InTransition Magazine
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InTransition Magazine : Transportation Planning, Practice & Progress

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About the Technologies in this Story

Ground-penetrating radar

GPR was introduced in the 1970s and has since been refined as a tool for surveys. The device consists of two parts: an antenna (also called a transducer), shaped like a large square box; and a digital control unit that produces radar energy and allows the user to view reflections from within the ground on an attached LCD screen. The antenna can be dragged along the ground with a rope, or placed on a lawnmower frame with the control unit in the handle area.

The antenna sends high-frequency radio waves into the ground which then reflect back whenever discontinuities are encountered. The reflections are received at the antenna and transmitted to the control unit, where they are stored as digital data. Reflections from within the ground are measured in the elapsed time from when they are sent, travel in the ground and then received back at the surface, measured in nanoseconds. The reflected waves are pictured as peaks and troughs of different color bands on the screen. When many hundreds or thousands of waves are stacked together, the position of buried objects or boundaries in the subsurface are visible in the profile. The antenna also records differences in wave strengths that are reflected off buried materials, which can tell something about the composition of these buried materials.

Magnetometers and Proton Precession Magnetometers

The first magnetometers were developed in the early 1830s and later refined by Nikola Tesla, after whom the magnetic field measurement unit, the tesla, was named. The technology has been tailored through the years for use in archaeological reconnaissance by modern engineers. There are many different types of magnetometers. They all work by measuring changes in the earth’s magnetic field as influenced by subtle variations in the ground. These measurements are collected in a storage unit and displayed.

A proton precession magnetometer, or a proton magnetometer, looks like a small metal rocket with plastic fins. A long wire enclosed in plastic tubing is wrapped around the rocket and a box that serves as a control device with a display screen is attached to the wire. The machine works when a radio frequency is turned on and passes through the rocket. The rocket holds a container full of a liquid that is rich with hydrogen atoms. Electrons that dissolve in the liquid get excited by the radio frequency. The electrons pass their energy on to the hydrogen atoms’ nuclei (protons) and alter their spin. After this occurs, the radio frequency is removed, and the protons spin back to their original position. The frequency of their spins, or “precession,” is measured with the coil. The spins show the presence of objects or features that change the earth’s magnetic field.

The changes, or distortions, show in the area that has been surveyed. The larger the object or feature is, the farther away from the object the distortion occurs. The device is operated by passing the rocket over earth or through water. The size of the object or feature can be measured by the pattern of distortion that is detected by the magnetometer. One of the best ways to map distortion is to walk up and down in straight lines with the magnetometer in the area to be surveyed.

Contributing: Jessica Zimmer; Larry Conyers, University of Denver; Chris Gaffney, University of Bradford, United Kingdom.

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