# AUV Adaptive Sampling Methods: A Review
## 3 Target of Interest
### Chemical Features
#### Hydrothermal Vent  
A hydrothermal vent plume is generally characterised as a neutrally-buoyant plume [55] since it continues to rise through the water column until the plume is denser than the surrounding seawater. It features physical and chemical signatures: temperature anomalies, cloudy water by solidified minerals, low redox potential due to low oxygen level and common chemical elements such as methane, manganese, iron sulphide, barium, calcium and silicon. In general, the plume is detected first by chemical anomalies even at significant distances from the vent sites. Then, the emitting sources are discovered by following its physiochemical signature. [PERSON] [54] and [PERSON] [56] listed useful sensors to detect a hydrothermal vent plume, as shown in Table 7.  
[PERSON] et al. [57] put an emphasis on the utility of a less precisely defined survey approach. They used the Autonomous Benthic Explorer (ABE) AUV to search for redox potential (Eh) for the activity of electrons to identify the nascent, hence buoyant, hydrothermal plume. The AUV was programmed to initially follow a lawn-mower trajectory. Then, every time it detected Eh anomalies, it intensively surveyed the spot by performing a pre-designed spiral reconnaissance. This deterministic manoeuvring design was referred to as a triggered spiral prospecting (TSP) strategy. Although the TSP method implements an adaptive behaviour, the robustness in the mechanism, relying on the triggering thresholds, might decrease when surveying the area of high-intensity anomalies.  
[PERSON] [54] introduced a less exhaustive search method in comparison with the moving-the-lawn (MTL) approach, the chemotaxis-based algorithm, a concept which was first reported in 1881 [58]. It was developed based on the behaviour pattern found in the male silkworm moth [59]. The method has three action guides: surge (heading directly toward the upstream direction), spiral search (with increasing radius) and redirection (toward the centre of the area). Beginning with an MTL path initially, the vehicle performs surging for six time steps once it contacts a plume. Then, it continues to the next spiral search step. When a new vent is detected, the behaviour is repeated from the beginning. To exit  
\begin{table}
\begin{tabular}{c c c} \hline
**Tracer** & **Sensor** & **Principle** \\ \hline Potential temperature and salinity anomalies & Conductivity–Temperature–Depth (CTD) sensor & Identify the potential temperature and salinity in the vicinity of the vent. \\  & & Estimate the particle (preipitated mineral \\ Particle contents & Light scattering sensor/nephelometer & that results in cloudy water) concentration \\  & & using optical backscattering. \\ Chemical tracer: methane, manganese, iron and others & System Used to Assess Vented & Measure the enrichment of chemicals in \\  & Emissions (SUAVE)/Zero Angle Photon & hydrothermal fluid in comparison with \\  & Spectrophotometer (ZAPS) & non-hydrothermal sea water \\ Redox potential (Eh) & Oxidation–Reduction Potential (ORP) & Measure the chemical reactivity of the \\  & & water. Hydrothermal water exhibits low Eh \\  & sensor & due to low oxygen and observable only in \\  & & the vicinity of the vent \\ \hline \end{tabular}
\end{table}
Table 7: Physical and chemical signatures (tracers) and their corresponding sensors.  
the survey area at any point in time, the vehicle heads toward the centre of the area through a linear path at a randomly selected angle.