INTEGRATIVE REMOTE SENSING TECHNIQUES TO DETECT COASTAL FRESH-WATER SEEPS

The semi-arid countries of the Arabian Peninsula are known for the prevalence of fresh water seeps in their coastal waters. Recent findings indicate that these seeps are detectable using a combination of remote sensing data. Examples are presented using two prominent plumes observed in TM and ASTER thermal data. These data were collected after sunrise in the diurnal heating-cooling cycle.

The thermal plumes are located in the Gulf of Oman. The first is called the Tiwi plume and extends from 22˚50'N, 59˚15'E for 68km. It was originally observed in a TM image dated from 14 April, 1992. The second, called the Dibba plume, extends from 26˚11'N, 56˚15E for 30km. It was observed in an ASTER image from 2 March, 2003. In both cases, the plumes project from inland fracture zones, mapped from optical images, implying the faults acted as conduits to channel rain water from the surrounding mountains before expelling it into the sea.

To test the validity of this deduction, Tropical Rainfall Mapping Mission (TRMM) data, available from January 1998, were retrieved. These data show a large rainfall event occurred 4 days prior to the emergence of the Dibba plume. Other ASTER and Landsat scenes available in the Dibba study area were then processed. Of the 2 scenes that contained a plume, rainfall events were observed less than 15 days prior to image acquisition. The 2 scenes that did not contain a plume show no rainfall event up to 49 days prior to image acquisition. Thus, the plumes appear to be rainfall related events.

The relative temperatures of the Dibba and Tiwi plumes compared with the surrounding sea water are different. To confirm that this is a real observation, absolute temperature data were retrieved for these plumes that were collected by different sensors (MODIS, AVHRR, ASTER and Landsat) on different dates. The direction of the temperature anomaly is seen to persist. The Dibba plume is consistently 1 to 1.5˚C colder and the Tiwi plume 1 to 1.5˚C warmer. One possible explanation for the directional change is that the warming effect at Tiwi is a consequence of deeper and longer groundwater transit times as is the case for hot springs. This hypothesis is being tested using freely available MODIS, AVHRR and TRMM data covering these areas. If plumes are present, their extent will subsequently be refined using higher spatial resolution ASTER and Landsat data.