PSN-L Email List Message

Subject: Re: Real time traces/ Vault design
From: "Dave Nelson" davefnelson@.......
Date: Fri, 14 Aug 2009 18:00:59 -0000

I think Its time for me to join the conversation.

The installation   for my broadband seismometers is a concrete pier set on 
the local bedrock . The pier is poured with the traditional 50/50 sand 
cement mixture  with 2 or 3 inches of clearance between the pier and the 
sides of the hole . The pier is topped with a granite slab bonded to the 
pier with  a layer of thinset mortar.

The pier is enclosed by a commercial plastic building with an interior door 
separating the building into two compartments creating an insulated and 
sealed inner "vault" for the instruments. No part of the plastic building is 
in contact with the pier. Power is brought into the building from an 
external UPS and multiple internet connections are also internal to the 

The computers are remotely located connected to the A/D with  an  Edgeport 
USD to serial converter and USB over Cat 5 extenders. I have two of Larry's 
A/D's in the vault. All instruments and in the vault are powered by a 
battery with a floating charger.

In spite of being in the Los Angeles urban area the noise below 5 Hz is 
quite low. The Palos Verdes Peninsula is a large monolith which is 
essentially a part of the coastal mountain range. The Los Angeles "basin" is 
alluvium which seems to isolate urban noise from the rock I sit on.

I am 200 M above sea level and surrounded on three sides by ocean (the land 
mass is a peninsula). The closest beach is about 4 km.

Currently I am running a modified  3 axis Guralp borehole instrument of 
early (1987) vintage along with two of my fluid based FMES horizontals which 
respond from 4 Hz to 20 seconds and are seismic background  noise limited.

  I have two vertical force balance instruments as Brett is describing in 
the vault in pressure sealed cases. The enclosure for these instruments and 
the method  for employing them  for  is extremely important to 
reduce/eliminate  atmospheric buoyancy noise without introducing flexure in 
the instrument base plate.

The Force Balance Verticals  are still somewhat developmental but work 
extremely well. I have a new version of the circuit board with minor 
revisions about to be installed and some ideas for simplifying the 
mechanical design which will be tried in the next month or two.

Dave Nelson   (Los Angeles)

----- Original Message ----- 
From: "Kay Wyatt" 
Sent: Friday, August 14, 2009 17:03
Subject: Re: Real time traces

> Brett,
> I am curious about the installation enclosure for your seismometer.  It 
> looks like a granite slab surrounded by a plastic enclosure.  Is this 
> located in a basement or is it outside.  How do you insure it is 
> waterproof? What is the granite slab setting on?  What type of plastic 
> enclosure are you using.
> Thanks so much,
> Kay Wyatt
> ----- Original Message ----- 
> From: "Brett Nordgren" 
> To: 
> Sent: Friday, August 14, 2009 9:51 AM
> Subject: Re: Real time traces
>> Meredith,
>> We certainly don't want to be accused of torture, though we'll gladly 
>> plead guilty to some discreet teasing.
>> For a little more tease, see a 
>> photo and a couple of sketches.  The photo shows a slightly earlier 
>> configuration, but is basically what Dave is running now.
>> We want to go slowly in regards to encouraging folks to start trying to 
>> make something like this until we feel we're able to do our part to help 
>> with any problems.  Building one of these would definitely not be a good 
>> first project, though I think we are coming to the point where we can 
>> provide some more detail if there is any particular interest.
>> Regards,
>> Brett
>> At 11:09 PM 8/13/2009 -0600, you wrote:
>>> guys sure know how to torture people...with the 2 mysterious 
>>>unseen undefined experimental instruments.
>>>How about a web photo's "sneak peek" of the verticals?  :>)
>>>Meredith Lamb
>>>On Thu, Aug 13, 2009 at 1:47 PM, Brett Nordgren wrote:
>>>>Hello All,
>>>>We just put up a Web page which displays some near real time outputs 
>>>>from two of our experimental vertical feedback seismographs.  There are 
>>>>displays showing three different combinations of sensitivity and 
>>>>low-pass filtering, which highlight different types of ground motion.
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