Chapter 21, Problem 68RPP

BIO Magnetic induction tomography (MIT) Magnetic induction tomography is an imaging method used in mineral, natural gas, oil, and groundwater exploration, as an archaeological tool; and for medical imaging MIT has also been used to measure topsoil depth in agricultural soils. Topsoil depth is information that many farmers need for instance, corn yield is much higher in soil that has a deep topsoil layer above the underlying, impermeable claypan. Using a trailer attached to a tractor, a farmer can map an ${\text{80,000-m}}^{\text{2}}$

(about 20-acre) field for topsoil depth in about 1 hour.

Figure 21.28 shows how MIT works A time-varying electric current in a source coil (Figure 21.28a) induces a changing magnetic field that passes into the region to be imaged—in this case, the soil (Figure 21.28b). This changing magnetic field induces a weak induced electric current in topsoil and a stronger induced current in the more conductive claypan soil at the same depth (Figure 21.28c the current direction here is drawn as though the source current and source fields are increasing). This changing induced electric current in turn produces its own induced magnetic field (Figure 21.28d). The induced magnetic field passes out of the region being mapped to a detector coil (Figure 21.28e) near the source coil. The nature of the signal at the detector (its magnitude and phase) provides information about the region being mapped A strong signal returned to the detector coil indicates a claypan layer near the surface, a weak signal returns if the clay layer is deeper below the surface.

Why is the detected signal from an MIT apparatus greater if a moist conductive layer is near the surface?

a. The signal is reflected better from the top of a nearby conductive layer.

b. The induced current is greater if the soil is moist and conductive.

c. The induced magnetic field from the induced current is bigger if its source is near the detection coil.

d. All three of the above reasons

e. b and c