First, we increase the signal to noise ratio. Suppose
we have N traces with signal amplitude A and random noise amplitude A (ie signal to noise ratio 1). If we can align the signal on the traces and then sum, the amplitude of the signal will be NA but the amplitude of random noise will be NxA, so the new signal to noise ratio is N.
• Second, it turns out that aligning the signal as we look at traces recorded with different source-receiver separations will give us information about the Earth.
• This does assume we know what is signal and what is noise
SOES6004 acquisition and geometry
Velocity estimation
• Because the normal moveout equation includes velocity, we can use the intent of the equation backwards to estimate how velocity varies
• Apply nmo at a range of velocities and see what is “corrected” (ie has the same time after applying the equation at all offsets)
• Aim to determine velocity as function of time and position that gives best stack