Helicopter-borne time-domain electromagnetic (HTEM) systems with a concentric horizontal coil configuration have been used increasingly in mineral exploration. Conductivity-depth imaging (CDI) is a useful tool for mapping the distribution of geologic conductivity and for identifying conductive targets. A CDI algorithm for HTEM systems with a concentric coil configuration is developed based on the pseudolayer half-space model. Primary advantages of this model are immunity to altimeter errors and better resolution of conductive layers than other half-space models. Effective depth is derived empirically from the diffusion depth and apparent thickness of the pseudolayer. A table lookup procedure is established based on the analytic solution of a half-space model to speed up processing. This efficiency makes generation of real-time conductivity-depth images possible. Tests on synthetic data demonstrate that the pseudolayer conductivity-depth-imaging algorithm maps a wider range of conduc-tivities and does a better job of resolving highly conductive layers, compared with that of the homogeneous half-space model. Effective depths are close to true depths in many circumstances. Field examples show stable and geologically meaningful conductivity-depth images.

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Defects in composite laminates are difficult to detect because of the conductive and paramagnetic properties of composite materials. Timely detection of defects in composite laminates can improve reliability. This research illustrates the preliminary analysis and detection of delaminations in carbon fiber laminate beams using a single layer of magnetostrictive particles and noncontacting concentric magnetic excitation and sensing coils. The baseline analytical models also begin to address the intrusive nature of the magnetostrictive particles as well as relate the applied excitation field with the stress and magnetic flux densities induced in the magnetostrictive layer. Numerical methods are used to begin to characterize the presence of magnetostrictive particles in the laminate and the behavior of the magnetostrictive particles in relationship to the magnetic field used during sensing. Unidirectional laminates with embedded delaminations are used for simulations and experimentations. A novel, yet simplified fabrication method is discussed to ensure consistent scanning and sensing capabilities. The nondestructive evaluation scanning experiments were conducted with various shapes and sizes of damages introduced into carbon fiber-reinforced polymeric composite structures. The results demonstrate high potential for magnetostrictive particles as a low-cost, noncontacting, and reliable sensor for nondestructive evaluation of composite materials.

Gradient measurement of the magnetic field vector, especially full-tensor magnetic gradiometers (FTMGs), provides various advantages over gradient components derived from measurements of the total magnetic intensity (TMI). These advantages include higher spatial resolution, directional information, and thus more detailed anomaly delineation and a significantly better-constrained solution space for magnetic inversion and interpretation. However, the airborne application of FTMG instruments requires exceptionally high sensor resolution and low levels of motion noise to maximize these advantages and to achieve high exploration depth. Superconducting quantum interference detectors (SQUIDs) are an effective option for FTMG sensors, now available commercially in the system discussed herein. This SQUID sensor system comprises intrinsic planar-type gradiometers that produce data with sufficiently low noise for use on an airborne platform. The evolution and advancement of the system and its predecessor over the past two decades has produced a robust commercial system that produces high-quality full-tensor data sets from a helicopter-towed-bird implementation. Because the SQUID sensors measure directionally sensitive data, the processing of the acquired data is significantly more challenging than for TMI sensors. Noise induced by the motion of the bird during flight, especially rotational noise, must be monitored and compensated. The introduction of a more robust and aerodynamic bird has significantly reduced the noise of the system. This noise reduction translates into greater sensitivity and accuracy and, thus, heightened confidence in the use of the survey data sets. While much of the early use of the system has been in diamondiferous kimberlite exploration, the system has successfully flown surveys in mineral exploration for a variety of targets, including gold, nickel, and iron ore. These data sets provide greater confidence in the geologic interpretation across the survey areas. Other applications for FTMG surveying include infrastructure mapping, unexploded ordnance detection, and compensation of electromagnetic data sets in marine environments.

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