Page 271 - Geochemical Remote Sensing of The Sub-Surface

244 H. Yang, F.D. Van der Meer and J. Zhang

Stress symptoms such as leaf chlorosis are also observed. Cwick et al. (1995) searched
for these biogeochemical effects using from an altitude of 1667 m, an approximate
spatial resolution of 2 m and with narrow bandwidth filters in the spectral ranges of
yellow-green (0.543-0.552 ~tm), red (0.656-0.664 ~tm) and near-infrared (0.815-0.827
~tm). Although single-band video data were generally not sensitive to biochemical
variations, transformed video data, in particular those in the near-infrared/red, exhibited
significant correlations with biogeochemical Mn concentrations and Mn / Fe ratios.
These results suggest that multispectral video data may have the potential to detect
vegetation stress associated with hydrocarbon microseepage.

Other anomalies

Everett and Petzel (1973) reported that some oil and gas fields in the Anadarko Basin
of the Texas Panhandle and western Oklahoma are manifested on imagery data as unique
features, appearing to be smudged or erased, which they term "hazy anomalies". Moore
and Anderson (1985) found a circular tonal anomaly on a Landsat TM image of the Herg
oil field of the western Hardeman Basin, north Texas, which did not correlate with any
single soil type or groups of soils, and suggest that it is possibly related to vertical
migration of hydrocarbons causing chemically-altered soils or anomalous vegetation
growth. Carter and Koger (1988) processed MSS and Landsat TM data from an area
with hydrocarbon prospects. They suggest that, in the near-surface and surface rocks,
soil and vegetation, structure and alteration (possibly due to hydrocarbon microseepage)
can be detected by tonal and spectral anomalies in various filtered, contrast-stretched and
edge-enhanced formats, in ratios of various bands in both colour and black-and-white,
and in false colour composites.

PROBLEMS AND FUTURE TRENDS

Oil and gas exploration methods based on what are assumed to be hydrocarbon-
induced alterations to rock and soil date back several decades. However, the processes
that produce the observed effects are not well documented. Hydrocarbons are but one of
the possible causes of the alterations and may not always be the most probable cause.
The nature and extent of the alteration can vary significantly not only laterally and
vertically but also temporally. Schumacher (1996) concludes that considerable research
is needed before we understand the many factors affecting the formation of theses
alterations in the near surface. We must evaluate seemingly "significant" alteration
anomalies carefully to determine if they are related to hydrocarbon seepage. This
requires answers to the following questions. Is the anomaly a function of geology or an
artifact of culture'? If geology, is the observed alteration syngenetic or authigenic? If
authigenic, is the anomaly seep-related or of non-seep origin'? If seep-related, does the