Author: Dan Kelley (https://orcid.org/0000-0001-7808-5911)
Date: 2024-08-17

# 3. Analysis of acoustic-Doppler data

#### Dan Kelley (https://orcid.org/0000-0001-7808-5911)

#### 2024-08-17

Abstract. This vignette explains the basics of using the oce package to handle measurements made by both mooring and ship based acoustic Doppler profilers. As in the related vignette about working with data-quality flags, the material is organized to reflect typical workflow. See the main oce vignette for general information about oce. The sample code provided here relies on the oce package, loaded as follows.

    library(oce)

# 1 Reading adp files

Acoustic Doppler profiling instruments are called ADCPs by some manufacturers and ADPs by others. The class name `adp` is used in oce, to handle both types. It is important to realize that there are many data formats for data of this type, and that the files are normally in a dense binary format that cannot be read with a text editor. It is necessary to use software to work with such files. Although oce can handle many file formats, and although its authors do their best to keep up with changes to file formats (at least for popular instruments), users will sometimes be forced to use manufacturer software to read the data, or at least convert to a format that can be manually read into R, and then converted to an `adp` object using the `as.adp()` function.

Suppose `f` is a character string naming a data file. In many cases,

    d <- read.oce(f)

will read the file, because `read.oce` will look inside the file to try to guess the format. If this does not work, one ought to try e.g.

    d <- read.adp(f)

which, again, will try to determine which subtype of adp file has been provided. If that fails, it might help to try a specialized function, such as `read.adp.rdi()` for an RDI file, `read.adp.nortek()` for a Nortek file, or `read.adp.sontek()` for a Sontek file. (Actually, each of these can handle a variety of sub-formats).

If any of these `read.` functions is called with just a single argument naming the file, then the entire dataset will be read in. This can be slow for large files (e.g. taking of order 0.1s per Mb), and so it can help to use the `by` argument, e.g. for a 138Mb file on the author’s computer,

    f <- "/data/archive/sleiwex/2008/moorings/m09/adp/rdi_2615/raw/adp_rdi_2615.000"
    dall <- read.oce(f)

takes 15.2s of user time, but

    d100 <- read.oce(f, by = 100)

takes 0.2s. It is also possible to window the data, using the `from` and `to` arguments, which (like `by`) can be integers that count ensembles (or “profiles”) or a POSIX time (see `?read.adp`). For example, the adp dataset provided by oce was read with

    read.oce(f,
        from = as.POSIXct("2008-06-26", tz = "UTC"),
        to = as.POSIXct("2008-06-27", tz = "UTC"),
        by = "60:00",
        latitude = 47.88126, longitude = -69.73433
    )

with `f` as defined above. This also illustrates that it is possibly to specify the location of an instrument; the further processing that was done to create `data(adp)` will be explained in due course.

+Note: Users who are not familiar with ADP data may noticed that there is a combination of many file types when dealing with ADP data. Some of these include: .ENR, which are raw data only received from the ADP, .ENS, similar to .ENR but also contain data merged from other file types without processing, .ENX, merged data that are processed including calculations to transformed data including east, north, and vertical velocities, .STA, which are a short term average of measurements, and .LTA, which are a long term average of measurements.

# 2 Summarizing adp data

The generic function `summary` provides a useful overview of an adp data object. For example,

    data(adp)
    summary(adp)

(results not shown the goal is to encourage the user to try this!) shows the sort of information normally present in an RDI file. Other manufacturer types will have somewhat different summaries, because both the metadata and the data differ from instrument to instrument. In all cases, there will be a vector holding time (named `time`), another holding the distance from instrument to the centre of a measurement bin, measured along a line equidistant between the beams (named `distance`), along with an array holding velocity (named `v`). (Other arrays may include, as in this RDI case, `q` for data quality, `a` for backscatter amplitude, and `g` for percentage goodness.)

Note that the summary reveals that instrument had been set up to record in `beam` coordinates, but that processing had been done to convert to `enu` (east-north-up) coordinates.

Exercise 1. Determine the name of the `metadata` item that holds the coordinate system of an adp object.

After completing Exercise 1, the user will notice that both `latitude` and `longitude` are available in the metadata for this built-in, moored, adp dataset. It should be noted that if this dataset was a ship-based adp dataset, then `latitude` and `longitude` would not be included. Instead, the data would include `firstLatitude`, `firstLongitude`, `lastLatitude`, and `lastLongitude`, which provide an indication of the ship drift during sampling.

# 3 Changing coordinate systems

It is critical for analysts to be aware of the coordinate system of an adp dataset, simply because those systems are so different. In what oce calls the `beam` system, velocities in each of the acoustic beams are measured along the slant angle of the beam. These data may be combined (using a transformation matrix which is part of the metadata displayed with the `summary()` function) into what oce calls a `xyz` system that is Cartesian and fixed to the instrument. Since instruments record heading, pitch, and roll, the `xyz` velocities can be converted easily to `enu` velocities.

