“Minimize Core Compression”The VibeCore-D (core sampler) is a result of research at SDI to improve the vertical correlation of sediments in the core sample with the actual depth of layers in the sediment. When the depth of penetration of the core tube into the sediment is compared to the length of the recovered core there is typically discrepancy of 40% to 60% in most core samplers. This is called “core compression’. The VibeCore-D was specifically designed to minimize this “core compression” and to provide a more portable and easier to operate core sampling device. This “core compression” term is a bit of a misnomer. Water is virtually incompressible and usually even less compressible when dealing with water saturated sediments. The core material does not compress. A more accurate description of this process is that the sediment fails to enter the core tube. Let’s take a look at the physical properties of what is going on during forcing or vibrating a core tube into the sediment. We attribute this “compression” to volume displacement, the degree of sediment liquefaction adjacent to the core tube wall thus resistance in movement of the sample into the core tube and the resistance to flow into the core tube due to check valve loading.
“Volume Displacement”During the process of inserting a core tube into the sediment, the core tube is displacing material of a volume equal to the volume of core tube entering the sediment. The thicker the core tube inserted, the more the volume of sediment that must be displaced. Some of the large vibrating core sampler devices use a thick-walled pipe with a plastic liner inside the pipe and a cutter head on this arrangement. The total thickness of the core pipe and liner plastic can exceed 1/4″ wall thickness. These coring devices typically produce large “core compression” in that the length of the sediment core recovered inside the core tube is considerably less than the depth to which the core tube was inserted. Let us consider how this problem is created and how it becomes worse the deeper you go into the sediment. At 1 meter below the seafloor you must displace a volume of sediment that is equal to the cross-sectional area of the core tube and liner times the 1 meter length of the core tube. The thicker the tube and liner the more volume you must displace in the sediment. This material must go somewhere. Unless you have a lot of trapped gas in the sediment so you can compress the gas, the only way to create this volume is to push the surrounding sediment upward enough to create this volume. The deeper you go into the bottom the more difficult it is to push the surrounding sediment upward to create the new volume for the core tube. Sediment characteristics define how “fluid” the sediment is and thus the resistance of the surrounding sediment. As the sediment characteristics change the ability to create this motion in the sediment changes. It becomes obvious that the less volume you need to make for your core tube to penetrate the bottom, the less the requirement for surrounding sediment movement. A thinner total wall thickness requires less sediment volume displacement
“Liquefaction of Sediment”Another factor in “core compression” is the friction between the core tube and the surrounding sediment. Unlike a gravity core or drilling equipment a vibracore relies on vibrating motion of the core tube to liquefy the sediment adjacent to the tube and thus allow the tube to fall into the sediment. The degree of liquefaction affects the residual friction between the sediment and the core tube. The longer the core sample, the greater the resistance to material entering the core tube due to the result of friction between the core material inside the core tube and the core tube itself. The material that is inside the core tube resists moving further inside the core tube and the result is that less material enters the core tube the further down into the sediment you try to take the core. This is most significant in gravity cores as the resistance inside the core tube is many times that of a vibracore device. The VibeCore-D (core sampler) works by vibrating the entire core tube at such a high frequency that the interstitial water in the sediment particulate becomes a lubricant for the particulate and the material becomes liquid. This effect is localized to the material immediately adjacent to the core tube. The thickness of this layer is dependent on the grain size, grain size distribution and the water content. However with sufficiently high vibrating frequency and reasonably small grain size, the typical disturbed layer is only a mm or two thick and the rest of the core is undisturbed. The high operating frequency of the VibeCore-D aids in this liquefaction of the sediment. Using a thin wall rigid core tube on the VibeCore-D, you can typically achieve 90 to 95% or more capture of sediment inside the core tube as compared to the penetration depth. Thicker walled vibracore devices and gravity cores achieve considerably less sample length to penetration depth ratio. This creates an unknown as to what material came from what depth below the surface. The effect is also non-linear with more “compression” occurring at deeper penetration due to the difficulty in moving buried sediment out of the way. It also changes with material characteristics. A thin wall core tube driven by a vibracore operating at high frequency minimizes this sample unknown and sample loss. You get a more accurate representation of how deep the sediments are and where vertically things occur in the sediment. This is the reason the Vibecore-D uses a thin wall tube when vibrating to depth and one of the main reasons the small and light Vibecore-D can outperform much heavier and more power hungry systems. A vibracore penetrates sediment due to the liquefaction of the sediments adjacent the inner and outer surfaces of the core tube. The degree of liquefaction affects the penetration of the core tube and the resistance of flow of the sediment into the core sampler. This liquefication of the sediment is a result of the vibration of the tube against the sediment causing the interstitial water in the sediment to become a lubricant between the particles making up the sediment. There are a number of factors affecting how well this works. The grain size and distribution of grain size, the water content and the frequency and displacement of the vibration are leading factors in how well the sediment will liquefy and thus allow the core tube to displace sediment and sink in the bottom. Too low as water content and there is insufficient water to lubricate the particles making up the sediment. Too large a grain size without some smaller grain material and particles will remain in contact and thus prevent the tube from penetrating the bottom. SDI performed a series of tests with.
