ABSTRACT

Distortion of ocean-bottom cores, as evidenced by thin smear zones along the sides or by the disturbed regions often disclosed by X-radiography, are well known. Evidence was obtained on one mode of core disturbance that cast further doubt upon the utility of the customary surface-operated piston corers for acquiring accurate data on sea-bottom sediment mass physical properties. Inadvertent rupture of a plastic bag of 3 mm diameter lead pellets, part of the equipment used to photograph a piston corer in action, resulted in pellet drag to nearly one meter along the side of one core without significant lithologic drag effects showing up on the X-radiograph of the whole core. This discovery suggests a mode of surficial coarse particle distortion which may be more common than heretofore re lized. Consequently, care should be exercised when these types of cores are used for studying the mass physical properties of sea-floor sediments especially when textural and mineralogical parameters are correlated with acoustic properties, and for sound velocity measured normal to the core.

X@INTRODUCTION

Much of the knowledge of the physical properties of marine sediments as related to acoustic effects is based on measurements of various properties of bottom core samples (for example: Buchan et al., 1967; Hamilton, 1956; Hamilton et al., 1956; Shumway, 1960). The accuracy of such measurements is dependent upon the degree of the undisturbed nature of the samples, and therefore various investigations have been conducted to determine the nature of possible distortions of core samples (Kallstenius, 1958; Emery and Hulsemann, 1964; Richards and Parker, 1967; Ross and Riedel, 1967; Bouma and Boerma, 1968). For example, drag (and subsequent smearing downward along the inner surface of the core barrel) resulting from wall friction has been suggested as a factor contributing to c re shortening.

A project to obtain photographic data on the piston coring process with the Cable-Controlled Underwater Research Vehicle (CURV) (Heller, 1966) was begun in 1968. Sea tests during December 1968 obtained these data as well as a number of core samples aboard the USNS Charles H. Davis (AGOR-5) in Santa Monica Bay, California, using the experimental design shown in Figure 1.

DISCUSSION

During the course of photographing the coring process, evidence was inadvertently obtained on one mode of core disturbance that tends to question the utility of piston cores for certain precise sediment property measurements. As indicated in Figure 1, plastic sacks of small lead pellets (approximately 3 mm diameter) were hung as weight around a colorbanded shield that was used to enhance recognition and visibility. During one of the coring

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drops, a sack was punctured and the pellets were carried down into the sediment by the corer. This occurrence was discovered when one of the unsplit cores (in a CAB liner) was examined by X-radiography, before sectioning, to determine lithological characteristics and to disclose any evidence of core disturbance resulting from the coring process. The X-rays were taken in three orthogonal orientations in a plane normal to the core axis, as shown in Figures 2 (a), (b), and (c). Radiographic and sediment analyses show that the sediment is highly stratified with large numbers of slightly dipping, well-defined layers of alternately clayey, sandy silt and sandy silt to silty sand. The dip of the bedding is considered approximately equivalent to that of the in situ sediment because of the app rent vertical penetration of the core barrel revealed by photography. Below approximately 15 cm, the sediment is highly stratified and compacted, and it may be seen that the core catcher was firmly imbedded.

Although the extreme edges of the core are lost in Figure 2, relatively straight zones of multiple dark specks (probably mica and possibly other ferromagnesian minerals in the more clayey zones), and only a slight downward curvature of the lithologic units suggests that a

FIG. 1. Corer-photography system, (a) Piston corer-identification shield system. Note two of three bags which contain lead shot hanging at points outside the identification shield, (b) Mooring arrangement for acquiring photographic data of a piston corer in action.

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FIG. 2. [Grey Scale] Radiograph of piston core in three orientations. (a) 0°, (b) 90°, (c) 180°. X-rays taken with Picker X-Ray Corporation Model 6000 equipment at 36 inches using 72 kv, 8 ma, and 2-min exposure on Kodak AA-2 industrial X-ray film.

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marked drag effect is not present. Core surface examination revealed only a thin surficial smear of the sea-bottom material. It is interesting that spherical objects such as the lead pellets would be carried so far without more apparent drag effects showing on the lithology. The pellets are seen to have been dragged along the side of the entire length of the core. The position of the pellets was verified by examining the core and was not inferred from the X-ray photography alone.

Some of the characteristics of the Navy Oceanographic Office modified Ewing piston corer are shown in Table 1. This corer was not equipped with a piston immobilizer, nor were the clearance and area ratios optimum in the sense of drive samplers (without pistons) as defined by Hvorslev (1949). The poor area ratio characteristic of the corer may have been partly responsible for the observed drag effect. However, it is believed that the degree of sediment particle drag phenomenon disclosed by the radiographs may be more common and of a magnitude not previously recognized for some types of samplers. Distortion of the type illustrated here may occur with piston corers when coarse grain particles (e.g., manganese or phosphorite grains or pebbles) are present in the cored section, and if such coarse particles are found along the outer edge of the core, they cannot be assumed to be in place. Moreover, this type of distortion could seriously affect the measurement for in situ mass physical properties when taken from such disturbed zones.

Since sediment particles are normally of irregular shape, it is not obvious how their drag will differ from that of spherical pellets of the same mean diameters. Nevertheless, if there were sediment particles of comparable dimensions on the surface, it would seem reasonable to conclude that data on grain size distribution as a function of depth could be misleading.

Sediment analysis and X-radiography may reveal particle aggregates in improper lithological sequence, but this cannot always be assured. X-radiographs taken with different exposures of some phosphorite pebbles (of sizes comparable to the lead pellets) extracted from the core, showed varying degrees of translucency to opaqueness. Because of the contrasting densities and exposures, the pebbles were readily visible in some cases while in others they were lost. Thus, some out-of-place sediment particles within the core could be masked, and wall-drag disturbance of this type may not be detected by X-rays on a whole core (taken through the liner).

Although unreliable data resulting from drag distortion along the outer part of the core may be avoided by trimming the surface of the core to a reasonable depth, under some circumstances and in very soft, unconsolidated sediment it may not be known to what depth into the core the drag has effected a change in the physical properties of the sediment. X-ray analysis may or may not be of value in this case. Measurements of acoustic properties such as compressional wave velocity are taken with the core (or core segment) still encased in the liner; hence, the degree to which surficial distortion may result in unreliable data is not known but may be significant, when affected by marked changes in such properties as mean grain size and porosity. It is acknowledged that skill in coring and i laboratory procedures and care in core handling, aside from corer design, are very important to the reliability of the core analyses data. However, it is believed that the type of core disturbance discussed above may be unavoidable regardless of the degree of operator skill, and that the core study objective should determine the degree of consideration for core disturbance.

TABLE 1. Characteristics of U. S. Naval Oceanographic Office Modified Ewing Piston Corer

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CONCLUSIONS

In addition to the well known very thin smear zone that contaminates the outside of cores, the core analyst must be alert to the possibility of coarse particles which are displaced and have caused distortion at the outer part of the core to a depth dependent upon their size and the properties of the sediment.

Measurement of sediment mass physical properties particularly those that are to be correlated with other factors such as sound velocity, and sound velocity itself, should be taken at the central (relatively undisturbed) portion of the core.

When sound velocity is measured through the liner, the effects of possible disturbance, especially along the side of the core, should be considered and evaluated.