PSN-L Email List Message

Subject: Re: Vertical vs. Horz.
From: Brett Nordgren brett3nt@.............
Date: Wed, 23 Dec 2009 09:38:16 -0500

Hi Chris,

At 09:32 PM 12/22/2009 -0500, you wrote:
>In a message dated 22/12/2009, brett3nt@............. writes:
>> >1  I know this is an over simplification, but does the  P arrives at an
>> >angle (bottom to top) and this influences the spring?
>>What you suggest I believe is true, that P waves may have some vertical

i.e., they arrive at some angle to the horizontal, as you say.

>      It may have even more to deal with the curved paths that the rays 
> travel inside the spherical earth!

>> >The obvious advantage of a vertical is they see many earthquakes, the
>> >disadvantage they are limited and also see a lot of other things which are
>> >not earthquakes.  All of my verticals have been 1 to 2.5 seconds.
>> > A typical vertical sensor recording will have a trace of  20
>> >mins vs. a 3 hours trace on the Lehman.
>     This must be an observational error. The noise on an uncased, 
> uncompensated vertical may be over 20x that on a horizontal. The noise on 
> verticals is only less at longer periods since atmospheric noise is 
> excluded in contemporary seismometers by the hermetically sealed case.

For a good comparison of vertical vs horizontal noise see  which shows the vertical and 
N-S channels of the Trillium over a couple of hours this morning.  They 
have the same gain and are both low pass filtered at 0.08 seconds to cut 
out most microseisms.  The N-S trace is the red one.

Don't even think of making a sensitive vertical without a pressure 
case.  As you rightly point out, it won't work.

>>The only problem is that the movement of a seismometer in response to an
>>error force, such as from small temperature changes, etc. increases as
>     Even more critical are air density changes effecting a vertical 
> sensor and also wind noise. Temperatures should change quite slowly 
> inside an insulating case, assuming that the sun does not shine on it!

Air density changes are readily dealt with by the pressure case.  Wind 
noise has not often been a problem (except for yesterday), certainly less 
than for the horizontals.  Temperature change rates are a big problem, with 
instruments responding to milli-degrees per hour.  A 500 sec high pass 
takes care of that, though you need to make sure that the instrument isn't 
creating significant "DC" levels at its output.  Thick insulation is rather 
effective at slowing down temperature rates.

>>It is way harder to make a stable instrument for 50
>>seconds than for 10 or 20 or 6.  Long period verticals almost certainly
>>need to use a feedback design if they are going to be sensitive enough to
>>see distant quakes while at the same time, insensitive enough to
>>temperature and other variations to not wander off to maximum output.
>     Uh, Uh! Now define what you are calling long period?

Ones with lower corner frequencies in the 20-120 second range, though I 
suggest that the use of feedback becomes more beneficial the lower you go.

>      The Roberts' period compensating amplifier circuit is probably the 
> easiest technique for amateurs to use. It has constant gain from the LP 
> filter down to ~ the resonant frequency rf of the vertical sensor, at say 
> 1/2 Hz. See 
> It was first used on geophones.
>     Below this to about rf / 10, (= 1/20 Hz) the gain increases as 1 / 
> f^2, so compensating the f^2 falling output to give a flat 
> characteristic. So you can extend a 2 second period vertical sensor out 
> to about 20 seconds quite easily. Extending the period much beyond x10 
> quickly runs into noise problems with a coil + magnet velocity detector. 
> You need two of these stages with maximum gains of x10 (total x100 at 20 
> seconds) linked by a 2 pole high pass filter at ~ 1/30 Hz. Lennartz use a 
> system like this. So do I and it works fine.
>     Direct digital period compensation probably works best with 24 bit 
> ADCs. The French use it on their schools system. 16 bit ADCs may only 
> show a few counts (1/100 the rest of the signal) at x10 period, unless 
> the gain is quite high and it may be partly masked by electronic noise.

I agree, that's a good technique.  Dave uses it in his miniature horizontal 
designs with great success.  My only concern, is that you are limited in 
how much you can use for the reasons you suggest.  Sensitive verticals may 
not be sufficiently stable in the DC sense to allow for longer-period 
instruments using that technique.  They would have a tendency to drift to 
the stops.


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