Geoff,

I'm just curious as to why an omnidirectional instrument is of 
interest.  A vertical is sort of omnidirectional, as waves coming 
from any azimuth affect the instrument in similar ways.  By the way, 
quakes themselves are generally quite directional in the 'antenna 
pattern' of their vibration.  One will shake your seismo a lot while 
an equally large quake at a similar distance but oriented slightly 
differently won't be visible all.

Also it helps visualize how things work if you note that the motion 
of a point on the ground caused by a quake will be three dimensional, 
generally sort of circular or elliptical, with its plane likely 
changing direction all the time, not just back and forth in straight 
lines.  Now that I think of it, a purely circular motion should show 
no oscillation at all on an omni. instrument, I believe it would just 
come out as a constant DC, but the more elliptical it was, the larger 
the signal.  I'll have to think about that.

The first thing you will need is enough sensitivity and, in 
particular, low enough noise to see quakes at all.  Only the 
well-known sensor designs have been found to achieve both of them 
sufficiently well to reliably see distant quakes, and I believe all 
have been single-axis devices.  There are, indeed, novel and cheap 
one-axis instrument designs, like Dave's Fluid Mass Electrolytic 
Seismometer, that do quite well, but not omni ones that I know 
of.  The compromises needed to achieve omnidirectional performance 
might mean that, though omnidirectional, it would llikely see very 
little.  The usual approach would be to record three perpendicular 
directions and use digital processing to vector-sum them, assuming 
there was a reason it was needed.

Regards,
Brett


>sorry to butt in like this,
>
>There must be some kind of strain gauge where you simply hang a 
>weight like a pendulum
>and any swing however slight would create a signal.
>
>????
>
>Then the signal would be omnidirectional which is what I am looking for.
>
>A single plane oriented sensor which produces an omnidirectional signal.
>
>A hoovering mass like the globes you can buy
>suspended within a magnetic field
>
>
>????
>
>A gyroscope of light where you measure phase differences between 
>input and output
>A laser beam traveling through a fiber optic coil.
>?????
>
>
>A foam pad which changes resistance with compression
>so you just put a mass like a gallon of water on it
>to measure vertical forces
>
>???
>
>Isn't there something novel and cheap and easy to try ???
>
>Something which requires X10,000 or less in gain.
>
>
>regards,
>geoff


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