The canopy effect in AEM revisited : investigations using laser and radar altimetry

Abstract

This study considers a specific issue, often termed the canopy effect that relates to our ability to provide accurate conductivity models from airborne electromagnetic (AEM) data. The central issue is one of the correct determination of sensor height(s) above the ground surface (terrain clearance) to the appropriate accuracy. The present study uses the radar and laser systems installed on a fixedwing AEM system to further investigate the effect. The canopy effect can arise due to a variety of elevated features below and in the vicinity of, the flight line. The most obvious features are welldefined forest and copse zones together with domestic, commercial and agricultural buildings. Such features may cause the terrain clearance to be underestimated and this has the potential to introduce resistive artefacts and incorrect interface depths into conductivity models. Correct determination of terrain clearance is also important for the accurate processing of the other geophysical data sets acquired by airborne surveys. Radar and laser altimetry offer two very different physical measurements of height above ground. Airborne radars detect the range to the nearest reflecting object. They do this over a cone of influence that may have a radius (at the ground surface) of ~55 m (assuming a survey height of 60 m). Reflections from objects that are off-line are thus a distinct possibility. In direct contrast, a laser ranging device with low beam divergence provides a highly focused measurement. Laser accuracies (typically <2 cm) are far greater than those of radars (typically ~0.5 m). In addition, the rapid sampling of laser ranging devices (e.g., up to 2 kHz) allows both real-time and post-processing algorithms to be applied to estimate the maximum range recorded across appropriate time/spatial windows. In our case a 2 kHz (maximum) dual-pulse laser when sampled at 200 Hz provides a ×50 oversampling in relation to the 4 Hz sampling of the EM components. Our studies from recent surveys indicate that the radar altimeter often provides significantly underestimated values of terrain clearance. Such estimates may be in error by up to 10 m or 15 m across dense woodland. The issues of accuracy and reliability raised by the routine application of laser altimetry to AEM surveys and conductivity models are evaluated. It is demonstrated that it is now possible to obtain reliable laser estimates of true height above ground surface across: a) most forms of canopy encountered, b) built structures of limited spatial scale and c) bodies of water