LOCATING GROUND ZERO
Using the shape of the magnetic field
ADDED NOTES December 2011:
The procedures described here are for the long
range "DQ" receiver and high-power beacons. With the Basic-1
and 2 radiolocators, the physics of locating ground zero remains the
same, but some considerations are different.
- The receiver is not as sensitive and therefore will not be
bothered as much by atmospheric noise at night. The
receiver does not have calibrated dials, alarms, null balance
and bandwidth controls, etc to worry about!
- The beacon can be left running for several hours without
running the battery down. The underground crew can map or
explore instead of waiting around.
- The surface crew may may need to wander around the general
area listening for the beacon due to it's short range, instead
of sitting in 1 place. The receiver can be turned on early
and left on. Using multiple surface receivers is a good
option.
- Once you have located the beacon, and the underground crew
switches to receive, you can let them know that you have
succeeded, by using simple prearranged codes, and they can let
you know if they are going to the next location or exiting the
cave. This flexibility is not possible with the 1-way DQ
radiolocator without carrying extra downlink communications
gear.
Radiolocation requires practice. Even though
this gear often gives a
useable signal close to 1 km away, always try to position yourself
as close
as possible to the expected location by using topographic
overlays, GPS,
compass course and distance from an entrance, etc. Try to always
be higher
than the cave passage, ie wait uphill rather than downhill. If you
are
within a horizontal distance of about 1.4 times the expected depth
of the
beacon, the locating will be very easy.
-
The best time of day for radiolocations is during the morning,
from about
8AM to 1-2PM. The worst time is during the night when "skip"
brings the
noise of far distant thunderstorms. This assumes no local storm
activity.
For this same reason, winter is much quieter than summer. Even
so, it is
possible to do moderate depths on a summer evening. I find it
hard to even
get cavers moving before the "crack-of-noon"!
- When the underground party arrives at the location, their job
is to locate or create a level spot at the intended Ground Zero
location, then position their loop precisely horizontal by using
the attached or supplied bubble level or a line level.
Eyeballs will not do this job! If the loop is offset from
ground zero for practical reasons such as a deep pool or uneven
ground, the crew can survey between the 2 spots while they are
waiting after turning the beacon on.
-
Coordinate times with the underground party so you know the
earliest time
they will "turn on". In smaller caves or with experienced
cavers, you can
have them operate on a schedule. At first, plan to have the
beacon turned
on for 30 minutes or more. With careful planning, I have used a
little
as 4 minutes with divers in springs. You go to the first
location, turn
on a little ahead of time, and null the receiver (after
disconnecting the
loop, set the switch to low gain and the dial below 5.0, or
simply turn
down the gain on the simplified receiver), then reconnect the
loop using
quite high RF gain, 1 Hz bandwidth, and the loop on the ground.
The alarm
works quite well (complete receiver only), although false
phase-locks can
occur with strong interference from Thunderstorms (atmospheric
noise),
power lines, or electric fence controllers. Simply reduce the RF
gain until
there is only an occasional false alarm. Don't wait directly
under a noisy
power line or close to a metal fence line!
-
When the beacon is heard, hold the loop vertical by its handle
and slowly
rotate it for a null, adjusting volume as desired. RF gain
should remain
high enough to give deep nulls. Ground Zero lies along the null
line, which
is known at a Line Of Position, or LOP. See the above
sketch showing
several lines of position passing thru ground zero.
Gamblers can
simply walk in one direction along the null line while
continually swinging
the loop thru the null to update direction. If the signal gets
weaker,
then simply turn around and walk back the other way. In an open
field,
one can walk perpendicular to the original null line a short
distance and
then null again. If you walked far enough, the two null lines
(LOPs) will
not be parallel and will intersect near Ground Zero. This is the
2-LOP
method. If all else fails, you may be very far from the Beacon.
In this
case make a quick signal strength measurement with the loop on
the ground
(complete receiver only) then walk 50-100m along the null line
and measure
again to see of the signal really is getting stronger or weaker.
