In May 1988 Hydrosearch commenced work on the ‘SIGNATURE’ project which concentrated on the usage of a computer workstation for the detection of shallow gas at proposed drilling locations.
Phase I of the project, which concentrated on analysis of post-stack data, was part-funded by the Offshore Supplies Office of the UK Department of Energy with the remaining funding being provided from the Hydrosearch R & D budget.
Phase II of the development programme involved further work on stacked data together with some analysis of pre-stack data. This phase of the project was supported by two major international oil companies.
Some key aspects of the work undertaken during Phase I and II of the ‘SIGNATURE’ programme are summarised in the following figures and commentary
The authors regret that time constraints have prevented a full manuscript from being prepared.
(Fig. 1 is available in full paper) The overall objectives of the ‘SIGNATURE’ development programme are shown in Figure 1.
The diagnostic procedures include analysis of both pre- and post-stack data and require a standard methodology to be followed and optimum display parameters (scales, colour palettes, etc) to be utilised.
The objective has been to develop an approach which can provide a detailed analysis of up to four lines from a shallow gas survey grid within 72 hours of receipt of the SEG Y stacked data (Fig. 2 is available in full paper) Before describing the ‘SIGNATURE’ developments to date it is worth looking at the relative stages of development of the key components of shallow gas survey technology.
This slide shows quite clearly that the key acquisition components are at a relatively mature stage in their development although there is still considerable scope for new methods of application (e g. ?engineering? 3D). It is in the areas of Computer-Aided (CA) Interpretation and Data Analysis that development effort now needs to be concentrated if shallow gas detection is to Improve. The enabling mechanism for this is the use of workstations for interpretation and analysis of gas hazard data.
It is interesting to note that the engineering geophysicist is lagging behind his exploration colleague by about five years in the routine use of such tools.
(Fig. 3 is available in full paper) Phase I of the ?SIGNATURE? development programme concentrated on the analysis of stacked data. The primary diagnostic criteria used to establish whether or not an anomaly is caused by the presence of gas are shown in Figure 3.
On the basis of the use of these criteria three main conclusions were drawn from the work undertaken during Phase I
All the diagnostics must be tested for every data set and although different confidence weighting can be placed on the various criteria each one can contribute to the overall conclusions. The key issue is "use all the available information".
There are limitations in the level of discrimination which can be achieved by post-stack analysis eg. gas sand vs lignite.
If data are to be analysed on a workstation, acquisition and processing quality control must be rigorous.
