Hi All -
Can someone please answer this question? There must be analogies in
the atmospheric community, but I don't know of anyone putting this kind
of ocean data into CF.
What is the best way to provide the information needed to extract the
geophysical quantity sound speed from the measured quantity of signal
travel time?
It has been proposed to be done with cell_methods='Z:sum', or with
a coordinate reference frame that includes the orientation of the
instrument.
Is there a 'best practice' for this?
Thanks - Nan
On 6/7/13 3:57 PM, Nan Galbraith wrote:
Hi Matthias and all -
Are cell methods the right way to document this? The 'sum' cell method
indicates
that you've summed a number of measurements, and I don't think that's
the case
here.
I'd have thought that providing the the instrument depth and
orientation (upward)
would make it more clear.
This isn't a feature I use routinely, so I could easily be missing
something.
Cheers - Nan
On 5/30/13 6:12 PM, Matthias Lankhorst wrote:
Hi,
thanks already for these comments. Roy's suggested name sounds pretty
to my
ears.
Trying to explain my "cell_methods" thing:
The instrument is sitting at a fixed spot on the seafloor, so unlike
the echo
sounder on a ship, the distance does not change (well, there are
tides, but we
filter them out). The remaining signal variance is variability in
environmental sound speed, which is mostly a measure of sea water
temperature.
The data, although measured by an instrument at one spot, are
dependent on the
vertical distance that the acoustic signal travels, i.e. represent
some space
other than a single point. Chapter 7 of the CF document that I found
online
explains it this way: "When gridded data does not represent the point
values
of a field but instead represents some characteristic of the field
within
cells of finite "volume," a complete description of the variable should
include metadata that describes the domain or extent of each cell..."
In my example, let us assume my IES sits at 4500m depth looking up. The
acoustic signal travel time (roundtrip) will be about 6 seconds
(sound speed
is ca. 1500 m/s). My data will be numbers that are closer to 5.9
seconds if it
is warm (faster sound speed), and more like 6.1 seconds if it is cold.
If my instrument were instead sitting in the same body of water at
3000m depth
(let's assume there is a mountain nearby), all of my numbers would be
something close to 4 seconds. Now... I don't want the user to think I
am still
in 4500m depth in outrageously hot water!
Bottom line: I need to tell the user what depth range I am covering
(0-4500 or
0-3000), and in my limited understanding of the situation this is
done via the
cell_methods and cell_bounds attributes.
Best wishes, Matthias
On Thursday, May 30, 2013 09:49:29 am Lowry, Roy K. wrote:
Hi John,
Not exactly. The travel time in both water column echosounding and
seismics is a proxy for distance and therefore provides information
on the
vertical distribution of returned signal intensity.
Cheers, Roy.
________________________________________
From: John Graybeal [[email protected]]
Sent: 30 May 2013 15:22
To: Lowry, Roy K.
Cc: [email protected]; [email protected]
Subject: Re: [CF-metadata] standard_name for acoustic travel time
from echo
sounder
+1 for Roy's choice.
Can you explain the following for the acoustically naive? "I assume the
data would need some additional description to denote the vertical
extent
of the measurement, such as cell_bounds and cell_methods='Z:sum'."
John
On May 30, 2013, at 06:45, "Lowry, Roy K." <[email protected]> wrote:
Dear All,
Of Matthias's suggestions I have a strong preference for a slight
extension of roundtrip_acoustic_travel_time_in_sea_water, namely
acoustic_signal_roundtrip_travel_time_in_sea_water. 'two-way' is a
possible alternative to 'roundtrip' but I think the former carries
unfortunate seismic semantic implications, so 'roundtrip' is better
for
IES. Including 'in_sea_water' is also essential to clearly
distinguish
it from seismic data.
Cheers, Roy.
________________________________________
From: CF-metadata [[email protected]] On Behalf Of
Matthias Lankhorst [[email protected]] Sent: 30 May 2013 13:16
To: [email protected]
Subject: [CF-metadata] standard_name for acoustic travel time from
echo
sounder
Dear CF,
I have oceanographic data from IES instruments (inverted echo sounder)
that I would like to publish via OceanSITES in a CF-compliant form.
The
data in question are acoustic travel times from the echo sounding
device. This means the time it took for the acoustic signal to run
from
the instrument (which sits on the seafloor) to the sea surface and
back
to the instrument. These data are commonly used as a proxy for ocean
heat content.
I don't think there is a suitable CF standard_name out there, and
ask for
your help in finding/creating one. Which of the following sound good?
acoustic_travel_time
vertical_acoustic_travel_time
roundtrip_acoustic_travel_time_in_sea_water
echo_sounder_acoustic_travel_time
...I could think of a couple more combinations using the words
above, but
would like to hear other people's opinions.
The canonical units would obviously be seconds.
I assume the data would need some additional description to denote the
vertical extent of the measurement, such as cell_bounds and
cell_methods='Z:sum'.
Any comments?
Kind regards, Matthias
--
*******************************************************
* Nan Galbraith Information Systems Specialist *
* Upper Ocean Processes Group Mail Stop 29 *
* Woods Hole Oceanographic Institution *
* Woods Hole, MA 02543 (508) 289-2444 *
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