Name 6 potential constraints to taking a seismic survey
- Time
- Weather
- Economics
- Geography
- Politics
- Geological/geophysical parameters
How do 2D and 3D seismic surveys compare in terms of how they are acquired?
2D: Shot in grid pattern, spacing in order of kms
3D: Acquired as swaths of more closely spaced points for better resolution, spacing in order of tens of ms
What is the cost breakdown for an average survey?
Reflection seismic technique - 70%
Data Processing - 10-15%
Interpretation: 10%
2D seismic surveys are acquired as regular orthogonal grid lines parallel and perpindicular to direction of dip.
What influences the spacing between lines in terms of structure to be mapped?
What does the orientation of the grid of dip and strike depend on?
Spacing influenced by size of structure:
- Needs to be at least half the wavelength of the fault block
- Wavelength of structures estimated by previous seismic
- Essential to intersect any wells that have been drilled in area both in dip and strike directions
Orientation dependent on:
- Trend of structures
- Spacing and size of structures
- Cost constraints
When planning and preparing a 2D seismic survey what should the designer do if possible?
- Keep line spacing equal
- Keep lines straight
- Intersect well in two directions
- Avoid restricted areas
- Leave out areas of no importance
- Allow lines to extend 2km beyond area of interest to allow migration of dipping reflectors during processing
What field equipment is used needed for seismic surveys?
Positioning systems Sources Receivers Cables Recording Instruments
What does a static correction allow? How does it account for variations in height of different source data location and effect of shallow weathering?
- Allow all shots and receivers to be related to a common reference datum
- Corrected by means of a fixed time shift
What does Normal-Moveout (NMO) account for?
Why is this done?
Compensates for time delay resulting from the varying horizontal separation of the source and receiver
Done as we only want to compare the vertical component of travel times
How is dynamite used to generate an energy signal on land?
Shot hole is drilled to below the weathered layer then a controlled explosion is let off
Give pros and cons of using dynamite
Pros: Good signal usually produced, thickness and seismic velocity of the weathered layer can be measured directly for static corrections
Cons: Time consuming, expensive, not viable near buildings and fragile ecosystems
Vibroseis is the main alternative to dynamite. Explain how it is used to generate a signal
- Employs 4/5 trucks with heavy metal pads beneath that can be lowered onto ground surface
- Pads used to generate a known seismic signal (sweep)
- Sweep lasts 20s and goes from low to high frequency
- Trucks line up and generate the sweep together before moving to the next position
- Each reflection will consist of a low delayed, low amplitude copy or inverted copy of the original sweep
- Raw data needs digitally corrected with the initial signal
- Thickness and velocity of weathered layer is calculated from the time of refraction arrivals
Give three pros and two cons of Vibroseis
Pros: Environmentally friendly, minimal disturbance, can be done alongside roads
Cons: Vehicles need access (poor terrain), slower process
What does a geophone do?
Converts ground motion into electrical voltage by relative motion between a sprung coil and a magnet
Why do geophones only allow for vertical motion?
Only P-waves of interest
How are geophones arranged/planted? What imfluences the number and spacing of geophones?
12-120 geophones with 5-10cm spike
Planted in good contact with earth in regular pattern
Number and spacing of geophones in the pattern will have been designed to suppress horizontally travelling ground roll and random noise
What test is done to establish noise problems in the area?
How is this done?
Noise spread field test
Single geophones arranged closely together, each geophone response recorded separately to produce detailed field report with no noise cancellation
What is the role of cables in seismic acquisition?
Geophones grouped together in strings of six
Main cable runs entire length of active spread
Made up of a series of sections so that as the recording effort passes the active stations they can be disconnected and geophones reused further along line
How is data recorded on magnetic field tapes?
Each receiver is sampled in sequential order at set time intervals and these values are written to tape in multiplex format
What do recording trucks allow the operator to do?
- Control recording sequence of every shot
- Monitor the progress of the survey
- Communicate with theVibroseis truck drivers and party manager
