Seismogram Relation between seismic trace and geology Wavelets Multiples Resolution Vertical resolution Horizontal resolution

Seismogram of an earthquake mid atlantic ridge measured in Brasil magnitude 5 (sept 2002) 3000km Amplitude Unfiltered: Time (s) Filtered: P PP PcP S SS Rayleigh

From geology to seismogram

Waves Amplitude t or x Wavelength l Period t Phase 1 2 p Period t == --- ------ f w Wavelength l Period t Phase 2 p Wavenumber k = --- l Velocity v = f× l

Important wave forms Zero-Phase Minimum Phase 0 time 0 time

Minimum phase and zero-phase wavelet Minimum phase wavelet: Zero phase wavelet: Energy is maximally front-loaded (Energy does not arrive before zero time) Symmetric with respect to zero time and peaks at zero time (Energy arrives also before zero time)

Important waveforms Minimum Phase Zero Phase

Display of Seismogram

Example of a shot Reynolds, 1997

Multiples

Air gun record Keary and Brooks

Surface Riverbed Sediment Bedrock Reynolds

Resolution Resolution refers to the minimum separation between two features such that we can tell that there are two features rather than only one. Vertical resolution Horizontal resolution

Comparison of resolution of different data sets 30 Hz wave gamma log Big Ben (ca. 123 m) at 2000 m/s through oil field (Emery and Myers)

Vertical resolution Rayleigh’s-criterion: Vertical resolution depends on: - Frequency - Velocity

Resolution of two boundaries depends on wavelength

l/4=12 ms V1 V2 V3 < > or R12 R23 R12/R23= pos. V3 V1 V2 < > or R12 R23 R12/R23= neg.

Horizontal resolution w for z>>l

Sampling theorem At least two samples per (horizontal) apparent wavelength must be obtained in order to recognize Features (remember the time sampling theorem!!). For example, to recognize a stream channel on a horizontal slice generally requires bin sizes no larger than 1/3 or ¼ the channel width.