Norman R. Carlson

BURIED LANDFILL DELINEATION WITH INDUCED POLARIZATION: PROGRESS AND PROBLEMS*

In recent years, the use of induced polarization (IP) data for delineating buried landfills has increased significantly, due to technological advances that have made this method faster and less expensive, and therefore more applicable to the environmental industry. As the database of IP information grows to include information over waste of differing composition and in differing environments, interpretation has improved significantly. Problems and limitations still exist, of course, but the IP method has become an efficient and economic tool in evaluating waste sites. We discuss here progress in interpretation, including methods to process large amounts of data rapidly in order to decrease costs, and we discuss problems that still exist, such as electrode stability, which still limit the field survey speed.

Early‐Time, Multi‐Component Mobile TEM for Deep Metal Detection

Data examples from a recent project show very interesting and useful characteristics of the earlytime data from the horizontal components of TEM surveys used in deep metal detection for targets such as USTs, UXOs, and utilities. Normally, most deep metal detection surveys utilize a system in which one or two TEM measurements of the vertical component (Hz) are acquired. These systems usually acquire data at relatively late times (hundreds of microseconds after transmitter turn-off), to allow the background earth response to decay to zero. A good example is the popular Geonics EM-61 system. By recording data at numerous time windows for all three components (Hx, Hy, and Hz), however, from early times (a few microseconds) through late times, additional significant information is acquired. In one recent project, a 55-acre site was surveyed (by another contractor) with an EM-61, and several subsurface targets were identified for excavation. Small areas around these targets, totaling only 2.25 acres, were re-surveyed using a multi-component, early-time system. In addition to verifying the targets, four additional anomalies that were not evident in the Hz data were detected, including two buried powerlines. Examination of the horizontal component data of the field also appears to be particularly useful in discriminating targets. For example, linear features such as pipelines and powerlines are easily distinguished from 3-dimensional targets with only a single line of data, instead of requiring an array of lines to interpret the targets based on the geometry of anomalies on adjacent lines.

A sewer sinkhole study using TEM

On 7 September 2002, two large sinkholes opened suddenly on a major, five-lane roadway in Tucson (Arizona), USA (Figures 1 and 2). The exact cause of the sinkholes is still controversial, but the sinkholes were apparently related to failures near 90° bends in a 35-year-old, 6-m deep, 1.1-m diameter sewer line known as the outfall interceptor.

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

Fluid-flow mapping at a copper leaching operation in Arizona

At the San Manuel copper mine in southeastern Arizona, recovery of copper from the oxidized portion of this porphyry mineral resource is being achieved through a large in situ leaching operation using weak sulfuric acid solution. In the past, this activity was coordinated with open pit and underground mining, but in todays economic climate only the in situ operation continues. The acid solution (20 grams/liter) is injected into wells (unpressurized at varying depths up to several hundred meters), usually at rates of only a few tens of gallons per minute. The copper-bearing pregnant leach solution (PLS) is recovered either in nearby recovery wells or in collection areas in the underground workings 350–500 m below the surface.

Case histories of controlled‐source audio‐frequency surveys

Sulfide systems can be identified in controlled source audio-frequency magnetoteLhuic (CSAMT) surveys by steep to vertically bounded zones of low apparent resistivity (pa) that pmvkle deftitive horizontal resolution of the lateral extent of the suhides. The method proved most effective iflow cost, preliminary self-potential (SP) surveys were used to defme the centers of the sulfide svstems orior to the CSAMT surveys. The technique can L&O be u&d to I&Y the electrical stratigraphy, and dips 260’ have been traced. The ability to map subsurface StNCtUrC and stratigraphy also has applications in oil exploration, engineering, and hydrology.