The Cretaceous Edwards Aquifer of central Texas is a critical groundwater resource for human and ecological needs. Protecting the quality of this vital resource is the responsibility of all Texans. For many years, groundwater has been pumped from water wells in the region and the Edwards aquifer is a sole source aquifer for the City of San Antonio. Over the years, many wells around homes, farms, industrial sites, and urban areas have been abandoned without being properly plugged. These abandoned wells become potential avenues for groundwater contamination, but they can also constitute a safety hazard for children and animals. Part of the solution is first to locate these wells, most of which are concealed below the ground.
Use of magnetic surveys can easily locate ferrous metallic anomalies (in the shallow surface consisting of soils and sediments) that can be attributed to buried metallic objects such as well casing from abandoned wells. Magnetic surveys were conducted to locate four abandoned water wells and two abandoned oil & gas wells in the state of Texas. Three of the abandoned water wells were located near residential places and were below the ground surface. Results from ground magnetic surveys indicated major magnetic anomalies in and around the steel well casings. Maximum signals from these three wells were in the range of 2,000 nanoTesla (nT). Sources for these anomalies are interpreted to be from the steel-casings of the abandoned wells, and their locations were delineated in the field. Excavations of the marked locations encountered these abandoned wells at about 3 to 4 feet deep below land surface. It appeared that none of the wells had well heads, indicating they had been improperly plugged and abandoned. The fourth abandoned water well was suspected to be within a shopping center in an area of an acre. Magnetic, EM61 and GPR surveys were conducted previously, but the well was not located. We mapped the site with a vertical gradient magnetic survey and also scanned the site and marked magnetic anomalies on the ground. The well was discovered beneath one of the marked anomalies.
Several abandoned oil and gas wells were mapped and/or scanned by magnetic surveys, and their locations were successfully located. Magnetic anomalies usually displayed a dipole shape with positive magnetic values, up to10,000 nT or more across a well with a casing. However, in some cases, the well casing showed a monopole shape with negative magnetic values with respect to the background magnetic values. Depths to the top of those buried wells varied between few feet to 20 feet, and most of them were unplugged.
Primary Author / Conference Presenter:
Dr. Mustafa Saribudak
Environmental Geophysics Associates
Austin, Texas, USA