In a message dated 17/05/06, kd6iwd@......... writes:

> I have built a hammer seismic system which incorporates a memory scope. I
> can hammer the ground any number of times and sum the returned signal. This 
> gives vastly increased sensitivity and immunity to noise. Can anyone suggest
> sources of information on how to measure the depth to bedrock with this type 
> of device?

Hi Jim,

       Can you use two high frequency piezo disks with a light centre load 
and a summing amplifier? You may have to damp the disks with polyurethane foam. 
Put one disk close to the hammer plate and the other several feet away. The 
timing in between the two disks will ~measure the ground velocity and then you 
look for reflections. But you need to reckon on 3 to 5 km / sec for the ground 
waves - you are likely to be measuring time intervals in milliseconds, if not 
microseconds. I think that commercial type equipment will probably use a 
variable gain amplifier (transconductance type) triggered by the initial pulse. The 
reflected signal drops off at about the 4th power of the depth. By separating 
the two sensors, you can put a higher gain on the remote one. 
       This might be an application for silicon accelerometers? They may have 
a higher natural frequency than piezo disks. Do you have any estimate of the 
depth to the bedrock?

       Regards,

       Chris Chapman
In a message=20=
dated 17/05/06, kd6iwd@......... writes:



I have built a hammer seism=
ic system which incorporates a memory scope. I

can hammer the ground any number of times and sum the returned signal. T=
his gives vastly increased sensitivity and immunity to noise. Can anyone sug=
gest

sources of information on how to measure the depth to bedrock with this=20=
type of device?




Hi Jim,



       Can you use two high frequency piez=
o disks with a light centre load and a summing amplifier? You may have to da=
mp the disks with polyurethane foam. Put one disk close to the hammer plate=20=
and the other several feet away. The timing in between the two disks will ~m=
easure the ground velocity and then you look for reflections. But you need t=
o reckon on 3 to 5 km / sec for the ground waves - you are likely to be meas=
uring time intervals in milliseconds, if not microseconds. I think that comm=
ercial type equipment will probably use a variable gain amplifier (transcond=
uctance type) triggered by the initial pulse. The reflected signal drops off=
 at about the 4th power of the depth. By separating the two sensors, you can=
 put a higher gain on the remote one.=20

       This might be an application for si=
licon accelerometers? They may have a higher natural frequency than piezo di=
sks. Do you have any estimate of the depth to the bedrock?



       Regards,



       Chris Chapman