GEOPHONE- AND STRING TESTING
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!Introduction
There are really two scenarios:
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# A thorough testing of the following geophone- and string parameters, prior to deployment:
## %alpha% Resistance of geophone coils
## Natural frequency of coil
## Damping of coil
## Sensitivity of coil (in Volts/meter/sec)
## Distortion limit in percent
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# During the survey, testing for cable breaks and short circuits - it's not usual to test parameters mentioned above.
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!Pre-survey testing
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We have these options:
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* Using a separate geophone tester.
* Modifying the GEODE data acquisition unit so it includes test facilities.
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!!Separate geophone tester
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!!!'''SMT-300'''
Attach:SMT-300.jpg"SMT-300 Geophone Tester"|''SMT-300 Geophone Tester, from [[http://www.geophone.com/Category.asp?CatID=1&SubCatID=6# | I/O Sensor Nederland]].''
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* Manufacturer: %newwin% [[http://www.geophone.com | I/O Sensor Nederland bv]]
* Datasheet: [[(Attach:)New_SMT-300_Brochure2.pdf | New_SMT-300_Brochure2.pdf]]
* Can test strings or individual geophones
* Download test data to PC for documentation
* Budget price: NOK 97'
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:'''Comments by W.W.''': %green%The Sensor solution is limited: The SMT-300 tests ONLY the geophones, one at a time. The SMT-GRIP tests ONLY the lines, and does not record the data.%%
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:'''Comments by O.M.''': %green%After closer scrutiny of SMT-300 datasheet, it seems that the SMT-300 can also test string of geophones (page 1, second section).%%
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!!!'''GRIP Geophone String Analyzer'''
Attach:GRIP.jpg"Grip String Analyzer"|''Grip String Analyzer, from [[http://www.geophone.com/Category.asp?CatID=1&SubCatID=6# | I/O Sensor Nederland]].''
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* Datasheet: [[(Attach:)GRIPStringChecker.pdf | GRIPStringChecker.pdf]]
* Budget price: NOK 18'
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!!!'''SMT-200'''
Attach:SMT-200.jpg"SMT-200 Geophone Tester"|''SMT-200 Geophone Tester, from I/O Sensor Nederland (discontinued).''
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We can also purchase or borrow a second-hand SMT-200 (this model is not made any longer). We have used this model in the past, with good results.
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* Datasheet: [[(Attach:)SMT-200.pdf | SMT-200.pdf]]
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!!GEODE modification
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* The Geode comes optionally with a built-in test oscillator. This allows full analog performance testing of the Geode as well as testing of the geophones, including damping, frequency, resistance, etc., similar to what the SMT does.
* Costs:
** Analog performance testing by itself is $4,400 (NOK 23') per 24-channel module.
** Line and geophone testing by itself is also $4,400 (NOK 23') per 24-channel module.
** The two together would be $8,000.
* Our Geodes need to be shipped to the factory for hardware upgrade.
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'''Comments by W.W.''':
->%green%The only real problem I see with the in-geode tester is that it does not allow you to test another Geode, so you need the tester to be in each of the Geodes. However, two Geodes can be converted for less than the price of one SMT-300. In fact, three Geodes can be converted for a bit more than the price of an SMT-300 and GRIP.
->%green% (also) .... the process is automated in the Geode, creates a record of the test result, and gives more results, such as cross talk.%%
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!Testing during the survey
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The most common problems encountered during the survey will be line breaks or short-circuits. This is due to the snow-streamer connectors - probably the weakest link in the chain.
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The question is whether the Geode noise monitor window is sufficient to detect open and shorts on the geophone or string lines or not.
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The GEODE noise monitor window looks like this, with 8 of the 24 possible channels activated. It's like a multi-channel oscilloscope, with selectable amplitude scaling (in this case 5 µVolts/division):
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Attach:geode_test2.png"Geode noise monitor window. Disturbance in ch.3 made by lightly touching the wire."|''Geode noise monitor window. Disturbance in ch.3 made by lightly touching the wire.''
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From past experience with the previous generation of Geometrics Seismographs (the ES-2401), it's fairly easy to spot geophone line breaks with the noise minitor window. The noise signal amplitude will be much higher in this case.
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A short-circuit on signal wires in the snow streamer, or a defective geophone which causes short-circuiting, manifests itself as low-amplitude or flat traces on the seismograph shot gather window.
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!Conclusions (by O.M.)
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* Purchasing a new SMT-300 is too expensive.
* Contact local company that has the old SMT-200 and discuss rental or purchase. This model is sufficient for our usage.
* All geophones/strings tested with SMT-200 prior to survey. Also add a number of spare geophones/strings.
* During the survey the most likely problems will be open and shorts in the geophone/string wiring. We can rely on the GEODE noise monitor window to detect open circuits. Shorts will be seen in the GEODE shot gather window.
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