TUTORIAL SÍSMICA REFRACCIÓN

Summary

The video demonstrates how to set up and run a standard seismic-refraction survey. Ground vibrations are recorded by geophones to estimate subsurface conditions, including the depth and continuity of bedrock and the characteristics of the overburden. Differences in wave-arrival times are used to construct a time–distance curve and interpret the subsurface profile.

Equipment

The equipment described for the survey includes:

• A 24-channel seismograph and power cable
• 24 geophones, arranged in two groups of 12
• Two 12-connection seismic cables
• A starter geophone and its extension cable
• A striking plate and a 5–10 kg sledgehammer, or a seismic rifle and cartridges
• A fully charged car battery, measuring tape, rubber mallet, tester, toolbox, gloves, and a screwdriver

Check the equipment against the supplied inventory and secure it during transport. Protect the measuring instrument from impacts.

Survey Setup and Acquisition

1. Lay out the profile

   • Stretch and secure the measuring tape.
   • Place 24 geophones at equal, predetermined intervals.
   • Prepare the ground at each geophone location and secure the geophones firmly.

2. Lay out and connect the seismic cables

   • Run the first cable from geophone 1 to geophone 12 and the second from geophone 13 to geophone 24.
   • Connect each geophone using the matching wide and narrow clamps, ensuring good electrical contact.
   • Where practical, keep connections off the ground to reduce electrical noise.

3. Set up the measuring station

   • Position the seismograph near the center of the profile.
   • Connect the battery, observing correct polarity and ensuring secure contacts.
   • Connect the cable for geophones 1–12 to the instrument’s left connector and the cable for geophones 13–24 to its right connector. Reversing these cables would invert the profile.
   • Connect the starter-geophone extension cable and run it to the source locations.

4. Check the system

   • Turn on the seismograph and use its geophone test function.
   • Confirm that all 24 geophone indicators are green. If any fail, inspect the connections and repeat the test.
   • Use the instrument’s noise-monitoring function to check for environmental disturbance before recording.

5. Record the planned source points

The standard profile uses seven source locations:

  - An external direct point, typically six geophone spacings beyond geophone 1
  - A direct point at geophone 1’s position
  - A central direct point between geophones 6 and 7
  - A central point between geophones 12 and 13
  - A central inverse point between geophones 18 and 19
  - An inverse point at geophone 24’s position
  - An external inverse point, typically six spacings beyond geophone 24

For shots at the geophone 1 and geophone 24 positions, temporarily move the relevant end geophone halfway toward its neighbor. Return it to its original position after the shot.

1. Energize and record

   • For hammer shots, seat the striking plate at the source point and place the starter geophone nearby.
   • The video also demonstrates a seismic rifle as an alternative source. This specialized equipment and its cartridges should be handled only by qualified personnel under applicable safety procedures.
   • The operator starts the recording, checks for excessive ambient noise, and coordinates the shot with the person operating the source.
   • Each recorded seismogram contains 25 traces: one for each profile geophone and one for the starter geophone.
   • Record the seismogram number at every source point.

2. Finish and pack up

   • After all planned shots, collect the measuring station, geophones, cables, and other equipment.
   • Replace connector caps and coil cables without dragging them.
   • Gather the geophones into two groups of 12 and load the equipment for transport.

Speakers and Sources Featured

• Unnamed instructional narrator/voice-over: Explains the equipment, setup, and survey procedure.
• Background music: Present during portions of the video.
• No other identifiable speakers are featured.