In a message dated 29/05/2009, DSaum@............ writes:

>  Since Rayleigh waves are vertical but vertical geophones are more  
designed for vertical, higher >frequency motions, would the geophone  still pick 
up these Rayleigh waves very well?

It is a common  misconception that 4.5 Hz geophones can not pick up seismic 
signals below 0.1  Hz, or that they need special post processing to do so.

I often pick up  0.05 Hz or lower signals from strong teleseismic events 
with my horizontal 4.5  Hz geophone systems that do not have any spectral 
emphasis processing on the  lower frequencies



Hi Dave,
 
    Sure you can occasionally pick up 20 second waves  on a 4.5 Hz 
geophone, but the sensitivity is a factor of x8,100 down.  However, most of us want 
to be able to sense the long period waves all the  time. It is relatively 
difficult to extend the period of a velocity sensor by  more than x10. You can 
get more, but you need a very low noise amplifier. I  extend the period of 
4.5 Hz geophones down to 0.5 Hz to pick up P and S waves  better.
 
    If you simply change your technology to Piezo  sensors 102-1170-ND @ US 
$1.62 instead of 4.5 Hz geophones, you can do this  fairly easily. You do 
need to make a suitable FET input amplifier; the  piezo disk needs to be kept 
dry and at a fairly stable temperature. I use a  honey jar with a metal 
lid, a moulded rubber seal and a sachet of silica gel. I  stick a PCB disk with 
soldered electrical feed through pins over a  hole in the lid, using two 
part acrylic adhesive. The amplifier is mounted  inside the lid of the jar, to 
keep it dry. The input impedance is about 50 M  Ohms.
 
    Regards,
 
    Chris Chapman
 






In a message dated 29/05/2009, DSaum@............ writes:
> 
  Since Rayleigh waves are vertical but vertical geophones are more 
  designed for vertical, higher >frequency motions, would the geop=
hone 
  still pick up these Rayleigh waves very well?

It is a common=
 
  misconception that 4.5 Hz geophones can not pick up seismic signals belo=
w 0.1 
  Hz, or that they need special post processing to do so.

I often=
 pick up 
  0.05 Hz or lower signals from strong teleseismic events with my horizont=
al 4.5 
  Hz geophone systems that do not have any spectral emphasis processing on=
 the 
  lower frequencies


Hi Dave,
 
    Sure you can occasionally pick up 20 second=
 waves 
on a 4.5 Hz geophone, but the sensitivity is a factor of x8,100 down.=
 
However, most of us want to be able to sense the long period waves al=
l the 
time. It is relatively difficult to extend the period of a velocity sensor=
 by 
more than x10. You can get more, but you need a very low noise amplif=
ier. I 
extend the period of 4.5 Hz geophones down to 0.5 Hz to pick up P and S wa=
ves 
better.
 
    If you simply change your technology to Piezo=
 
sensors 102-1170-ND @ US $1.62 instead of 4.5 Hz geophones, you can do thi=
s 
fairly easily. You do need to make a suitable FET input amplifie=
r; the 
piezo disk needs to be kept dry and at a fairly stable temperature.=
 I use a 
honey jar with a metal lid, a moulded rubber seal and a sachet of silica=
 gel. I 
stick a PCB disk with soldered electrical feed through pins over=
 a 
hole in the lid, using two part acrylic adhesive. The amplifier is mo=
unted 
inside the lid of the jar, to keep it dry. The input impedance is about 50=
 M 
Ohms.
 
    Regards,
 
    Chris Chapman