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Searching for Ground Zero
Using the 3496 Hz "D-Q" Receiver
       The author is shown attempting to locate Ground Zero on a West VA hillside ~700 feet above the Dream Lake siphon in Culverson Creek Cave. This particular location turned out to quite difficult and required a special technique to find Ground Zero. The equipment shown is the original prototype D-Q Receiver with the 18 inch loop. A belt clip is used with a top-mounted digital signal strength readout.  The earphones are 700 ohm units originally used with an electronic stethoscope. The yellow object below my right hand is a cylindrical level removed from a plastic hand level and remounted with silicon rubber in an adjustable mount. It is used in the final stage of locating the point where the magnetic field lines are precisely vertical. The low-stretch cord emerging from the center of the loop is used for Ratiometric depth measurements up to about 120 feet.  It is precisely 6 feet long with its free end tied to a stick which the operator stands on while the loop is held horizontal overhead with the cord tight.
        The usual procedure for locating Ground Zero is to first find the null direction (LOP1), which is left-to-right in the picture.  Two general methods may be used to determine which direction to walk in:

Method 1):  Walk perpendicular to the LOP (into or out of the picture) a short distance to get LOP2. If the LOPs are in different directions then Ground Zero will be near where they cross. If not, then walk farther and try again.  Now just walk down LOP2  following the null and watching the signal build up.

Method 2):  Lay the loop on the ground and note the signal strength.  Walk along LOP1 in either direction and repeat. If the signal is weaker, you are going in the wrong direction.

        As you approach ground zero, stop and turn the loop 90 degrees so it is perpendicular to the LOP. Now tilt the loop towards and away from you to find a null. If the top of the loop tilted towards you, then you have not reached ground zero. If it tilts away, you have walked past it. You just keep repeating this process until the null occurs when the loop is precisely vertical, using the bubble level, no matter in what direction the loop is rotated. This method seems generally more accurate than just relying on crossing LOPs.