\section{Repeat Mapping}\label{repeat-mapping}

Repeat mapping is a technique in which highly resolved surveys are
collected, and then repeated on a later deployment, in order to observe
measurable change to the area of interest. Many geological and
biological processes of interest (e.g.~fault creep, sedimentation, scour
by low intensity currents, growth or movement of benthic animals) can
produce seafloor changes on the order of 1 cm/year. The ability to
conduct repeated mapping was a primary reason why this system was
developed, and those changes have driven design requirements. At the 1-m
scale, this technique has been employed using data from the Dorado class
AUV in the past for mapping large scale lava flows from Axial volcano
\cite{caressAxial}. One example will be presented here to illustrate the
usefulness of extending this analysis into the scale collected by this
low altitude survey system.

The Monterey Canyon contained within the Monterey Bay National Marine
Sanctuary is a highly dynamic system of sediment transport. The active
canyon is comprised of features that can be observed though these
imaging efforts. Using this payload, it is possible to determine a wide
array of geomorphic change. This site is of particular interest to MBARI
scientists. A multi-year operation, known as the \emph{Coordinated
Canyon Experiment} \cite{CCEwebsite} has emplaced a network of sensors
in the Monterey Canyon targeted at detecting benthic events associated
with canyon dynamics studies. On January 15th, 2016, an event was
detected by that network of sensors.

The imaging payload has conducted surveys on this area before and after
the event. Figures \ref{fig:novtopo} and \ref{fig:maytopo} represent two
5-cm resolution topography maps collected using the 400-kHz multibeam
sonar before and after the incident. The benthic event in this area
involved sediment transport entering the survey area from the upper
canyon wall to the east and creating clear changes in the seafloor. Most
notable changes are in the southern regions where there are new
depressions. Overall, more texture appears to be present in November's
survey when compared to May.

\begin{figure}[htbp]
\centering
\includegraphics[width=0.45000\textwidth]{./img/CCEBIN_20151116_Topo5cm_ShadeB.jpg}
\caption{Multibeam sonar 5cm resolution map of the 1850m Bin Site,
collected in November 2015. \label{fig:novtopo}}
\end{figure}

\begin{figure}[htbp]
\centering
\includegraphics[width=0.45000\textwidth]{./img/CCEBIN_20160509_Topo5cm_ShadeB.jpg}
\caption{Multibeam sonar 5cm resolution map of the 1850m Bin Site,
collected in May 2016. \label{fig:maytopo}}
\end{figure}

\begin{figure}[htbp]
\centering
\includegraphics[width=0.45000\textwidth]{./img/CCEBIN_20160509m20151116_TopoDiff5cm_NoShadeB.jpg}
\caption{Observed change between data shown in Figures \ref{fig:maytopo}
and \ref{fig:novtopo} \label{fig:topodiff}}
\end{figure}

Since these measurements are all made by self-consistent and highly
accurate systems, we are able to quantitatively compare the two surveys.
Figure \ref{fig:topodiff} depicts the measured difference in topography.
Total change measures up to nearly 1.2-m of deposition, and nearly 0.9-m
of erosion in some areas.

These types of data are vital to understand the total change over this
100-m by 100-m area. This location was chosen for detailed mapping
because there was a cluster of monitoring instruments deployed nearby.
Thus the maps and direct physical measurements can ultimately be
compared.
