Ted-
    We did something like this in 1994 in Santa Cruz to estimate site response.
We put energy from a repeatable source into the ground and measured the
absolute groundmotions that resulted to estimate the compliance of the
near-surface materials. In a gross way, our results confirmed the site-response
estimates we made from aftershock records of Loma Prieta in 1989.
-Edward

ted@.......... wrote:

> Although it is pretty clear that the Mars Polar Lander (and the two
> microprobes, "Scott" and "Amundsen") are lost, I came across an article in
> the NY Times online which mentioned that the microprobes were instrumented
> with accelerometers which were to record the deceleration as they slammed
> into the Martian surface and came to rest about 4 feet down.  Arrival speed
> at moment of impact was expected to be about 400 mph.  Variations in the
> deceleration force would contain information about differences in density
> of layers in the first 4 feet of soil, which would later be broadcast back
> to earth via the lander, or the MPL, or the Mars Global Surveyor, currently
> in orbit.
>
> This must have been tough gear.  If my math is right, 400 mph is about 600
> ft/sec.  To decelerate from 600 ft/sec to 0 (assuming uniform deceleration)
> means that the avg speed during deceleration is 300 ft/sec.  At 300 ft/sec
> it takes about 14 msec to go 4 feet.  Therefore the deceleration must have
> been 300 / 0.014 = 21,400 ft/sec/sec or about 670 G's.  The science team
> was prepared to lose either probe if it hit a rocky surface (which would
> result in about 10,000 G deceleration I'd guess).  But they tested the
> units to thousands of G's on earth by firing them into the earth from
> airborne cannons.  (Why didn't I go to work for NASA?)
>
> Does anyone know any more about these accelerometers, the instrumentation,
> and in particular what kind of sampling rate they wanted to get the fine
> structure of the subsurface geology?  If you wanted resolution to 1 mm
> you'd need about 1300 samples in 0.014 sec or about 92,000 samples/sec.
> How do you design an accelerometer which will report 100,000 times per
> second as it records accelerations between 0 and 1000 Gs?  Strong motion
> indeed!
>
>  Imagine little Lehman sensors in the probes!  Now imagine the
> technicians on Earth performing the final assembly as one says to the
> other, "Should these little magnets attract or repel each other?")  ;)
> 
>
> Regards, Ted
>
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--
Edward Cranswick                Tel: 303-273-8609
US Geological Survey, MS 966    Fax: 303-273-8600
PO Box 25046, Federal Center    cranswick@........
Denver, CO 80225-0046  USA      E.M. Forster said, "Only connect".




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