“Cutter Head Design”Another problem with the other vibracore units comes from the use of a “cutter” on a core tube that is sloped outward from the center of the core. Using this cutter, you are forcing the displaced material to move away from the core sample and thus less material enters the inside of the core tube. The thin wall core tube naturally cuts through obstructions without a thick-walled cutter that reduces the material entering the core tube.
“Core Compression reached with the VibeCore-D”Typical “Core Compression” achieved with the VibeCore-D is less than 10% with cores of 10 feet in sediment penetration producing recovered core sediment samples of 9.3 to 9.6 feet. This significant reduction in core compression allows the user better determination of the depth of layers in the sediment. Total sediment above the point of refusal is more accurately determined.
“Operating Water Depth”The VibeCore-D is effective from virtually no water depth to about 100 feet or optionally up to 200 feet if the deep water option is selected. The vibrating head goes underwater so there is no need for longer core tube than the sediment sample needed. Other systems you may be familiar with clamp onto the side of a core tube and the coring head needs to stay above water. This is not the case for the VibeCore-D
“Core sample length”We have taken 18 foot long samples with a 20 foot core tube on the Vibecore-D. We reached the point of refusal at about 18.5 ft. The VibeCore-D will reach refusal if it encounters large gravel, a root, rock or sediment with very low water content. Generally we find we reach refusal in man made lakes a few inches into the pre-impoundment material as the pre-impoundment material water content drops below 10 to 15% and has rocks and debris not usually transported a significant distance from the edge of a reservoir after impoundment. Longer cores have been recovered in reservoirs with deeper sediment layers. Addition of a core tube coupler can extend core tube lengths to more than 40 feet. Core samples with recovered sediment lengths of greater than 23 feet have been recovered with the VibeCore-D
“Capture of soft upper sediment layers”Another subject we addressed in designing this VibeCore-D unit was the check valve that closes when the core tube is retrieved. The older existing designs we investigated used a spring-loaded valve that stays closed until the pressure of the sediment entering the bottom of the tube forces the valve to open. Opening of this valve is required during core sampling as the water in the tube must escape out of the top of the core tube to allow the sediment to enter the bottom of the core tube. There is a problem with light aqueous mud which many times will reside on the top of more firm sediment. This aqueous mud has a low shear strength and will not generate sufficient pressure to open the valve. This means the water trapped inside the core tube will not displace and the light material is rejected. We designed a light check valve for the VibeCore-D that virtually floats open as the VibeCore-D is lowered through the water column thus assuring that the low shear strength sediments can enter the core tube. If you are interested in capturing the light deposits on the top of the sediment this will allow you to do so. The valve consists of a light aluminum plate with a series of polymer cast half o-rings molded to the bottom of the plate. It functions well as a check valve upon retrieval but improves the ability of the VibeCore-D to capture the entire sediment column.
