Basic Procedures and Methods of Seismic Exploration
Pre-field
Procedures:
Prior to doing any geophysical survey, it is
important to understand the basic concepts and limitations of the method and the
basic operation of the instrument. The parameters of the survey should be set
up in advance and should be optimized for the desired output. For instance,
spacing of geophones will depend on the desired target depth.
Types of information to be determined in
planning the survey include:
- Goals of survey and targeted depth of penetration
- Understanding of geological or other controls on subsurface
features
- Location and orientation of survey line(s)
- Spacing of geophones (depends on depth of penetration)
- Number and position of "shots"
In
Field Procedures:
Set-up
Basic Procedure for Laying
out a Seismic Line
Laying out a seismic line can be a very quick and easy
procedure. With a small amount of
equipment and easy connections, a seismic line can be connected and ready to
take data in a minimum amount of time.
Equipment (use this as a checklist before heading into the
field):
- SmartSeis
seismograph
- 12 V
battery and hookup cables
- 12
geophones
- seismic
cable with 20 ft spaced connectors (on trolley)
- Sledgehammer
with detector
- strike
plate
- trigger
extension cable for shot source (hammer)
- measuring
tape (with pins)
- field
notebook
- compass
- map
Part One-Setting Up Your Line:
- Set
out measure tape, making sure your line is as straight as possible.
- Roll
out your seismic line carefully, having in mind the connecter spacing that
is desired (the shorter the space between connectors, the shallower the
readings). You will have
excess cord between each connector if your spacing is less than 20
feet.
- Take
the twelve geophones and connect them to the seismic line, and place the
spike and main body of the geophone directly next to the foot marker of
your desired spacing. Make
sure that the spike of the geophone is completely in the ground. The clip of the geophone goes onto
the connecter easily; just match the small end of the clasp with the
smaller part of the connector, and the fat part with the larger side of
the connector.
- Your
point sources will be x ft (connector spacing) away from the end of your 1st
geophone, the middle of your line, between geophone 6 and 7, and x ft away
from your last geophone.
These are the points where you will place your strike plate.
Part 2-Connecting the Seismograph:
1. Connect the seismic line to the seismograph. This connection is on the side. Line up the pins and the line on the
outside of both the male and the female connections, then screw the connector
on once the pins are secure.
2. Connect the
12 Volt battery to the seismograph.
Red is positive (this goes to the left electrode), and black is negative
(right electrode). The end of this
connection cable has a pin connection.
This goes to the furthest left pin connection on the back of the
seismograph. Once this is properly
connected, you can boot up the seismograph.
3. The next
cable to be connected is your source point (hammer) to the extension cable for
your sensor. The sensor is taped
onto the sledgehammer, and should always point away from the ground when being
used. The extension cord is
connected to its furthest pin slot on the right in the back of the seismograph.
Data collection and storage
Data analysis
Equipment pack-up and care
The procedure for pack-up care is
reverse of setting up your seismic array.
You may disconnect all peripherals from the SmartSeis, and turn off your
display after saving your information.
All geophones are disconnected from the seismic line, and placed on
their corresponding rack. Then the
seismic line can be rolled onto its trolley. Tape measures can be removed at anytime after the seismic
line is sufficiently shot. It is
to be remembered that the equipment in use is sturdy, but should still be
handled with the utmost care. Make
sure all material is secured when transporting back. All materials are stored in 252A Stillwell in care of Dr.
Genny Peterson.
Problems in the Field
The noise that is generated naturally and
anthropogenically should be taken into consideration. Even though the SmartSeis
corrects for many noise with low and high pass filtering, it is only noise
reducing. Noise can be generated not only from passing vehicles, but also by
planes, and even the wind. It isn't actually the wind causing the noise, but
the interaction between wind and plant life, like trees, which move in response
to the wind. There are many other noise factors that can cause unclear data,
but along with the filtering and being aware and observant of the environment
when taking your readings can give good results. Signals can also be stacked to
a certain extent if some of the noise is giving interference with the
seismograph. 2
One of the most important parts in the field
for Seismic exploration is setting up the geophone array over the desired
stretch of land. One of the big questions is what will the interval between
each geophone be? The cables being used had receptive connector that can be
removed and moved, but are usually spaced at regular intervals. For example,
25-ft spacing between geophones is standard for 50-100 ft deep explorations. As
a general rule, the length of the cable spread should be three to five times
what you expect your depth to be. Lie out your seismic line and place your
connected geophones into the ground.
Next in consideration is where should you
place the shots, or place your energy source. The energy source SHOULD NOT BE
IN LINE WITH THE CABLE. It is best to offset the shot perpendicular to the
cable. This allows for the times it takes for ALL of the geophones. It is also
advised to take another shot near the center of your geophone cable (also
offset) for more accurate information. 1
Another option for laying out your seismic
like is to take two shots on either side of the cable, then move the cable down
the line in a "leap-frog" fashion, taking another two shots at the
end. This technique can be repeated for as long as necessary.
Sources: 1 Redpath, Bruce B. Seismic
Refraction Exploration for Engineering Site Investigations Explosive
Excavation Research Laboratory, Livermore, California. 1973.*
2
Laymon, Douglas E. and Robert H. Gilkeson. Application of Seismic
Refraction Methods to Evaluate Regional Groundwater Resources*
*These guides are available upon request at www.geometrics.com
