414 REMOTE SENSING/Active Sensors REMOTE SENSING Contents Active Sensors GIS Passive Sensors interpretation There are three main application areas relevant to geology: structural and geomorphological mapping, exploitation of the ability to measure fine surface roughness, and the measurement of surface motions including those associated with earthquakes (see Tectonics: Earthquakes) and volcanoes (see Volcanoes) Active Sensors G Wadge, University of Reading, Reading, UK ß 2005, Elsevier Ltd All Rights Reserved Introduction Radars, Lidars and Sonars The amount of solar radiation reflected from rocks at longer wavelengths (>1 mm) is very small To properly exploit this microwave part of the electromagnetic spectrum for remote sensing, artificial sources of radiation are used These artificial, or ‘active’ techniques allow measurements to be made at night as well as during the day The longer wavelength of microwave radiation compared to that in the optical part of the spectrum also makes clouds transparent, allowing microwave radiation to ‘see’ the ground surface in all weathers, day and night In radar (Radio Detection and Ranging) remote sensing, a series of timed pulses of microwave energy are transmitted through the air, strike targets (e.g., the ground) and are reflected back to the radar which receives the returned signals There are parallels between radar and seismic exploration techniques The principle of ‘ranging’ is common to all active sensors in remote sensing described here Because the energy source is under the control of the instrument designer, the frequency and wavelength of the radiation is chosen with a very narrow range so that it can be recognized more easily on reflection and its arrival timed accurately Knowing the two-way (to the ground and back to the sensor) travel time (t) and the speed of travel through the medium (c), the distance or range (R) to the ground is given by: Radars send pulses of microwave radiation (wavelength usually 3–30 cm) from antennas through the atmosphere to the ground, in our case, and receive the return signal Lidars the same but with optical wavelength light (800–1000 nm) from lasers Lidars not work through cloud, as radars Sonars are the oceanographic equivalent to radars that send pulses of energy (1–15 cm wavelength) through the water column to strike the seabed Figure shows the typical sensing arrangement of the three techniques Radars have made the most impact on geology Lidars have been relatively little used in geological applications to date but the technology is newer Usually flown from low flying aircraft, they are used as a simple track or swath altimeter with a small footprint With their high vertical ($10 cm) and horizontal ($1–2 m) resolutions, detailed digital elevation models (DEMs) can be created They will become increasingly useful to studies requiring detailed geometric knowledge of the terrain, such as process-based simulations Sonars have a much longer history of sea-floor mapping The sensors are towed behind a ship at various levels above the seabed Absolute positioning is a more involved process than for space-borne sensors The range information received by the sonar is used to form bathymetric maps with accuracies of a few tens of metres in some cases As with radar, the magnitude information is a function of the geometrical relationship of the sensor, and the seabed and the surface roughness Particularly valuable results have been achieved at spreading ridges, accretionary prisms, and in mapping the apron of gravity collapse deposits around oceanic islands R ẳ ct=2 ẵ1 After briefly reviewing the three allied ‘active’ techniques of radar, lidar, and sonar, attention is focused on radar and the basic principles behind its use and