What is Vibracoring?
Vibracoring is a state-of-art sediment sampling technology to obtain undisturbed cores of unconsolidated, sediment in saturated or nearly saturated conditions by driving sampling tubes with a high-frequency-low-amplitude vibrating device. During sediment coring, the high-frequency vibration transfers the energy to the sediment and aids in the liquefaction of the surrounding sediment. It greatly reduces the friction between the core tube and sediment and eases the core tube to penetrate into the sediment layer. Comparing to non-vibratory coring devices, such as box cores, gravity cores, and piston cores, vibracore has higher core sample recoveries. Vibracorers are effective in both shallow and deep environments. They retrieve core samples with different lengths depending on sediment lithology.
Benefits of Vibracoring with SDI VibeCore D
The key benefits of vibracoring with SDI VibeCore D are:
- Unique near zero “core compression” for accurate true depth of sample recovery
- Out-performs non-vibratory coring devices, such as box cores, gravity cores and pistons cores, equal or better the performance of heavier vibracores.
- Operates from two simple, safe low-voltage car batteries and carries enough charge for a full day of operation, no generator or compressor needed.
- Lightweight body (only 75 lbs.) and portability of VibeCore D makes it suitable for one-man operation on small vessels.
SDI VibeCore product line contain lightweight, high energy motor which we seal and pot inside a rigid aluminum light frame. This assembly vibrates very well but is too light 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 that we get the necessary downloading without losing energy in accelerating the weigh 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 12 V car batteries. Since the VibeCore-D is on 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.
By way of a little more background, we decided to build the VibeCore as a result of a customer that used our acoustic sediment mapping system to survey 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 into the lake. The acoustic survey showed a hard layer about 6 feet below the bottom with the possibility of 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. This deposited material masked significantly more sediment below the layer. The additional dredging cost was high and unanticipated. It was obvious there was a need for a more transportable core sampler. We decided to design a more efficient coring device that could be hand carried and yet return a similar core sampling capability.
Looking at the design of the then present coring devices it was apparent there could be improvements. They typically use a large AC motor in pressure housing with added weights and an AC generator to supply enough energy to vibrate this large mass. The VibeCore-D was designed to use a lightweight, high energy motor which we seal and pot inside a rigid aluminum light frame. This assembly vibrates very well but is too light 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 that we get the necessary downloading without losing energy in accelerating the weigh 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. The Vibecore-D still requires a lot of power but for a very short period of time. This is exactly what a car battery is good at providing. Since the VibeCore-D is on 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.
Operating water depth
The VibeCore is effective from virtually no water depth to about 100 feet or 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.
Core sample length
We have taken 18-foot-long samples with a 20-foot core tube on the VibeCore. We reached the point of refusal at about 18.5 ft. The VibeCore 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.
Core Tube Sizes
The VibeCore has interchangeable adapters so you can use many types of core tube on the same VibeCore. We offer 2″ plastic and aluminum, 3″ plastic, aluminum core tubes. The new 4” VibeCore is now available for 4” aluminum or plastic core tube. The plastic is clear and comes as either Acrylic or Polycarbonate depending on your needs. We custom make core tube adapters for the core tube available locally to the user and find this an advantage for overseas users. The adapters can be changed on the boat in about 5 minutes should you need to take multiple cores for differing applications.
Another factor in core sampling is “core compression” 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 is that the material fails to enter the core tube.
Taking a look at the physical properties of what is going on during forcing or vibrating a core tube into the sediment, you are displacing material of a volume equal to the volume of core tube you force into the bottom. The thicker the core tube you use the more volume of material you must displace. Some of the large vibracoring 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 have large “core compression” in that the length of the core 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 3 feet below the seafloor, you must push a volume of material out of the way that is equal to the cross-sectional area of the core tube and liner times the 3-foot length of the core tube. The thicker the tube and liner the more material you must make room for 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 up the surrounding sediment enough to create this volume.
The deeper you go into the bottom the more difficult it is to push up the surrounding sediment to create the new volume of the core tube. There is more sediment on top of you the deeper you go. Sediment characteristics define how “fluid” the sediment is and how easy it is to create this motion 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 of a problem this surrounding sediment movement becomes. 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.
Material Displacement with Other Vibracores
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 on the VibeCore-D naturally cuts through obstructions without a thick-walled cutter that reduces the material entering the core tube.
A lesser effect, but still worth considering, is the friction between the core tube and the surrounding sediment. 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 VibeCore device.
The VibeCore 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 dependant 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 a considerably lower ratio of sample length to penetration depth. A significant problem created by this displacement is the depth of the material inside the core tube is not well related to the depth at which it originally occurred. This 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 VibeCore operating at high frequency minimizes this sample unknown and sample loss. You get a more accurate representation of the sediment depth and the true vertical distribution of the sediment.
Capture of soft upper sediment layers
Another subject we addressed in designing this VibeCore unit was the design of a 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. When open, the water in the tube can escape out the top of the core tube thus allowing the sediment to flow into the core tube. There is a problem with light aqueous mud on the top of firm sediment as this mud generates insufficient pressure to open or to fully open a spring loaded valve. The water trapped inside the core tube will not displace and the softer sediment materials encountered at the bottom of the core tube are rejected.
A light check valve allows the flow to stay open as the VibeCore is lowered through the water column. The light material 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 cast polymer half o-rings molded to the bottom of the plate. It functions well as a check valve but improves the ability of the VibeCore-D to capture the entire sediment column.
SDI offers a float and weight ring option that assists taking cores in currents or deep waters. A set of floats suspend above the VibeCore head from a set of weight rings around the bottom of the core tube. Turn the VibeCore on and it slides down these ropes. This keeps the VibeCore vertical until it is well into the sediment. In most cases, this solution is optional.