Refraction Microtremor (ReMi) Field Tutorial
John Louie of the
CEMAT project at the
Nevada Seismological Lab
30-meter Shear Velocity With Two Hours Effort
10 September 2001
You simply record noise from your site, on your existing
seismic refraction gear. With no seismic source required, you can
collect good data at the noisiest urban sites using two person-hours
or less. It is now possible to get a quality shear-wave assessment of
shallow earthquake site effects in just a few hours, at one-tenth the
cost of a logged borehole.
- Equipment Needed:
All you need is 24-channel digital refraction gear with 8-12 Hz single
- ASTM Standard D5777 on seismic refraction (link at bottom of page)
advises in paragraph 6.1.1 that field personnel have specialized training
and experience. Please discuss any needs you may have for seismic training
or personnel with John Louie.
He will be able to recommend consultants, or training opportunities such
as those provided by the
Environmental and Engineering Geophysical Society.
- Most digital refraction seismographs built since 1990 should be adequate,
digitizing 24-bit fixed-point or 21-bit floating-point samples. The recorder must
have enough memory to hold 24-channel records with a length of 4 seconds or more.
StrataView R and
Seistronix RAS-24 are examples.
SmartSeisTM can only record a maximum 1-second record length.)
- A ReMi survey can be made with a 12-channel seismograph; but the
15% velocity accuracy may not result.
- The geophone array should be 200 meters long, and must be more than
100 meters. The refraction cable must have this length. A 24-channel
cable with an 8-to-10-meter takeout interval is ideal. A 5-meter takeout
interval is too short.
- Only single-channel vertical-motion seismometers (geophones) are
- Geophones should have a resonant frequency of 8 Hz or less.
12.5-Hz geophones have yielded good results, but 4.5-Hz geophones
are needed to assure ReMi velocity resolution to 100 meters depth.
- A set of 4.5-Hz Mark Products L10B single refraction geophones with 4-inch
spikes and 3-foot leads is ideal.
- Amplitude or frequency-response calibration of geophones is not
needed - as with refraction, ReMi uses only the phase information
in the recorded wavefield (ASTM Standard D5777 also does not specify
any geophone calibration).
- Siting the Array:
Put the cable across your site in a line 200 m (600 ft)
long, with about 8 m (25 ft) spacing between phones.
- You need to locate a reasonably straight stretch of flat ground
at your site at least 200 meters long. Center it on your foundation
or borehole location if possible.
- The absolute minimum array length needed for 15%-accuracy
ReMi is 100 meters.
- Avoid known underground cavities 3 meters or more in diameter
- pass beside but not over them.
- Avoid concrete slabs; thin pavements are OK as long as you are
able to set the geophones on them.
- For ease of deployment, avoid water more than 6 inches deep.
- An easy urban layout is to run the array along the sidewalk,
with the geophones planted into the parking strip or cracks in
pavements. If the seismic cable must cross a street or driveways
that cannot be blocked during the survey, put it between 2x4s nailed
to the pavement. Protect any geophones installed in a street with
a traffic cone, or note any channels that cannot be installed.
- A ReMi array can work extending into the ground floor of a building.
- ASTM Standard D5777 specifies the straightness and surveying of
the geophone array in paragraphs 18.104.22.168 and 22.214.171.124. For ReMi
a deviation in the line of 5%, or 10 m from a 200-m-long line,
will not affect the stated 15% velocity accuracy of the method.
- This accuracy applies to elevations as well - in fact the line
can have a constant inclination that can safely be ignored, as long
as geophone elevations do not deviate more than 10 m from the incline.
- Simply chain out the geophone locations with a tape measure.
- Survey the geophone locations to 1-meter accuracy if the array
is more than 10 meters out of line, or 10 meters off constant incline.
Include the survey data with the seismic records if this is the case.
- Installing the Array:
- Unreel your geophone cable along the array line, setting
takeouts at the chained, marked geophone locations.
- ASTM Standard D5777 has general instructions for setting
geophones in paragraph 126.96.36.199.
- The geophones can be as much as
15 degrees off vertical without compromising ReMi data quality. Visual
inspection of verticality is thus adequate.
- Our tutorial on geophone setting.
- If you must use multi-element geophone groups, plant all phones
from each group in one spot. Dig a 2-foot-wide hole and pot them
all in that, then bury it all. Or try extending the group arrays
perpendicular to the line direction, if you have enough labor available.
- Attach your multichannel seismograph and test for electrical
continuity, and signal from all geophones.
- Recording Data:
Take 3 to 10
records of background noise, 20-60 seconds long each.
- Set the record to have 24 channels (if your recorder allows more).
- Set a time sample interval of 2 milliseconds (ms) or longer, up to 10 ms.
- Set each record to have a time duration of 20-30 seconds.
4 seconds is the absolute minimum. Records longer than 30 seconds
can be difficult to translate between formats - configure each recorded
trace to have no more than 16,000 samples to avoid such problems (8-sec
record at 0.5 ms sampling; 32-sec
record at 2 ms sampling; or 160-sec record at 10 ms sampling).
- Turn off any filtering before digitizing or plotting. If the recorder
does not allow this, set the lowest possible low-cut filter frequency
(hopefully 4 Hz or less) and a high-cut frequency equal to half the
sampling frequency (e.g.: with 2 ms samples, sampling frequency is 500 Hz,
so a high-cut filter at 250 Hz is OK as a reasonable antialiasing filter).
- ASTM Standard D5777 has a general statement on seismograph calibration
in paragraph 6.3.5. This procedure is adequate to assure ReMi's 15%
velocity accuracy. Projects requiring documented quality assurance
should obtain test oscillators from the seismograph's manufacturer, and
conduct a seismograph time calibration procedure at each site.
- Record 3 to 10 records, triggered manually. You may want to
wait for the passage of a good noise source like a train, heavy trucks,
or low-flying jet.
- Do not stack records in the seismograph's memory. Clear the stack
memory before triggering each record.
- Save each record separately to the seismograph's hard disk, or floppy.
- Transfer records to a laptop on-site, for easy data transmission later.
- If the site is quiet, activate some sort of source during each
record. Drive up and down the geophone line in a truck. Run up and down
the line. Pick up and drop 50-lb rocks. No timing or locating of the source
The ASTM Standard Guide D5777 for Using the Seismic Refraction Method for
Subsurface Investigation is available for purchase from the
American Society for Testing and Materials at
Commercial product names are included for informational purposes only.
No endorsement by the State of Nevada or other agencies is implied.
Copyright © 2001 John N. Louie