1 Seismic Reflection 3D seismic reflection image of the Nankai Subduction zone, Japan

2 Seismic Sources: Land SurveysMany options! Hammer, weight-drop, dynamite, vibroseis What frequencies are needed? What energy level is needed (what distances do you need to cover)? Image courtesy USGS

3 Seismic Sources: Marine SurveysHistorically, dynamite was used, but due to environmental concerns air guns are now used. If internet access is available, supplement with youtube video examples (Langhammer, 1994)

4 Source Signals Land - Vibroseis Source Marine – Airgun Source

5 Airgun array

6 Land Recording Each recording point is a small array

7 Marine recording- hydrophones on a “streamer”

8 Random Noise Noise that is transient in space or time. ExamplesEnvironmental noise (winds, swaying trees, wave noise, animals) Electronic interference (portable equipment) Human activity (traffic, people walking by, boat noise) Quiet! Seismic Equipment!

9 Coherent Noise Signals that repeat on many traces and/or at different times that are not desired. Typically represents a long-term feature or type of wave. Examples: Ground roll Multiples Electronic interference (power lines)

10 Does this reduce coherentStacking Stacking is the summation of recorded waveforms/seismograms/traces that reflected off of the same point. Does this reduce coherent or random noise? Duplicate records helps us raise the signal-to-noise ratio, giving us clearer images and improved resolution. Here, we are looking at a Common Mid-Point Gather (CMP Gather).

11 Stacking Stacking is simply adding & normalizing multiple traces or seismograms. Constructive Interference Destructive Interference + + =

12 Gathers – how to organize the data

13 Normal Moveout

14 Normal Moveout t2(x) = to2 + x2/v2

15 Now these traces can be stacked to reduce noise!Normal Moveout t2(x) = to2 + x2/v2 Now these traces can be stacked to reduce noise!

16 Velocity Analysis Ecker et al. 2000,1998