The Utility of Horizontal Component Measurements in Random-walk Tem Surveys

Laboratory tests and field examples of the horizontal field component measurements in shallow transient electromagnetic (TEM) surveys show the utility of these data in target characterization in reconnaissance, random-walk surveys for unexploded ordnance (UXO) and underground utilities. For example, prior work has shown that the Hx component (which we define as the horizontal field component in the direction of travel of the measurement system) is often useful in distinguishing small 3-D targets from linear features (such as buried pipelines or power lines). The current work expands on this capability, which is particularly important in UXO projects in which random transects are evaluated to determine a statistical estimate of density and distribution of potential ordnance and explosives. Anomalies from pipelines or power lines can skew the statistical evaluation toward over-estimating the number of potential UXO in a given area. The current research provides examples of the additional information that is available in the horizontal components of TEM surveys, particularly in the early time after transmitter turnoff. For example, the Hy component data are useful in estimating the orientation of linear features such as pipes and power lines with respect to the survey lines. Introduction The TEM method, using relatively small loops (a few square meters in area and less) mounted on wheeled carts is one of the most commonly-used techniques applied currently to UXO, underground storage tanks (UST), and utilities detection. In most cases, the vertical component (Hz) is measured at one or more heights above the ground after the system and background earth responses have decayed to near-zero levels, allowing detection of subsurface conductors. Hz-component cart-mounted measurement systems, such as the Geonics EM-61, have been extremely successful tools in UXO, underground storage tank, and utility detection. Characterization of targets is becoming increasingly important, however, particularly in UXO projects. Advance knowledge of the type of UXO, or the simple distinction between UXO and nonUXO metallic debris, can have important economic impact on excavation and removal procedures. Although characterization is somewhat less important in projects in which 100% of anomalies are to be excavated and removed regardless of the nature of the target, it is very important during random-walk, reconnaissance surveys, such as an Engineering Evaluation / Cost Analysis (EE/CA) project. These projects are intended to provide a statistical estimate of UXO targets in given areas for detection and removal cost estimates. One avenue of research into the characterization problem includes the use of the horizontal component, in addition to the vertical component, of TEM data. Also of interest is the use of more measurements along the decay curve, in order to make use of the time constant information. The directional dependence of the horizontal component measurement and its use is intuitive, and has been