Seismic reflection Seismic reflection profiling basically same principle as echo sounding But lower frequency used for greater subbottom penetration Trade off: –Lower frequency penetrates deeper –Higher frequency means greater resolution Many sound sources, but typically on the order of Hz, depending on application –Lower - deep oil prospecting –Higher - Hazard surveys, shallow subsurface

Source-receiver geometry Source is explosive –air gun –sparker –water gun –transducer –other Receiver is horizontal hydrophone array, “eel” –attenuates horizontal waves –amplifies vertical waves Rep rate sec

Seismic artifacts - bubble pulse Seismic section is in time, not depth –Velocity function often unknown –Velocity “pull-up” or “pull down” –Must correct for velocity/depth to get true geometry Bubble pulse masks details of layering –explosion creates cavity that collapses, rebounds, acts like damped oscillator –creates pulse train that is reflected from interfaces –Use “deconvolution” to remove bubble pulse

More artifacts - diffraction Diffraction - wave energy is scattered from edges to make hyperbolae Side-swipe - waves reflected from objects off to side What to do? Migration –Computer process that traces scattered energy back to source –need to know media velocity –works best on 3D data

More artifacts - multiples Multiple occurs when waves are reflected strongly from interface –ship’s hull –sea surface –peg-leg in layer

Single channel seismic profile Allison Guyot, Pacific Ocean; Ocean Drilling Program Leg 143 Two way traveltime

Low-fold digital seismic profile km Two way traveltime 250 ms Shatsky Rise, Pacific Ocean, depth =3412 m 6-channel, 4-air guns, stacked, filtered & migrated