In some applications, the scientific focus is primarily on the `enu` data, but it is a good idea to calculate and study the data at all stages of processing. Thus, although the `toEnu()` function will convert data measured in any coordinate system into an `enu` variant, many analysts will do something along the lines of

    beam <- read.oce(f)
    xyx <- beamToXyz(beam)
    enu <- xyzToEnu(xyz, declination = -18.1)

where the declination illustrated is the value used in creating the `data(adp)` dataset. (Compensating for declination is necessary because compasses are calibrated at a particular place and time, and compasses have no way of “knowing” the local angle between true north and magnetic north.)

# 4 Plotting

The generic `plot()` function is specialized to handle adp objects in a variety of simple but helpful ways. The type of plot is determined by the `which` argument. The default for `which` is to create a multi-panel image plot that shows time-space dependence for each velocity component. The number of panels depends on the number of beams used by the instrument, and each one has a label indicating the component name. For example,

    plot(adp)

has velocity panels named `east`, `north` and `up`, along with one named `error`, which is an estimate of the velocity-inference error.

Options to this plot variant can be used to control how distance from instrument is shown (which can be useful in visualizing data from upward- and downward-aligned instruments), what colours are used to represent the signals, etc.

Dozens of other plot types are also provided; see `?"plot,adp-method"` for details – but be warned that a complex plot requires many arguments!

When dealing with ship-based adp data, in order to get the true `east`, `north`, and `up` velocity, the user must remove the speed of the ship. To do this, use the `subtractBottomVelocity()` function, which subtracts the bottom tracking velocities from an adp object.

Exercise 2. Create a u-v scatterplot.

Exercise 3. Write the steps to plot the `east`, `north`, and `up` velocity without the speed of the ship for a file named `COR2019002_20190818T064815_007_000000.ENS`.

# 5 Subsetting

Adp data can be subsetted in a wide variety of ways, e.g.

    plot(subset(adp, distance < 20))

plots only data within 20m of the instrument. See `?"subset,adp-method"` for more on subsetting.

Exercise 4. Plot the adp data for the first half of the sampling interval.

# 6 Accessing data within adp objects

All oce objects share a common framework, with so-called “slots” for metadata, data, and processing log. It is possible to examine, and modify, the elements of each of these slots using base R constructs, but it is usually better to use oce accessor functions. The details are provided with `?"[[,adp-method"` and `?"[[<-,adp-method"`, but a few examples should suffice to sketch the general pattern.

For example,

    time <- adp[["time"]]
    distance <- adp[["distance"]]

extracts the times and distances of the profiles within `data(adp)`, while the array holding the velocities is extract with

    v <- adp[["v"]]

Altering values uses the `[[` to the left of the assignment operator, but this is not commonly required in simple work, so it is best for readers to focus first on accessing data. The exercises may help with this.

Some of the fields contained in adp objects, particularly the amplitude, correlation, and percent good fields (`a`, `q`, and `g`) are stored as `raw`-type. This is done primarily to save memory space, as expanded all of those fields into `numeric` types can be slow and they get large quickly. However, to actually work with the numbers, users can extract a “numeric” version by supplying the second argument to the `[[` function, e.g.

    a <- adp[["a", "numeric"]]

This has the advantage of returning an object with the same matrix dimensions as the original (note that functions like `as.numeric()` typically discard dimension information, thereby converting an array to a vector).

Exercise 5. Plot a time series of mid-depth and depth-averaged eastward velocities.

Exercise 6. Perform an eigen-analysis of eastward and northward velocity.

Exercise 7. Plot a time series of pressure variations, to reveal tidal height.

# 7 Solutions to exercises

Exercise 1. Determine the name of the `metadata` item that holds the coordinate system of an adp object.

Solution.

With adp as loaded in the text, we may the names in the metadata with:

    sort(names(adp[["metadata"]]))

(results not shown), after which a keen eye will notice that two elements relate to coordinates:

    adp[["originalCoordinate"]]
    #> [1] "beam"
    adp[["oceCoordinate"]]
    #> [1] "enu"

and comparison with the output of `summary(adp)` reveals that the former is the coordinate system in which the data were originally measured, while the latter is the coordinate system of the transformed data. Using

    processingLogShow(adp)
    #> * Processing Log
    #>
    #>     - 2019-08-12 15:29:36 UTC: `read.oce("/data/archive/sleiwex/2008/moorings/m09/adp/rdi_2615/raw/adp_rdi_2615.000", ...)`
    #>     - 2019-08-12 15:29:36 UTC: `beamToXyzAdp(x = beam)`
    #>     - 2019-08-12 15:29:36 UTC: `xyzToEnuAdp(x, declination=-18.1, debug=0)`

reveals the steps in the data transformation.

Exercise 2. Create a u-v scatterplot.

Solution.

    plot(adp, which = "uv")

Exercise 3. Write the steps to plot the `east`, `north`, and `up` velocity without the speed of the ship for a file named `COR2019002_20190818T064815_007_000000.ENS`.

