204
N. A . JlOLME
lwcw lost i l l t liis way. In it.s original form it, can only be worked under
very favourable conditions, ill calm weather, but an improved version
is mentioiicd on p. SOX.
The Knudsen sampler is possibly assisted in its netra ration by a
lessening of the cohesion of the sand, which would tend to be lifted
within the tube owing to thc tendency of water to be sucked upwards
through the sand. This raises the question as to what extent i t might
be possible to loosen thc substratum before sampling, making use
perhaps of its thixotropic properties (cp. Chapman, 1949) to assist
penetration. This would p d i a p s ticcessitate a vibrating coring tube,
and such 811 instrument is in fact described by Kudinov (1957). His
instrument consists of R iiarrow coring tube whose penetration is
assisted by a commc:rcial elcctrically-driven vibrator.
2. Retention of the sample
In the majority of instances there is no difficulty in raising a
sample of mud, as it readily adheres to the sampling instrument and
has sufficient cohesion not to fall out, of a narrow coring tube. The
cohesion of most other se.diments is so small, however, that the sample
may be lost during hauling.
Grabs. In the grab type of sampler in which two scoops close beneath the surface of the sediment there is a very real danger that a
stone or shell become wedged between the jaws so preventing complete
closure. Much of the sample may then be lost before reaching the
surface (Davis, 1925; Holme, 1949; Thorson, 1957s). This effect is
diminished when as in the van Veen grab (particularly when fitted
with endless warp rig) considerable leverage is exerted to close the
jaws. In some grabs (c.g. unmodified orange-peel grab, Baird's grab,
some Petersen grabs) the tops of the grab bucket8 are open and unprotected against washing out, but usually a cover of gauze or canvas
can be contrived. Losses from the top of the grab are not usually
as significant as those from imperfectly-closed jaws, and indeed an
amusing incident occurred with Baird's grab in which a large sole was
brought up still lying on the surface of the sand, rtlthough there was
no cover over the instrument.
An attempt to overcome the loss between grab jaws is shown in
Holme's (1 949) scoop-sampler in which a single semi-circular scoop
rotates through 180" to take a sample (Figs. 22, 40). The sample is
completely enclosed underneath, and there is a cover over the top of
the scoop to prevent washing out. A small loss of material which may
take place a t the sides could be minimized by fitting rubber flanges.
There is prohably less washing-out with this instrument than with
N. A . JlOLME
lwcw lost i l l t liis way. In it.s original form it, can only be worked under
very favourable conditions, ill calm weather, but an improved version
is mentioiicd on p. SOX.
The Knudsen sampler is possibly assisted in its netra ration by a
lessening of the cohesion of the sand, which would tend to be lifted
within the tube owing to thc tendency of water to be sucked upwards
through the sand. This raises the question as to what extent i t might
be possible to loosen thc substratum before sampling, making use
perhaps of its thixotropic properties (cp. Chapman, 1949) to assist
penetration. This would p d i a p s ticcessitate a vibrating coring tube,
and such 811 instrument is in fact described by Kudinov (1957). His
instrument consists of R iiarrow coring tube whose penetration is
assisted by a commc:rcial elcctrically-driven vibrator.
2. Retention of the sample
In the majority of instances there is no difficulty in raising a
sample of mud, as it readily adheres to the sampling instrument and
has sufficient cohesion not to fall out, of a narrow coring tube. The
cohesion of most other se.diments is so small, however, that the sample
may be lost during hauling.
Grabs. In the grab type of sampler in which two scoops close beneath the surface of the sediment there is a very real danger that a
stone or shell become wedged between the jaws so preventing complete
closure. Much of the sample may then be lost before reaching the
surface (Davis, 1925; Holme, 1949; Thorson, 1957s). This effect is
diminished when as in the van Veen grab (particularly when fitted
with endless warp rig) considerable leverage is exerted to close the
jaws. In some grabs (c.g. unmodified orange-peel grab, Baird's grab,
some Petersen grabs) the tops of the grab bucket8 are open and unprotected against washing out, but usually a cover of gauze or canvas
can be contrived. Losses from the top of the grab are not usually
as significant as those from imperfectly-closed jaws, and indeed an
amusing incident occurred with Baird's grab in which a large sole was
brought up still lying on the surface of the sand, rtlthough there was
no cover over the instrument.
An attempt to overcome the loss between grab jaws is shown in
Holme's (1 949) scoop-sampler in which a single semi-circular scoop
rotates through 180" to take a sample (Figs. 22, 40). The sample is
completely enclosed underneath, and there is a cover over the top of
the scoop to prevent washing out. A small loss of material which may
take place a t the sides could be minimized by fitting rubber flanges.
There is prohably less washing-out with this instrument than with