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The Seismic Exploration of the Mahakam Delta, or "Nine Years Shooting in Rivers, Swamps and Very Shallow Offshore"
The MAHAKAM River delta, located on the East coast of KALIMANTAN is a portion of the MAHAKAM Contract Area jointly held by TOTAL INDONESIE and JAPEX INDONESIA, LTD under a Production Sharing Contract with PERTAMINA.
Regarding the choice of the seismic techniques to be used, the steep talus of the modern delta front sharply delineates the intertidal swampy and shallow water (5 m) studied area from the deeper waters (25 m) where conventional marine seismic method is applicable.
The first seismic reconnaissance of the delta was initiated in 1969 by shooting along the main rivers and the tidal channels where the water depths reach about 10 meters or more. Single fold or three fold CDP seismic sections were recorded using a towed floating cable and an explosive source. The recorded results were generally poor.
In 1971/72 the bay-cable technique was experimented in the shallow water outer delta flats. The source was a highly repetitive small airgun allowing "vertical stacking" and the CDP coverage was increased to twelve folds. The results showed an improvement though still quite variable, being dependent on surface shooting conditions.
In late 1972 another attempt was made to try to improve the seismic response in the rivers by using, where possible, the conventional marine high CDP techniques (powerful air-gun and streamer).
This attempt proved to be unsuccessful and the decision was taken to increase the field effort by using heavy land methods. This consisted in cutting straight lines through the Nipah and Mangrove swamp, ignoring the rivers, drilling deep holes for shooting, and recording as high CDP coverage as possible. Although very expensive, this effort resulted in a spectacular improvement of the section quality.
Since then about 1650 km of new seismic lines have been recorded using land or bay-cable techniques, and many experimentations were carried out in order to try to design the best appropriate shooting parameters.
This paper presents TOTAL INDONESIE's conclusions which result from this nine year research for interpretable seismic records in the severe surface conditions of the MAHAKAM River delta:
1. It is confirmed that a great field effort including a high CDP coverage and shot point patterns is profitable even though this effort leads to a very expensive cost and a low production rate.
2. Unfortunately it appears to be practically impossible to design constant shooting parameters, the same being successfully applicable all over the studied area. The seismic response, being closely dependent on the surface conditions including depositional patterns of the modern delta as well as tidal effects, is rapidly changing from place to place. This contingency limits the efficiency of any routine method and it is rather difficult to design different shooting parameters at each shot point!
3. It is regrettable that too often the efficiency of the subcontracted seismic crews is limited by the lack of versatile equipment correctly designed to be operative in variable conditions. This remark mainly applies to portable drilling equipment, seismic cables and shallow water vehicles.
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