Helicopter Time-domain Research Articles

Kentucky coal plant gets SCR retrofit

Integration of information from the following sources has been used to produce a much better constrained and more complete four-unit geological/hydrological model of the Okavango Delta than previously available: (i) a 3D resistivity model determined from helicopter time-domain electromagnetic (HTEM) data recorded across most of the delta, (ii) 2D models and images derived from ground-based electrical resistance tomographic, transient electromagnetic, and high resolution seismic reflection/refraction tomographic data acquired at four selected sites in western and north-central regions of the delta, and (iii) geological details extracted from boreholes in northeastern and southeastern parts of the delta. The upper heterogeneous unit is the modern delta, which comprises extensive dry and freshwater-saturated sand and lesser amounts of clay and salt. It is characterized by moderate to high electrical resistivities and very low to low P-wave velocities. Except for images of several buried abandoned river channels, it is non-reflective. The laterally extensive underlying unit of low resistivities, low P-wave velocity, and subhorizontal reflectors very likely contains saline-water-saturated sands and clays deposited in the huge Paleo Lake Makgadikgadi (PLM), which once covered a 90,000 km2 area that encompassed the delta, Lake Ngami, the Mababe Depression, and the Makgadikgadi Basin. Examples of PLM sediments are intersected in many boreholes. Low permeability clay within the PLM unit seems to be a barrier to the downward flow of the saline water. Below the PLM unit, freshwater-saturated sand of the Paleo Okavango Megafan (POM) unit is distinguished by moderate to high resistivities, low P-wave velocity, and numerous subhorizontal reflectors. The POM unit is interpreted to be the remnants of a megafan based on the arcuate nature of its front and the semi-conical shape of its upper surface in the HTEM resistivity model. Moderate to high resistivity subhorizontal layers are consistent with this interpretation. The deepest unit is the basement with very high resistivity, high P-wave velocity, and low or complex reflectivity. The interface between the POM unit and basement is a prominent seismic reflector.

A helicopter time-domain EM system applied to mineral exploration: system and data

David Fountain, Richard Smith, Tom Payne, and Jean Lemieux (Fugro Airborne Surveys) describe recent progress in developing helicopter systems for time-domain airborne electromagnetic surveys, once the ‘domain’ of fixed-wing aircraft. The first operational fixed-wing airborne electromagnetic (AEM) system was introduced in 1948 and was followed by the first operational helicopter AEM system in 1955 (Fountain, 1998; Fountain and Smith, 2003). There has been parallel development of both fixedwing and helicopter AEM since that time and, up to the early 1960s, all these systems were frequency-domain. However, since the late 1970s, fixed-wing AEM systems have been primarily time-domain while the helicopter systems remained primarily frequency-domain. With the new millennium, and especially since 2002/ 2003, there has been significant development and introduction of helicopter time-domain AEM systems (Boyko, et al., 2001; Balch et al., 2003; Eaton et al., 2004; Sorenson and Auken, 2004; Vrbancich and Fullagar, 2004; Witherly et al., 2004). The pace of introduction has been such that, today, the number of time-domain systems in use is equal to the number of frequency-domain systems. Fugro Airborne Surveys and its predecessor companies have been leaders in the continuing development of fixedwing AEM systems, including the introduction of the GEOTEM, MEGATEM, and TEMPEST systems (Smith and Annan, 1997; Lane et al., 2000; Smith, Fountain and Allard, 2003). By the same token, Fugro has been involved in the continuing development of helicopter frequency-domain AEM systems including DIGHEM and RESOLVE. Based upon this combined experience and seeing the need for a broader band, high power helicopter time-domain AEM system, Fugro has introduced the HeliGEOTEM system in 2005. This system brings together the proven GEOTEM/ MEGATEM technology with the greater operational flexibility and improved lateral resolution of helicoptermounted AEM systems.