Using xpath queries against XML Datatype

Hello,

I have a XML document stored in a PostgreSQL column defined using a XML datatype.  A fragment of this document is included below:

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<sml:SensorML xmlns:sml="http://www.opengis.net/sensorML/1.0.1"

xmlns:swe="http://www.opengis.net/swe" xmlns:gml="http://www.opengis.net/gml"

xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"

xmlns:xlink="http://www.w3.org/1999/xlink" version="1.0.1">

  <sml:member>

    <sml:System gml:id="CTD1454">

      <gml:description>M1 deployment of 10m CTD (Model SBE 37-SMP) with serial communication, internal memory and pump. The SBE 37-SMP MicroCAT is a high-accuracy conductivity and temperature (pressure optional) recorder with internal battery and memory. Designed for moorings or other long duration, fixed-site deployments, the MicroCAT includes a standard serial interface and nonvolatile FLASH memory. Construction is of titanium and other non-corroding materials to ensure long life with minimum maintenance, and depth capability is 7000 meters (23,000 feet).</gml:description>

      <sml:keywords>

        <sml:KeywordList

codeSpace="http://gcmd.nasa.gov/Resources/valids/keyword_list.html">

          <sml:keyword>MBARI</sml:keyword>

          <sml:keyword>OCEANOGRAPHY</sml:keyword>

....

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I would like to extract <gml:description> where gml:id="CTD1454".  In SQL Server

2008 I write the following query:

WITH XMLNAMESPACES ('http://www.opengis.net/sensorML/1.0.1' AS sml,

                              'http://www.opengis.net/swe' AS swe,

                              'http://www.opengis.net/gml' AS gml

                              )

SELECT

sensorML_xml_u.query('data(//sml:System[@gml:id="CTD1454"]/gml:description)')

FROM dbo.sensorML

How do I do this in PostgreSQL?  I'm struggling trying to find any documentation/examples.  The following does not appear to return anything, but I'm not sure if I have the syntax quite right:

select * from sensorML WHERE (xpath('//sml:System/@gml:id', "sensorML_xml", ARRAY[ARRAY['sml','http://www.opengis.net/sensorML/1.0.1'],ARRAY['gml','http://www.opengis.net/sensorML/1.0.1']]))::text

= 'CTD1454';

Thanks,

Robin