Application - Geometrics Forum

What are the key components of a seismic refraction system?

Gretchen Schmauder — Fri, 01 Sep 2023 15:10:23 +0000

Key components include:

Geode Seismograph

12V Battery

PC (required for Geode and ES-3000)

Digital cables (Geode and ES-3000)

Geophones (including spares)

Geophone cable(s)

Source (hammer, weight drop, explosives)

Striker plate (if using hammer)

Hammer switch and spare

Trigger extension cable

Measuring tape

Hand level

(IMPORTANT)

How many people are required for a refraction survey?

Gretchen Schmauder — Fri, 01 Sep 2023 15:07:41 +0000

In general, two people can get the job done. If you are using explosives or a weight drop, a third person may be required.

What geophones should I use in a refraction survey?

Gretchen Schmauder — Fri, 01 Sep 2023 15:06:19 +0000

10-14 Hz are the most common and the least expensive. Much more than 20-30 Hz and you start cutting into your signal. Less than 10 Hz and the phones become a somewhat fragile and less field-robust.

Is it necessary to bury geophones for seismic surveys?

Gretchen Schmauder — Fri, 01 Sep 2023 15:05:04 +0000

This is rarely necessary or helpful, even in high noise conditions. If you are having difficulty with air waves, burying the geophones might blunt their effect. If planted properly, wind is generally not a factor, and if it is, it is usually via vegetation vibrating around the geophones, which would not be mitigated by burial.

Using a Geode Seismograph to quantify vibrations

Gretchen Schmauder — Wed, 30 Aug 2023 19:44:27 +0000

We get occasional calls asking how to use one of our seismographs as a vibration monitor. The method for this is described below, but it should be noted that while true amplitudes can be obtained, this method of measuring them would probably not stand up in court. True vibration monitors – seismographs designed specifically for this task – have a built-in geophone. The voltage output of the geophone per unit vibration is known to a very high degree of accuracy, and the system is calibrated by the manufacturer regularly (usually once a year). If you are measuring vibrations in a situation in which litigation might be involved, you should use a true vibration monitor. One of the more popular ones is the Blastmate by Instantel.

Vibrations are generally quantified in units of particle velocity, the first derivative of displacement. Geophones are particle velocity sensors – output is directly proportional to particle velocity. If you know the response function (sensitivity) of your geophone – the voltage output per unit velocity input – you can convert voltage (as measured by the seismograph) to mechanical vibration in terms of particle velocity. The sensitivity of your geophone can be obtained by the geophone manufacturer, and will be expressed as a function of frequency. A typical graph of geophone sensitivity is shown below:

It is best to used a geophone that has a natural frequency at or lower than the lowest frequency of interest.

Seismic data files are stored in a SEG format. The first step is to convert the SEG output of the seismograph to an ASCII columnar format.

If you are using an ES-3000 or Geode, your controller PC should have this icon for Tape Reader on the desktop:

If not, download Tape Reader.

Run the program and click on File>>Open:

Read in the file you wish to convert to ASCII.

Now, click on File>>Save Displayed Data to Ascii File:

After making your format choices (be sure to convert to mV), press Export. The record will be written in an ASCII format that can then be imported to Excel. From here you can calculate the frequency spectra and particle velocities using the response function of the geophone.

Calculate the "True Amplitude" for waves/vibrations measured on a Geode Seismograph

Gretchen Schmauder — Wed, 30 Aug 2023 19:43:07 +0000

You can use the tape.exe application that was installed on your computer when you installed the Seismodule program. Read in the SEG-file, convert it to ASCII by selecting the File menu and Convert to ASCII. Check the box to convert to mV. Once you have converted the waveform to ASCII voltages, you can multiply by the sensitivity of the geophone to recover true ground amplitudes. Download Tape Reader. IMPORTANT - Be sure to back up your data files before running this program.

How do you decide what type of strike plate to use for a seismic survey?

Gretchen Schmauder — Wed, 30 Aug 2023 19:37:40 +0000

Below is a series of diagrams that can act as analogies for impacts. If the impulse is enacted rigidly (hard tip hammer, steel plate, etc.), the impulse will look something like the far-left figure. High-amplitude (height of the curve), narrow wavelength (width of the curve). This is because the impacted materials respond rigidly to the impulse, i.e. the hammer rebounds from the plate almost instantaneously. Therefore, as a result of the narrow-wavelength impulse, the transmitted waves will have relatively high-frequency (short wavelength) content.

As you use softer and softer impact materials, applying impulses of equal force will appear like the diagrams to the right (smaller amplitude, longer wavelength). The impacted materials are responding less-rigidly to the impulse, so the hammer spends more time on the plate due to the more absorptive nature of the impact. The same amount of energy has been put in (area under the curve), but the amplitude of the input (height of the curve) decreases to compensate for the input duration (width of the curve) caused by the impact absorption of the softer materials. Therefore, as a result of the wide-wavelength impulse, the transmitted waves will have relatively low-frequency (long wavelength) content.

Using a more rigid striker plate (like one made of aluminum) on a hard surface can cause the generated wave frequency to be too high at times given the survey goals, so we suggest using a polyethylene plate on a relatively solid material like asphalt.

Remember: lower frequency -> deeper signal penetration -> decreased signal resolution.