What is Seismic? A brief introduction.

What is Seismic?

It is a method of determining sub-surface information by analyzing sound waves originating from an energy source.

What is an energy source?

The energy source is typically either a small amount of dynamite in a shot hole, drilled to depths up to about 100 feet or vibrations generated from a series of "vibroseis" trucks.

How are the sound waves collected?

The reflected sound waves are detected by listening devices called geophones that are laid out along the seismic line. They are usually attached to cables, that connect into a recording truck / doghouse where the seismic data is recorded onto computers.

What is the difference between 2D & 3D?

2D seismic lines are single lines of regularly spaced geophone stations (e.g. every 165 feet). Energy source points are established along the line typically at every 2nd or 3rd station. The same line contains recording cables and geophones as well as source points.

3D seismic programs are generally a uniform and evenly spaced grid of lines. Receiver lines containing the recording devices (geophones) usually, but not always, run in a direction perpendicular to the source lines. The spacing between lines can vary from project to project. Spacing between energy source points will also vary from project to project.

Who will be on the program and when?

After landowners and Government agencies have granted the appropriate permission, the following operations will usually occur:

1. The seismic line will be established as indicated on a program map.
   A surveyor or advance person will refer closely to, and adhere to, all permit conditions and will supervise any bush cutting and locating of underground utilities (e.g. pipelines, telephone cable, etc). He / she will track-out and mark the line with lath or survey stakes and position signs and flagged arrows to show access routes.
   
   
2. Bush cutting and /or snowplowing will be done.
   If there is any bush on the proposed seismic line, the surveyor or advance person will supervise the cutting, where allowed, using cats or chainsaws. Receiver lines usually require a continuous straight-line path for cable and geophone layout. Source points can be "offset" to avoid cutting bush and soft or wet areas. When snowplowing is required, cats or 4 wheel drive tractors are utilized.
   
   
3. Utilities will be located.
   It is normally the responsibility of the utility owner to mark out the location of its pipeline or cable; however, sometimes a locating company will conduct this work. Survey stakes, lath or pin flags typically marked with yellow flagging or the utility company's logo will be placed along the route of the buried utility.
   
   
4. The points will be surveyed.
   The seismic line will be surveyed to enable an accurate co-ordinate and elevation to be calculated for each point along the line. Conventional surveys are performed by a surveyor traversing between survey control points using a Survey Instrument, which measures angles and distances to high standards of precision. On 3D programs several survey crews may be working in the area at the same time.
   
   
5. GPS layout, surveying & mapping may be done.
   The layout and surveying operation can also be accomplished by GPS (Global Positioning System) pack operators, who position and record points by receiving signals from a network of satellites that enable them to resolve an accurate position and height. Some of these crews may also collect cultural information for mapping (e.g. fences, water bodies, power lines, etc.).
   
   
6. The shot holes will be drilled.
   When dynamite is the energy source, each source point will be drilled and then an explosive charge, detonator / blasting cap will be loaded to the required depth. Capwire will connect the charge to the surface for later detonation. In areas of difficult drilling, patterns with 2 to 5 shallower holes containing smaller charges, may replace the normal single hole. All shot holes will be abandoned in accordance with the exploration regulations. The seismic drilling rig can be on a wheeled or tracked vehicle. There are many types and sizes of drills that are designed to work in the different areas where subsurface conditions can vary considerably (e.g. sand, shale, gravel, and rock). Terrain, bush and permit constraints will also affect the type of drill selected for each job. It is usual for more than one drill to be used. A drilling supervisor will coordinate seismic drilling operations. Some drills require water so may have to travel the line more than once. In some cases a water truck will ferry water to the drills as required.
   
   
7. The recording cable will be laid out.
   Cable, geophones (jugs) and recording devices (boxes) are normally carried aboard line trucks. The cables will be spooled out of the truck along the seismic line. In narrow bush cuts, over fences, across creeks and through wet or soft areas, cable will be hand carried by the line crew. The geophones and recording devices will be connected to the cable at the prescribed locations. When all of the geophones are in position and the recorder has connected to the cable the line is ready to be recorded. Note: On some jobs, helicopters are used to transport; cables, geophones and recording devices in heli-bags to planned locations, from where the line crew manually position them.
   
   
8. The seismic line is recorded.
   Where dynamite is the energy source, a shooter will sequentially visit each shot hole and detonate the charge. Where the seismic line is to be vibrated, typically 1 to 4 vibrator units mounted on trucks or buggies, will lower their pads and simultaneously vibrate the ground for a few seconds at each source point location. Often vibrators will vibrate over several locations on either side of each source point.
   
   
9. Cleanup of the seismic program.
   On completion of recording, the seismic lines will be completely cleared of all recording equipment, survey material and debris. In the case of frozen ground conditions, a cleanup crew will return to the seismic program when ground conditions are suitable, (non-frozen). They will ensure that all debris has been removed. All shot holes will be abandoned in accordance with the exploration regulation.

24-hour operations

Recording, explosive detonation and vibrating can sometimes occur at night to overcome weather disturbance problems such as wind, or to expedite the job.

Who do I contact in the case of a problem, query or complaint?

• The Crew Manager
  The Crew Manager is responsible for all field recording crew activity. He / she will often establish a crew office at a local motel from where operations are coordinated.
• Crew Supervisors
  The Crew Supervisor can be contacted at the Geophysical Company's head office.
• The Permit Agent
  The permit agent should be informed of any problems.

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