Microgravity Method For The Detection Of Abandoned Shafts Of The Schuyler Copper Mine In Northeastern New Jersey

The microgravity method has been used to detect subsurface<br>abandoned shafts of the Schuyler Copper Mine in the Borough of<br>North Arlington, northeastern New Jersey. The Schuyler Mine, a<br>potential hazard because of its highly urbanized location, was<br>known to have more than 40 mine shafts, all covered by<br>unconsolidated sediments. The applicability of microgravity for<br>detection of abandoned mines had previously been demonstrated at<br>the Lawrence Iron Mine in northern New Jersey. Synthetic<br>computer models were used to simulate the gravity anomalies<br>expected from shallow shafts in a sandstone bedrock.<br>Microgravity surveys at the Schuyler Mine were conducted at<br>21 possible shaft locations. Data were collected at 686 stations<br>along 38 profile lines. The stations on each profile were spaced<br>5 feet (1.52 m) apart. Special field procedures were followed to<br>minimize errors due to instrument drift, station elevation and<br>location. The resulting gravity maps and profiles showed<br>anomalous gravity lows ranging from 0.02 to 0.05 mGa1 which were<br>interpreted to be due to buried shafts. Some gravity profiles<br>were modeled using 2.5-dimensional gravity inversion software.<br>The models showed about 10 feet (3.05 m) of unconsolidated<br>sediments over a shaft and bedrock. The model5d shaft dimensions<br>ranged from 8 ft2 (2.44 m2)to 15 ft2 (4.57 m ). Data from 2 of<br>the 21 sites surveyed did not indicate shafts. Of the remaining<br>19 sites that indicated the presence of shafts, 15 were<br>subsequently confirmed and delineated by drilling. The newly<br>located shafts are to be filled with stone and capped.<br>The success of the microgravity method is attributed to the<br>shallow depth of the mine shafts, instrument accuracy, closelyspaced<br>stations, accurate station location and accurate station<br>elevation control.