# AUV Adaptive Sampling Methods: A Review
## 4 Mission Objectives of Sampling
### Mapping (Field Estimations)  
Another important objective of a sampling mission using an AUV is a field estimation. This is the construction of an estimate in a spatio-temporal varying distribution [95]. The mapping objective itself does not usually require an adaptive approach. It is often involved with an exhaustive survey following regularly-spaced track lines through the field of interest. However, when the exact location of the object of interest is unknown, or the feature has a dynamic attribute, a combined module of adaptive tracking and subsequent mapping offers an efficient solution. The following section describes some methods that have been used for such classes of problems. The magnetic field was surveyed using the Autonomous Benthic Explorer (ABE) AUV near the sea bottom in order to map lava flow thickness after a volcanic eruption that had occurred in 1993 on the CoAxial segment of the Juan de Fuca (JDF) Ridge [96]. The spatial extent and volume of the newly created lava flow could be obtained through a repeated-swath mapping technique of seafloor bathymetry and subsequent comparison between the pre-eruption and the post-eruption survey data. However, pre-eruption data is not always available. [PERSON] et al. [96] introduced a near-bottom magnetic mapping method, as an alternative option, to detect the new flow. It was to use the magnetic anomaly as a proxy. The lava flow generated a significant magnetic field anomaly as a result of the combined consequence of the new flow's magnetisation and the topographic effect. Utilising the inversion technique, the young lava thickness, as opposed to the old terrain, was calculated based on an estimate of its magnetisation. [PERSON] et al. [72] proposed a real-time adaptive algorithm aiming to selectively measure a thermal outflow plume from the Calvert Cliffs Nuclear Power Plant (CCNPP) in Lusby, MC, USA. The nuclear plant discharges the cooling water at a maximum temperature that is 5.6 \({}^{\circ}\)C greater than ambient at a maximum volumetric flow of 156 m\({}^{3}\)/s. This horizontally discharged jet eventually fades away and becomes a buoyant plume. They developed a plume indicator function in order to discriminate the plume from the ambient. The function worked on a binary decision with a threshold, indicating whether the sample is inside or outside the plume. The measurable parameters as an indicator were temperature, salinity, dissolved oxygen and flow magnitude. They also proposed an adaptive transect mapping algorithm, which adaptively determined three features based on real-time measurements: direction of the vehicle transect, the length of transect and the distance to the next transect. Table 13 explains each component of the algorithm. The boundary tracking algorithm was additionally introduced by simply modifying the length of the transect. Each transect was commanded to pass the plume boundary twice instead of passing the entire plume body. The largest scale offshore oil spill in history followed the Deepwater Horizon blowout. The complexity of plume formation in deep water due to the interplay between oil and gas in a multi-phased flow made it difficult to model and predict. [PERSON] et al. [97] deployed the Sentry AUV of the National Deep Submergence Facility equipped with a mass spectrometer that was able to measure 3500 samples and 10 independent chemical parameters. The spatial extent of the plume was unknown thereby, it had to be defined through mapping. While the initial survey was conducted at three discrete depths (1000, 1150 and 1300 m), the AUV detected elevated hydrocarbon peaks while performing a zig-zag survey at a depth of 1120 m. During the course of dynamic re-tasking over several deployments, the AUV mapped the three-dimensional natural gas (methane) plume. In addition, the Lagrangian transporting trend toward the southwest was also captured while mapping until it reached the edge of the plume located at a distance of 35 km from the source.