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 [graybeal at marinemetadata.org]
>>> Sent: 30 May 2013 15:22
>>> To: Lowry, Roy K.
>>> Cc: mlankhorst at ucsd.edu; cf-metadata at cgd.ucar.edu
>>> 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." <rkl at bodc.ac.uk> 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 [cf-metadata-bounces at cgd.ucar.edu] On Behalf Of
>>>> Matthias Lankhorst [mlankhorst at ucsd.edu] Sent: 30 May 2013 13:16
>>>> To: cf-metadata at cgd.ucar.edu
>>>> 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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Received on Tue Oct 08 2013 - 09:20:18 BST
