Reflection Seismic Method The hyperbolic form of the reflection travel-time: t2 = (4h2 + x2)/V02 This form describes NMO (normal moveout) V0 V1  V0

Recall ray paths in a 1-layer model Direct ray: t(x) = x/V1 Refracted ray: t(x) = ti + x/V2 where ti = 2z(V22 – v12)1/2/V1V2

Direct and Refraction ray travel times Direct ray: t(x) = x/V1 Refracted ray: t(x) = ti + x/V2 where ti = 2z(V22 – V12)1/2/V1V2

A dipping layer – refraction paths

Travel times – dipping reflector

Diffractions masquerade as reflections

Amplitudes of seismic waves Waves lose amplitude due to: Geometric spreading as waves move farther into volume: Amplitude = 1/(distance)2 Absorption of wave energy into heat: Amplitude ~ A0e-x Scattering -- reflections, refractions, diffractions from layer inhomogeneities and anisotropies

Automatic gain control We compensate for the ever decreasing wave amplitude by preferentially amplifying the low amplitude signals... Logarithmic amplifiers... the signal records as the logarithm of its amplitude Gain-ranging amplifiers... the signal records according to the average signal amplitude during a short time window

Seismic wave sources for reflection surveys Land surveys: Dynamite (geogel), hammer, thumper, dropping weights – shock sources are not easily controllable as to waveform Vibrational sources -- “Vibroseis” a controlled source

Generator for shear waves Small explosive source

Vibroseis system The Vibroseis “chirp” waveform

The “geophone”

Geophone response The geophone responds according to frequency of vibrations... The resonance: c = (m/k)1/2

Geophone arrays... Sensitivity according to apparent wavelength

Suppressing “ground roll” A geophone array is normally tuned to suppress “ground roll” -- the surface Rayleigh wave.

Marine seismic surveys

Marine source: Airgun

The airgun pulse shape The gas bubble released by the airgun oscillates causing a repetition of the pulse

Airgun arrays The pulse from 1 gun An array The pulse repetition is cancelled by interference

Hydrophone, streamers and recording

A marine seismic profile (i.e. 2d)

Digital recording of seismic signals The electrical signals from the geophone or hydrophone are converted into a stream of “digital” values with a fixed interval between samples. Each “time series” of samples is recorded for subsequent data processing. A high-resolution marine seismic survey might accumulate several Gigabytes of data per minute.

Aliasing by sampling To avoid aliasing, we must sample 2x per shortest period in data

Anti-alias prefiltering A filter is designed to cut out the high frequencies (short periods) that would be aliased in sampling.

S/N enhancement The signal-to-noise ratio (S/N) is increased by N1/2, where N is the number of data traces added.

Noise reduction – CDP (CMP) We assemble many pairs of shot-detector spacings that reflect from a “common depth point”... we add the signals together to average away “noise”.

Reflection seismic processes The “seismic section” is a metaphor of geological structure