On the Origin of the HTEM Species

By the year 2000, the airborne EM world was mainly dominated by two types of systems mounted on two very different types of platforms: the fixed wing AEM time domain systems and the HFEM systems. The idea of breeding fixed wing time domain AEM systems with HFEM systems became a reality. The objective was to get the best of the two worlds by combining the high transmitter power of fixed wing systems for depth penetration with the slower speed and flying altitude of the HFEM systems for higher spatial resolution and/or surveys over more rugged topography. During the last 5 years, the helicopter time-domain (HTEM) systems evolved rapidly to meet customer needs and finally took over their parent systems for most applications. Between 2000 and 2002, three HTEM systems were commercially operational: the AeroTEM by Aeroquest Ltd, the VTEM by Geotech Ltd, and the THEM by THEM Geophysics Inc. At about the same time, three major mining companies were developing their own in-house systems: Normandy Exploration Ltd with the HoisTEM, Newmont Mining Corp. with the NewTEM and Anglo with the ExplorHEM. A short while later, two additional systems were born: the SkyTEM of SkyTEM ApS and the HeliGEOTEM of Fugro Airborne Survey. The improvement in data quality, signal to noise ratio, depth of investigation, spatial resolution, conductance aperture and conductance discrimination, these systems achieved in such a short period of time is remarkable. However adaptation is context-sensitive; an evolutionary change that increases fitness in one exploration environment may decrease its ability to survive in another. This is why some systems have been designed to be more universal, whereas others are more target specific. In all cases, improvement in system efficiency can be attributed to major changes or fine tuning of basic specifications such as the increase of the dipole moment, a proper selection of the pulse waveform and base frequency, the possibility of measuring the on-time secondary field for B-field calculation and an efficient rejection of different sources of noise. Over the last 5 years the HTEM systems have adapted quickly and successfully to their exploration environment and to the client’s needs. However clients will always need more bandwidth, better resolution and greater depth of penetration at lower cost.