The location of buried unexploded ordnance (UXO), which includes both detection and mapping, requires geophysical surveys. A major impediment to effectively using the results of geophysical surveys for UXO cleanup is the inability to discriminate between geophysical anomalies caused by buried UXO and geophysical anomalies caused by other buried objects, which result in false alarms.
This SERDP Exploratory Development (SEED) project investigated the potential role of microgravimetry in buried UXO discrimination and identification. Specific objectives included (1) compiling mass and volume values for ordnance items, (2) developing a microgravity modeling capability for a prolate spheroid geometry, (3) generating model gravity anomaly signatures, (4) assessing detectability (anomaly magnitude versus depth for selected ordnance items), (5) determining spatial sampling requirements, and (6) assessing the potential for gravity inversion to provide model parameters.
Localized anomalies detected by gravity surveys are related to density contrasts and, hence, to the mass excess or deficiency of a localized feature relative to the surrounding material. Microgravimetry involves high-resolution, high-accuracy measurements of very small anomalies in the gravitational field, such as would be caused by the presence of buried UXO. Detecting the gravitational anomaly caused by a UXO or non-UXO item could allow the mass of the item to be determined. When coupled with ferrous volume estimates from magnetic surveys, UXO/non-UXO discrimination may be possible. The prolate spheroid gravity modeling program requires the development of a complete analytical solution for the gravity anomaly measured on the soil surface above the model in a layered half-space. The solution would be valid for any orientation of the prolate spheroid model.
A limited microgravity field investigation was conducted. Microgravity measurements were acquired above several buried ordnance objects. Results of the field surveys were used to assess the modeling program predictions. All of the buried ordnance items were detectable at the shallowest depth (i.e., buried horizontally at a depth to center equal to half the diameter or just below the surface). Only five items, 1,000-pound bombs and larger, were detectable at depths of roughly 0.5 meters or greater, and only the 16-inch projectile was detectable at a depth of 1 meter.
Except for the largest ordnance items buried at very shallow depths, microgravity surveying is not a viable technique for detection and discrimination of buried UXO in real-world settings. The gravity modeling capability developed may, however, be useful for other environmental geophysics applications, such as underground storage tank detection and condition assessment, landfill investigations, and other detection requirements for localized buried features.