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

Principles apply on many scales

SOES6004 acquisition and geometry

Airgun• Left end is reservoir, right end

mechanism• Air expelled through annular ports

SOES6004 acquisition and geometry

Commercial seismic boat• Specialised for one thing only…..

SOES6004 acquisition and geometry

3C ocean bottom cable boat• Even more specialised!

SOES6004 acquisition and geometry

Common midpoint gather

SOES6004 acquisition and geometry

Several CMPs

SOES6004 acquisition and geometry

Several shots

SOES6004 acquisition and geometry

Single shot gather

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

SOES6004 acquisition and geometry

Constant velocity panels

SOES6004 acquisition and geometry

Semblance analysis