Note that
this method may fail if you are within a distance roughly twice
the estimated
depth of the beacon from Ground Zero, where the field lines are
not vertical.
-
The above sketch shows a side view of a single line of
position.
You will be walking towards Ground Zero from the left or right
side of
the sketch, with the loop oriented for an audio null like the
green loops
shown.
-
It is easy to "overshoot" Ground Zero. As one gets close, within
1.4 times
the estimated depth of the beacon, the best way to check progess
is to
occasionally rotate the vertical loop perpendicular to
the null
line that you are walking along, then tilt the top of
the loop towards
yourself. See the blue loop positions in the above
sketch. At
first this null may occur with the loop nearly horizontal (very
confusing),
but as Ground Zero is approached, this null will occur with the
loop closer
and closer to vertical. Very close to ground zero the loop will
null while
rotated in any direction, but the signal will become incredibly
strong
when the loop is tilted even slightly from vertical. At this
point, for
highest accuracy, it is a good idea to re-null the
receiver. See
the sketch below for the magnetic field shape near ground zero.
-
Now the vertical loop is placed on the ground and tilted back
and forth
to find the null. At any nearby location except exactly at
Ground Zero,
the magnetic field will tilt towards you as it exits the ground.
The cylindrical
bubble level shows loop tilt. If the loop nulls when it is
tilted slightly
towards you, then move the bottom of the loop slightly away from
you and
re-null, repeating until the loop is precisely vertical when
nulled. The
plane of the loop is now precisely on a line of position that
passes thur
Ground Zero. With lots of practice, and quiet conditions, it is
possible
to detect a 6 inch (10cm) change in the position of a beacon at
300 ft
(90m) depth! Mark this line at the loop, then rotate the
vertical loop
90 degrees and repeat the process to obtain a second line, which
should
cross directly over the first line. Mark the intersection of the
lines
as ground zero. See the blue loop positions at the center of the
"overhead"
sketch above. To cancel out most of the errors in the
receive loop
and its level, repeat each measurement in the last step with the
vertical
loop rotated 180 degrees from its original position. This will
usually
give slightly different positions. The result will be a small
square box
with ground zero at the center.
Causes of Poor Nulls
-
There are several possible causes of poor nulls, which are nulls
that are
not deep and sharp. Use the narrow 1 Hz bandwidth mode for
the cleanest
signal. First, carefully adjust the audio null control on
the receiver
for minimum audio tone output, with the loop antenna
disconnected and the
RF gain turned down. If the tone does not totally
disappear, try
tweaking the internal "null balance" control very very
slightly with
a screwdriver thru the hole in the side of the case.
Readjust the
front panel null and repeat. The controls interact.
-
Interference from nearby power lines, electric fence
controllers, or distant
thunderstorms can make the null appear broader because the
signal disappears
into the interfering noise when you get close to the null.
-
Nulls tend to be broad when you are a long way from ground zero,
or when
the beacon is very deep, say 300 ft or more, or when there is a
lot of
highly conductive overburden such as deep wet soil, clays,
shale, etc.
-
The most exotic cause of poor nulls is the limestone
itself. The
effect is called anisotropy, which is a fancy way of
saying that
the conductivity of the rock is not the same in all directions,
ie is not
homogeneous. The assymetry may be explained by parallel sets of
vertical
fractures in the rock, called "joints". This effect is
obversed when
doing radiolocations under quiet conditions in deep dry
caves and
in nearly every location I have done in Florida Springs, where
the water
saturated limestone is highly conductive, although quite flat
lying and
uniform with essentially no overburden.
-
This anisotropic effect causes null sharpness to vary depending
on the
direction that you approach ground zero. In fact you will
always
find one Line Of Position which will always give a perfect
deep null.
This fact can be used to make the search easier. A line of
position at
right angles to this best LOP will always result in the
broadest,
shallowest null. This effect is very noticable at Ground Zero,
where different
loop directions may have to be chosen to obtain sharper nulls.
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