“Why the VibeCore-D and why does it perform as well or better?”By way of a little more background, we decided to build the VibeCore-D as a result of a customer that used our acoustic sediment mapping system to survey some reservoirs in upstate New York. These were very old reservoirs and no roads existed to them. All boats and materials had to be hand carried in to the lake. The acoustic survey showed a hard layer about 6 feet below the bottom but possibly more material below this layer. It was not possible to map anything deeper with acoustics so we recommended they take core samples. This would be very expensive as they would need to cut a road in to bring in a boat, a generator and a vibracoring device. They opted to not do so. The hard layer turned out to be gravel transported in during the 1954 hurricanes and there was a lot more material below this level. The additional dredging cost was high and unanticipated. We decided to design a more efficient coring device that could be hand carried and yet return performance similar to the heavy coring devices that required larger boats. Looking at the design of the then present coring devices it was apparent there could be improvements. These vibracoring devices typically used a large AC motor which rotated off-center weights to generate a vibration. This motor was enclosed in a pressure housing with added weights. The vibracore required an AC generator to supply enough energy to vibrate the combined mass of the motor, pressure housing and weights with sufficient energy to create the needed vertical acceleration of the core tube. The VibeCore-D was designed to use a light weight, high energy motor which we seal and pot inside a rigid but light aluminum frame. This assembly vibrates very well and couples the energy to the core tube well but did not have sufficient weight to drive the core tube. We added the necessary weight not by coupling it to the vibecore head but by casting a ring weight that sits on rubber bumpers. The bumpers are about 99% efficient springs. The result is we get the necessary down loading on the core tube without losing energy in accelerating the weight up and down. Most of the energy used to compress the bumpers is returned. We calculate that other machines get about 12% of their energy into the core tube while we estimate we achieve closer to 60% of the energy into the core tube. The increased efficiency allows us to run our VibeCore from a pair of car batteries. It still requires a lot of energy but for a very short period. This is exactly what a car battery is good at doing. Since the VibeCore-D is typically powered for less than a minute for each core, even a small set of car batteries can supply all the energy needed to take core samples for several days without recharging. Recharging every night is still recommended as it prolongs the life of the battery to keep it near full charge.
“Optional equipment available from SDI”
“Tripod Options”Looking at the boats and required lifting capability, you may have more than will be needed. We use a 2,600 lb. rated hand winch and probably rarely need to pull more than about 300 to 500 lbs. to break the core free from the bottom. A 20 foot core in the right sediment and the pull could go higher. There is the option of clicking on the vibecore for a second or two to break it free to ease recovery of the core. Of course, we like to limit this as it can cause a loss of care sample also. We do have keepers that can be installed in the core tubes but find probably 90% or more of the time they are not needed.
“Additional Tripod Options”We also offer a coring frame that gives you about 8 feet of lift above the deck and extensions for those that want more lift. We do make leg extenders which provide another few feet of lift. For long cores we don’t try to lift a full core length. Usually if you can pull the core tube free of the bottom and the VibeCore head a few feet or more above the deck, then we walk the coring head aft (coring off the bow) so we can get to the end of a long core and cap it. Shorter cores we lift above the deck to cap the bottom. Ask for our set of pictures from some of our users so you can see some of the many rigs they have used to collect and sample the cores.
“Core Tubes”Specialty Devices Offers Core tubes in various materials, diameters and length
|Material||2″ Diameter||3″ Diameter||4″Diameter||Length|
|Aluminum||√||√||√||1ft. – 24ft.|
|Stainless Steel||√||1ft. – 24ft.|
|Polycarbonate||√||√||√||1ft. – 24ft.|
|Acrylic||√||1ft. – 24ft.|
The problem is usually more one of trying to get the sample out of the tube than a problem keeping it in the tube. Some of the EPA users and those working for the EPA are required to use a standard procedure that includes the use of keepers. In most other applications core keepers are not needed. The VibeCore has interchangeable adapters so you can use many types of core tube on the same VibeCore.