investigating the causes of earthquakes in delaware a photo essay of geologists and geophysicists...
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Investigating the Causes of Earthquakes in DelawareA photo essay of geologists and geophysicists from the Delaware and United States geological surveys conducting a seismic reflection survey near the city of New Castle, DE in the summer of 1998prepared by:Thomas E. McKenna and Stefanie J. BaxterDelaware Geological Survey3/2000
The primary objective of the seismic survey was to identify faults that may be associated with moderate earthquakes in Delaware (up to magnitude ~3.6).
ENERGY SOURCEGEOPHONES (RECEIVERS)REFLECTED WAVESThe seismic reflection method produces an image of the subsurface. Seismic waves are initiated by an energy source at the surface. The seismic waves propagate downward and are reflected from contacts between subsurface layers back towards the surface. Geophones record the arrivals of the reflected waves at the surface.
The 3-kilometer seismic survey was run along an abandoned railroad track. Sources and receivers were laid out at marked locations (yellow flags) every 5 meters.
10-to-15-ft-deep shot holes were drilled for putting in explosive sources using the Delaware Geological Survey drilling rig and ...
... a contractors drilling rig.
The explosive sources that were placed in the shot holes are similar to those used for blasting during road construction.
A bobcat mounted jackhammer rig was used to drill 18-in-deep shot holes for another type of source - blank shotgun shells.
A few shallow holes were drilled by hand with a power auger.
Blank 8-gauge shotgun shells were used to initiate seismic waves in the shallow holes.
Sometimes it got crowded out there.
300 geophones were pushed into the ground to listen for the incoming seismic waves produced by the sources.
Signals from the geophones were transmitted to the seismographs along a string of cables.
Here is a different style of geophone ready to be deployed. The points of the phones are pushed into the ground.
Five portable seismographs were used to record and process data from the 300 geophones.
Stations were setup to facilitate reading the instruments in the bright sun and for protection from rain.
A Betsy Seisgun was used to shoot the shotgun blanks into the ground.
Here is a closeup of the Betsy Seisgun trigger. The red cap (and wire coupling) is inserted over the spring on the top of the handle and a shot is triggered by hitting the spring with the hammer.
The shooting team consisted of at least five persons: a shooter (pictured with the trigger hammer) to fire the shotgun, an ammunitions loader (pictured with ammunition bags) to load and empty shells, a radio operator, a person to prep the shotholes, and a person to assist the shooter in moving equipment from hole to hole.
The shooting team preparing to shoot the Betsy Seisgun. The man on the left braces the shooter (middle) and uses the radio to confirm that all stations are ready to receive the shot. Two people stand on a metal plate and cloth cover to minimize flying debris (soil).
The shot is fired...
reflected seismic waves are received by the geophones and transmitted along cables...
to the seismometer that records the incoming signals.
The used shotgun shell is removed from the bottom end of the Betsy Seisgun and a new one is loaded.
The shooting team moves to the next shot location 5 meters away.
Another source we used was a Pentalite booster. Shown here is the cylindrical booster (with energy equivalent to a few firecrackers) and an electrical wire with a metal blasting cap. The cap is put in the booster in a > 10-ft-deep shot hole and the explosion is triggered by an electrical impulse.
All explosive detonations were handled by a licensed blaster using detailed safety precautions.
Boosters with attached blasting caps and wires were buried before detonation to minimize soil blowout. Fill was shoveled into the hole after the placement of the explosive. A rod was used to tamp the fill in the hole. The man on the right is holding the wire to the buried blasting cap.
Precision clocks and detonators (pictured) were used to synchronize all recording stations (seismographs) with the time of detonation.
The blaster detonates an explosive. Note the station in the background where seismic waves are recorded with a seismograph.
Precise locations of all shot holes and geophones were surveyed with a total station (yellow instrument on tripod). The total station gun shoots a laser towards a prism held by rodpersons (in the distance). The prism reflects the laser back to the total station and the distance and angle to the prism are calculated.
The total station combines laser technology with digital recording of data for ease in precision surveying. Note the prism that reflects the laser that is propped up inside the truck.
Measurements of earths gravitational field enable geologists to model the density of underlying rocks, and these measurements help in interpreting the seismic survey. Here, a geologist reads the output from a precision gravimeter.
This is an example of the seismic data as collected in the field. The horizontal axis is the distance away from the shot and the vertical axis is the time that it took the reflected seismic wave to reach the geophones.
Note that the travel time increases with the distance from the shot because the wave must travel farther.
Fieldwork is completed and the shooting team celebrates. Extensive computer processing and interpretation of the data is ongoing.
DE Geological SurveyUS Geological Survey UD Geology Dept.Thomas E. McKennaRufus D. CatchingsSusan McGeary Stefanie J. BaxterMark R. GoldmanShane Detweiller Scott A. StrohmeierThomas BurdetteJames Black Charles T. Smith Alan Yong Jordan HegedusKimberly Gregg Jose RodriquezBart Wilson Lisa DonohoeJoe GrowRoland E. BoundsAndy JonesC. Scott Howard Michael KnoblauchFieldwork bySupported byDelaware Emergency Management AgencyUnited States Federal Emergency Management AgencyDelaware Geological SurveyUnited States Geological Surveyphotos by T. E. McKenna