1. SOES6004 Data acquisition and geometry
What do wave kinematics mean about the way that we do an experiment?
SOES6004 acquisition and geometry

2. 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

3. Principles apply on many scales
SOES6004 acquisition and geometry

4. SOES6004 acquisition and geometry
Airgun
Left end is reservoir, right end mechanism
Air expelled through annular ports
SOES6004 acquisition and geometry

5. Commercial seismic boat
Specialised for one thing only…..
SOES6004 acquisition and geometry

6. 3C ocean bottom cable boat
Even more specialised!
SOES6004 acquisition and geometry

7. Common midpoint gather
SOES6004 acquisition and geometry

8. SOES6004 acquisition and geometry
Several CMPs
SOES6004 acquisition and geometry

9. SOES6004 acquisition and geometry
Several shots
SOES6004 acquisition and geometry

10. SOES6004 acquisition and geometry
Single shot gather
SOES6004 acquisition and geometry

11. 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

12. 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

13. 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

14. Constant velocity panels
SOES6004 acquisition and geometry

15. SOES6004 acquisition and geometry
Semblance analysis
SOES6004 acquisition and geometry