The 1440 seismometers and sensors deployed along the two ENAM profiles will record seismic waves produced by 11 detonations located along the trace of the profiles. To ensure that most of the energy will travel in the subsurface rather than being lost at the surface, the explosives are detonated at a depth of about 70 feet. Each shot point location is carefully selected to match strict safety requirements and then permitted with state and local authorities. Once permits are authorized, drilling of the boreholes and loading of the explosives can begin. The seismic source team has been working ahead of the deployment team to prep all the shot points along the two lines.
Near surface geology along most of the ENAM lines consists
of unconsolidated sands and plastic clays.
These require mud rotary drilling techniques and some method to hold the
borehole open (i.e., casing) so the liquid explosive (called emulsion) is not
squeezed from the hole. In the ENAM project we case
only the explosive, a much more economical method than casing the entire
borehole. The white tubes in the photo
below are filled with emulsion and have been primed with detonators (orange wires)
and boosters (inside the tubes). They
are ready to be loaded into the hole.
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| Two charges ready for loading into the borehole. Charges contain 200 lb. each of emulsion and are 5 feet long and 10 inches in diameter. |
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| A charge being lifted by the drill before loading into the borehole. Note the two large piles of sand produced from drilling the borehole. |
(by Steve Harder and Beatrice Magnani)
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