Deep-sea corehead camera photography and piston coring

Abstract

Cameras were mounted in a newly designed corehead of a piston corer and used to photograph coring operations during 36 stations on CHAIN cruise 75 and 28 stations on ATLANTIS II cruise 42. Through the analysis of these photographs, the deep-water operation of a piston corer during its descent, tripping, impact with the bottom, and ascent has been studied, providing information on the corer's stability, effectiveness in obtaining a bottom sample, and influence on the nearby sea-floor. Accurate determinations of the amount of penetration were possible, allowing comparisons to be made with the more indirect methods of determining penetration and with the length of core recovered. Sediment clouds produced by bottom currents were noticed in many of the bottom photographs. A number of suggestions are made for future piston coring operations. The corer descends with little rotation and swinging. Free-fall and penetration generally take place in less than 5 seconds, with a rotation of 20-60° and an increase of about 6° in vertical deviation. During penetration, the corer disturbs the surrounding sea floor, producing both mounds and depressions around the core barrels. While resting in the bottom, the corer is very stable although some wobbling does occur. Considerable rotation takes place during both pull-out and ascent; frequent sediment discharges from the piston corer occur. No consistent relationship was found between the amount of penetration and the length of core recovered, and thus with the degree of core shortening. Comparisons between piston and pilot cores indicate that the piston cores have been shortened and disturbed relative to the pilot cores, and that as much as a meter of the upper portion of the piston core has been lost. The position of the mud-mark appears to be a reliable indicator of the amount of penetration; estimates by extrapolation of the thermal gradient to the surface are less reliable. The vertical deviation of the corer in the bottom does not influence the amount of penetration. Stratigraphic dips in the recovered cores correspond poorly to this vertical deviation in the bottom.