Airborne Light Detection and Ranging relates to an active instrument using a laser to record three-dimensional data points. Lidar produces highly detailed terrain models, over large areas and in a short amount of time. The instrument identifies and maps features with a topographic expression and may record the presence of archaeological features, even when situated beneath heavy vegetation. There is a version of Lidar that is able to penetrate water column to detect, record and map submerged features.
Multispectral sensors are passive, and capture image data at frequencies across the electromagnetic spectrum, including those beyond the visible light range. Four bands may be recorded, showing a characteristic signature of vegetation depending on its type, the environment and stress caused by underlying archaeological structures. These sensors are commonly mounted on orbiting satellites. They also may have regular cameras mounted. Satellites may also employ hyperspectral instruments, as described below.
Sound Navigation and Ranging produces a nearly photographic two-dimensional image of the seabed, and is active. The instrument is most commonly towed as a fish behind a vessel. Sonar actively senses the environment by sending out pulses of sound and measuring returns. It is used mainly for the detection of shipwrecks, imaging features that stand proud of the seabed. Multibeam uses multiple transducers, producing a number of sonar signals, or beams to record high-resolution images. These instruments are finding increasing us in discovering and managing submerged heritage sites.
A magnetometer is a passive instrument that can be used to detect metallic objects on, or beneath the seabed by measuring variations in the Earths magnetic field. The instrument may be airborne or towed from a vessel. Its main use in archaeology is for the detection of wreck sites and associated ferrous material.
Hyperspectral instruments are passive, and based on same principles as multspectral, however they measure electromagnetic radiation in a multitude of small spectral bands that are only a few nanometres wide. This enable a more detailed investigation of the environment, and in higher resolutions. The detection and visualizing of archaeological material is thus enhanced. Targeted environments may be either above the water mark, or submerged.
Aerial Photography may detect sites on land, in the intertidal zone and even underwater.
Sub-bottom profilers are active sensors, normally towed by a vessel, though may be hull mounted. Operating on principles similar to sonar, they can detect and image structures buried within the sediments of the seabed. The higher resolution systems can provide information about buried shipwrecks, although the exact location of the site needs to be known in advance. However, the instrument may be used to investigate anomalies detected by a magnetometer. Sub-bottom profiling is useful to understand the prehistory of the seabed. It shows deposits, cuts and surfaces of previously exposed landforms, and their sequences.