Solution.

    library(oce)
    adcp <- read.adp("COR2019002_20190818T064815_007_000000.ENS")
    enu <- toEnu(adcp)
    removeShipSpeed <- subtractBottomVelocity(enu)
    plot(removeShipSpeed, which = 1:3)

Exercise 4. Plot the adp data for the first half of the sampling interval.

Solution. (results not shown)

    plot(subset(adp, time < median(adp[["time"]])))

Exercise 5. Plot a time series of mid-depth and depth-averaged eastward velocities.

Solution.

    time <- adp[["time"]]
    v <- adp[["v"]]
    # The second index is for bin number, the third for beam number
    midIndex <- dim(v)[2] / 2
    eastMid <- v[, midIndex, 1] # third index is beam
    distance <- adp[["distance"]][midIndex]
    oce.plot.ts(time, eastMid, ylab = "Eastward velocity [m/s]")
    # Depth mean; note that na.rm, is passed by apply() to mean()
    eastMean <- apply(v[, , 1], 1, mean, na.rm = TRUE)
    lines(time, eastMean, col = 2)

Here, `oce.plot.ts()` is used instead of the basic R function `plot()`, because it provides a margin note on the detailed time interval.

Exercise 6. Perform an eigen-analysis of eastward and northward velocity.

Solution.

    u <- adp[["v"]][, , 1]
    v <- adp[["v"]][, , 2]
    ok <- is.finite(u) & is.finite(v) # remove NA values
    u <- u[ok]
    v <- v[ok]
    eigen(cov(data.frame(u, v)))
    #> eigen() decomposition
    #> $values
    #> [1] 0.48350161 0.01557049
    #>
    #> $vectors
    #>           [,1]       [,2]
    #> [1,] 0.5441538 -0.8389855
    #> [2,] 0.8389855  0.5441538

Note that this action is equivalent to principal component analysis, and it might be wiser to use

    pr <- prcomp(data.frame(u, v))

to do the analysis, which yields the same principal axes

    pr
    #> Standard deviations (1, .., p=2):
    #> [1] 0.6953428 0.1247818
    #>
    #> Rotation (n x k) = (2 x 2):
    #>         PC1        PC2
    #> u 0.5441538 -0.8389855
    #> v 0.8389855  0.5441538

but has the advantage of having associated methods, including for plotting.

Exercise 7. Plot a time series of pressure variations, to reveal tidal height.

Solution.

    time <- adp[["time"]]
    pressure <- adp[["pressure"]]
    oce.plot.ts(time, pressure)

As a matter of interest, a tidal analysis may be done with

    m <- tidem(as.sealevel(pressure, time))
    #> Note: the tidal record is too short to fit for constituents: SA, SSA, MSM, MM, MSF, MF, ALP1, 2Q1, SIG1, Q1, RHO1, O1, TAU1, BET1, NO1, CHI1, PI1, P1, S1, PSI1, PHI1, THE1, J1, SO1, OO1, UPS1, OQ2, EPS2, 2N2, MU2, N2, NU2, GAM2, H1, H2, MKS2, LDA2, L2, T2, S2, R2, K2, MSN2, ETA2, MO3, M3, SO3, MK3, SK3, MN4, M4, SN4, MS4, MK4, S4, SK4, 2SK5, 2MN6, M6, 2MS6, 2MK6, 2SM6, MSK6, M8

where the list of undetermined tidal constituents is long, because this tidal record is so short (see `?tidem`). The fitted constituents are as follows

    summary(m)
    #> tidem summary
    #> -------------
    #>
    #> Call:
    #> tidem(t = as.sealevel(pressure, time))
    #> RMS misfit to data:  0.06615227
    #>
    #> Fitted Model:
    #>         Freq Amplitude Phase       p
    #> Z0    0.0000   39.0465   0.0 < 2e-16 ***
    #> K1    0.0418    0.0689 334.9   0.089 .
    #> M2    0.0805    1.1730 203.8 4.6e-16 ***
    #> 2MK5  0.2028    0.0312 112.8   0.417
    #> 3MK7  0.2833    0.0265  87.8   0.618
    #> ---
    #> Signif. codes:  0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1
    #> * Processing Log
    #>
    #>     - 2024-08-17 09:38:22 UTC: `create 'tidem' object`
    #>     - 2024-08-17 09:38:22 UTC: `tidem(t = as.sealevel(pressure, time))`

(Note that it fitted for M2, but not S2, because the Rayleigh criterion prevents inferring both, and the conventional tidal analysis favours M2 over S2 if the time series is too short to fit for more than one semi-diurnal constituent.)

A useful step might be to draw a tidal fit along with the data. It makes sense to construct a finer time grid, to get a smooth curve, as in the following (see `?predict.tidem`).

    oce.plot.ts(time, pressure, type = "p", col = "blue")
    timePredict <- seq(min(time), max(time), length.out = 200)
    pressurePredict <- predict(m, timePredict)
    lines(timePredict, pressurePredict, col = "red")