February 17, 2011
Dear Editor,
Please allow space again to reply to the question asked by Sister YaYa Marin on Two Cents Cam. She asked “What is seismic testing?” and sadly, although key people are adamant about continuing oil drilling, the persons whose oil, if any more is found, do not even know what is seismic testing. It is also referred to as seismic surveys.
Seismic testing is a scientific method used to do two things: 1. predict the occurrence of earthquakes, and 2. discover natural gas. This method varies depending if you are testing for oil offshore or on shore.
And since the 1920 when it was first developed, the technology has vastly improved. It became popular in the 1930s.
Now, I must advise that explaining seismic is not simple, but I will try my best to put it in as lay terms as I possibly can. So I will explain onshore (on land) seismic first.
This testing can be done either in 2D or 3D – and just like what we know about television, the 3D will give you images that show length, depth, width, and like a 3D TV, show details a 2D would not give.
The difference in image comes from the fact that in 2D, the testing on the surface is done along parallel lines on the land area under petroleum contract.
The 3D, on the other hand, is done along a grid – so imagine 2D being the lines in your exercise book and 3D being the grid on your chest board or bingo card.
There are two ways or methods of testing, one more destructive, but cheaper and one little less destructive but more expensive.
1. Vibroseis – using high tech vibrators, trucks drive over cut out pathways or roads at a crawling pace, causing their vibration to send seismic waves (sound waves) deep into the earth.
A typical seismic survey in a forest preserve was described this way:
“Workers clear the vegetation and mulch down 4 ft. wide paths in a grid pattern throughout the woods being surveyed. Microphones are then connected via bright orange cables along the paths. Vibrator units mounted on buggies emit sound waves that go approximately five miles down into the ground. These waves reflect back off the underground rock formations, and are picked up by the microphones. The seismic data produced is then studied in order to decide where to drill.”
2. Explosive – this is where the crew detonates explosive charges in the ground. This is laid out in a grid pattern and shot points with small explosive devices laid out in one direction, and receiver cables connected to geophones go the other way. When the charges are detonated, they are 20-30 feet in the ground, and generate seismic waves that can be picked up by the geophones.
Usually, about one hundred shot points are used per square mile. The more shots you set off, the higher resolution imagery results.
In both cases, it is the resulting sound waves that bounce back to the surface and is captured by the geophones that are processed into a computer which uses special software to read the waves and indicate if there is any chance of gases being caught in the rocks below.
This is a simple explanation, but persons interested to learn more can go online and type “seismic testing or survey,” and endless material will show up.
Next week I will explain how seismic testing is done offshore.
Thanks, YaYa, for asking the questions and keep our people thinking.
Audrey Matura-Shepherd
VP Oceana in Belize