Experiment with "by-pass" capacitors on the input to your seismic monitor. A 
good quality AC power line filter on both units - seismic and transceiver. 
One problem that you might check into is what is called "an elevated 
ground". This is where your transmitter is grounded, but not truly 
grounded - confusing YES!. I have had to chase grounding problems in complex 
communication systems and this be frustrating. You start by making sure you 
have good quality grounding straps - I use the term straps to indicate 1" 
wide tinned copper braid or 1/16" thick x 1" wide flat copper stock in lieu 
of just a piece of wire. Fine stranded welding cable or high quality 
automobile battery straps have been used for grounding applications. 
Circular grounding wires can sometimes "radiate" and cause interference. 
Each and every grounding point needs to be cleaned and "no corrode" contact 
improver smeared on the connection points. You can get "no corrode" at 
electrical supply houses. Power companies and electrical contractors use 
this all the time.

Now for the fun stuff: When you connect grounds, or any else for that 
matter - make sure that you are not causing your own problems by using 
dissimilar metals. For example, don't mix carbon steel hardware with copper 
wire. In the 1970's a common practice in residential construction was to use 
cheap aluminum wire in lieu of copper. Over the years the aluminum wire 
expanded and contracted at different rate from the contacts on switches and 
outlets - this created a "high resistance" contact which later heated up and 
caused fires. Last summer I had help my neighbor on this same issue - she is 
lucky she did not have a house fire.

Here is a grounding story that went bad by design: I use to work on marine 
radar systems. The system engineer never considered that "his" pride and joy 
would ever have a problem. It seems that the engineer ignored electrolysis - 
he spec's called for aluminum waveguide to be connected to brass waveguide 
with steel screws - all on a boat in a salt water environment. Within six 
weeks the entire assembly was corroded beyond repair. In another application 
on an aluminum hull vessel the engineer had an HF radio connected to the 
hull using a copper grounding strap with a steel bolt - this was a real mess 
and caused considerable damage to the hull.

One point in closing: Power line ground is not always "ground" for RF 
purposes. Power line ground is for safety purposes only and not for RF. You 
may need to install your own ground rods.

Jim


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