Seismic Flashcards
1D seismic
- called well shot
- used to calibrate seismic and measured depth in a well
2D seismic
- shot in one direction only
- these days used for regional mapping and tie-line projects only
- as VSP in a well
3D seismic
- shot and acquired over an oil/gas field, or coal mine (PRAKLA/Seismos early 1980)
- for detailed reservoir and fault correlation/mapping
- forecast of reservoir/seal properties
- forecast of fluid properties
- AVO
- field development planning
4D seismic
- field monitoring
Body waves: P or Primary waves
- are longitudinal or compressional waves
- the ground is alternately compressed and dilated in the direction of propagation
- In solids, these waves generally travel almost twice as fast as S waves & can travel through any type of material
- In air, these pressure waves take the form of sound waves, hence they travel at the speed of sound
S or Shear waves
- transverse or shear waves
- the ground is displaced perpendicularly to the direction of propagation
- S waves can travel only through solids
Surface waves
- analogous to water waves and travel just under the Earth’s surface
- travel more slowly than body waves
- Rayleigh waves & Love waves
- low frequency, long duration, and large amplitude - the most destructive
Seismic sources
1. Explosives
At present used only onshore; dynamite or seismogelit, 500 g or less.
2. Vibroseis - onshore
A seismic signal is produced by a vibrating heavy metal plate
3. Airgun (offshore)
A highly compressed, and rapidly expanding air bubble
4. Drill-generated
Using the noise from a rotating drill bit
Seismic receivers
1. Geophone
only onshore; a kind of microphone with a magnetic coil; placed into soil. Several geophones are typically lined up in patterns and linked to a receiver/amplifier box with a certain number of channels
2. Marshphone
A special watertight geophone pushed into wet soil in marshlands. Linked to a station or a baycable
3. Hydrophone.
A watertight microphone operating on the principle of the piezo-electrical effect. Linked to a streamer
Seismic recording
1. Onshore recording
Geophones are grouped into clusters, and these are wired to a relay station, and from there to the recording truck. A modern digital recording truck can handle several cables with more than 1000 clusters/channels
2. Offshore recording
Traditional 2D recording requires only one airgun, and one streamer. A modern streamer has many groups of hydrophones (in linear arrays) and can be up to 8 km long. Modern marine 3D seismic involves several streamers (2, 4, 6 or 8) with several airgun arrays towed right behind the ship.
Essential components of a signal
- In seismology, a coherent wave response as recorded by the geophone is called signal. It is commonly distinguished from un-coherent seismic energy which is called noise.
- A signal has essentially three components:
- Frequency range
- Amplitude
- Phase
