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
- 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
Radiolocation requires practice. Even though
this gear often gives a
useable signal close to 1 km away, always try to position yourself
as possible to the expected location by using topographic
compass course and distance from an entrance, etc. Try to always
than the cave passage, ie wait uphill rather than downhill. If you
within a horizontal distance of about 1.4 times the expected depth
beacon, the locating will be very easy.
The best time of day for radiolocations is during the morning,
8AM to 1-2PM. The worst time is during the night when "skip"
noise of far distant thunderstorms. This assumes no local storm
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
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
on for 30 minutes or more. With careful planning, I have used a
as 4 minutes with divers in springs. You go to the first
on a little ahead of time, and null the receiver (after
loop, set the switch to low gain and the dial below 5.0, or
down the gain on the simplified receiver), then reconnect the
quite high RF gain, 1 Hz bandwidth, and the loop on the ground.
works quite well (complete receiver only), although false
occur with strong interference from Thunderstorms (atmospheric
power lines, or electric fence controllers. Simply reduce the RF
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
rotate it for a null, adjusting volume as desired. RF gain
high enough to give deep nulls. Ground Zero lies along the null
is known at a Line Of Position, or LOP. See the above
several lines of position passing thru ground zero.
simply walk in one direction along the null line while
the loop thru the null to update direction. If the signal gets
then simply turn around and walk back the other way. In an open
one can walk perpendicular to the original null line a short
then null again. If you walked far enough, the two null lines
not be parallel and will intersect near Ground Zero. This is the
method. If all else fails, you may be very far from the Beacon.
case make a quick signal strength measurement with the loop on
(complete receiver only) then walk 50-100m along the null line
again to see of the signal really is getting stronger or weaker.
this method may fail if you are within a distance roughly twice
depth of the beacon from Ground Zero, where the field lines are
The above sketch shows a side view of a single line of
You will be walking towards Ground Zero from the left or right
the sketch, with the loop oriented for an audio null like the
It is easy to "overshoot" Ground Zero. As one gets close, within
the estimated depth of the beacon, the best way to check progess
occasionally rotate the vertical loop perpendicular to
line that you are walking along, then tilt the top of
the loop towards
yourself. See the blue loop positions in the above
first this null may occur with the loop nearly horizontal (very
but as Ground Zero is approached, this null will occur with the
and closer to vertical. Very close to ground zero the loop will
rotated in any direction, but the signal will become incredibly
when the loop is tilted even slightly from vertical. At this
highest accuracy, it is a good idea to re-null the
the sketch below for the magnetic field shape near ground zero.
Causes of Poor Nulls
Now the vertical loop is placed on the ground and tilted back
to find the null. At any nearby location except exactly at
the magnetic field will tilt towards you as it exits the ground.
bubble level shows loop tilt. If the loop nulls when it is
towards you, then move the bottom of the loop slightly away from
re-null, repeating until the loop is precisely vertical when
plane of the loop is now precisely on a line of position that
Ground Zero. With lots of practice, and quiet conditions, it is
to detect a 6 inch (10cm) change in the position of a beacon at
(90m) depth! Mark this line at the loop, then rotate the
90 degrees and repeat the process to obtain a second line, which
cross directly over the first line. Mark the intersection of the
as ground zero. See the blue loop positions at the center of the
sketch above. To cancel out most of the errors in the
and its level, repeat each measurement in the last step with the
loop rotated 180 degrees from its original position. This will
give slightly different positions. The result will be a small
with ground zero at the center.
There are several possible causes of poor nulls, which are nulls
not deep and sharp. Use the narrow 1 Hz bandwidth mode for
signal. First, carefully adjust the audio null control on
for minimum audio tone output, with the loop antenna
disconnected and the
RF gain turned down. If the tone does not totally
tweaking the internal "null balance" control very very
a screwdriver thru the hole in the side of the case.
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
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,
the beacon is very deep, say 300 ft or more, or when there is a
highly conductive overburden such as deep wet soil, clays,
The most exotic cause of poor nulls is the limestone
effect is called anisotropy, which is a fancy way of
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
fractures in the rock, called "joints". This effect is
doing radiolocations under quiet conditions in deep dry
in nearly every location I have done in Florida Springs, where
saturated limestone is highly conductive, although quite flat
uniform with essentially no overburden.
This anisotropic effect causes null sharpness to vary depending
direction that you approach ground zero. In fact you will
find one Line Of Position which will always give a perfect
This fact can be used to make the search easier. A line of
right angles to this best LOP will always result in the
shallowest null. This effect is very noticable at Ground Zero,
loop directions may have to be chosen to obtain sharper nulls.