The Utility of Horizontal Component Measurements in Random‐Walk TEM Surveys

Abstract

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 overestimating 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.

A Fast 4-D TEM System for UXO Characterization

“…There is no coherent pattern on the lines that cross 1 m (nominal) north and south of the centerline. Figure 2.26 qualitatively illustrates an important point that we noted in our Blossom Point report and further amplified in recently presented technical papers [44][45][46]. For compact 3-D targets, the u-component (horizontal component in direction of cart travel) is bipolar with the negative peak occurring as the antenna approaches the target, a crossover occurring directly over the target and a positive peak occurring as the antenna moves beyond the target.…”

“…In this application, a large antenna array (Figure 2.3) profiles city streets beneath which are decaying sewer lines constructed of non-metallic concrete pipe. Early time transients measurements permit the system to sense areas where the conductivity of the soil is higher due to increased moisture in the vicinity of a leak, thereby mapping areas where the sewer pipe is leaking [16]. …”

“…Various experimental controls have been built in to our survey procedures that provide direct estimates of these quality parameters. 16 The radius of the data patches that we gather for target modeling is specified by the user. In the Calibration Lanes at ATC, many targets were buried on 2-m centers.…”

“…An example calibration run standard response plotted against time. The red profile is an Intermediate time composite gate for the vertical channel (uVsum3= dBz/dt, windows [12][13][14][15][16][17][18][19][20][21], and the green profiles are the 31 channels of the complete vertical field transient. The first peak is generated by passing one direction over the test sphere, the second peak from passing over target in the opposite direction, and the plateau is from parking the cart directly over the target for a 30 second static occupation.…”

“…Therefore in our modeling, we gathered data points falling within a 1-m-radius of the target under investigation for our target analysis. 16 Station locations over the entire area that we surveyed are shown in the Appendix: OpenFieldTargets.pdf.…”