SOES6004 Data acquisition and geometry

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

2. 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. 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. Several CMPs SOES6004 acquisition and geometry

9. Several shots SOES6004 acquisition and geometry

10. 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. 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. 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. Semblance analysis SOES6004 acquisition and geometry