What vibrocoring is. Vibracoring (or vibrocoring) is a technology and a technique for collecting core samples of underwater sediments and wetland soils. The vibrating mechanism of a vibracorer, sometimes called the "vibrahead", operates on hydraulic, pneumatic, mechanical or electrical power from an external source. The attached core tube is driven into sediment by the force of gravity, enhanced by vibration energy. When the insertion is completed, the vibracorer is turned off, and the tube is withdrawn with the aid of hoist equipment. A typical vibracoring rig is shown here. How vibracoring works. A variety of vibrahead types have been developed. In general the frequency of vibrations is in the range of 3,000 to 11,000 vibrations per minute (VPM), and the amplitude of movement is on the order of a few millimeters (mm). The vibrations cause a thin layer of material to mobilize along the inner and outer tube wall, reducing friction and easing penetration into the substrate. The liquid spaces in the matrix allow sediment grains to be displaced by the vibrating tube. Vibracoring works best on unconsolidated, waterlogged, heterogeneous sediments and soils. Silty sediments of mixed grain size are easiest to core. Vibracoring is less effective for relatively dry clays, packed sand or any consolidated (cemented) materials. Advantages over other coring methods. In non-vibratory coring methods a tube is simply pushed, dropped, propelled or hammered into the substrate. One might think that hammer-coring is essentially a very low-frequency, high-amplitude version of vibracoring, but the effect of slow hammering is not the same. In vibracoring, the high frequency vibration transfers more energy to the sediment, greatly reducing wall friction both inside and outside of the tube. The result is longer and more representative cores. In addition, vibracoring is quicker, less labor intensive, and the submersible units can be used over a wide range of water depths. Among the different types of vibracoring systems, the submersible electrical and hydraulic units are less constrained by depth than mechanical or pneumatic systems. The plugging or rodding effect. As a tube penetrates the sediment the material captured inside the descending tube moves upward at the same rate. At some point, however, the increasing wall friction inside the tube may exceed the bearing strength of the sediment. So, even though the tube penetrates further, the material inside stops moving up. Then the tube behaves like a solid rod. No more sediment is collected unless the tube penetrates to a harder layer and the friction inside is exceeded once more. As a result some intermediate layers of sediment may be bypassed -- unknown to the investigator. This effect is called plugging or rodding, and is illustrated in this diagram. Sediment disturbance in vibracoring. In a typical undisturbed sediment column the upper few inches may be a loose watery material that is easily resuspended by any nearby motion. This material will be stirred up by the vibracore tube (or other sampling devices) as it penetrates. In the deeper firmer layers of sediment, only a few millimeters adjacent to the tube wall are significantly disturbed by the low-amplitude vibrations and the movement of the tube. Within the "heart" of the core, details of structure such as layers and gas bubbles are normally still intact. Vibratory action vs. sediment types. Three types of vibratory action have been described for electrical vibracorers engineered by Andre Rossfelder, a pioneer in the field. In resonant action, the vibrohead creates a high frequency (8,000 - 11,000 VPM), low amplitude pattern of vibrations which move along the core tube. Resonant vibration seems to work best in finer grain, more homogeneous sediments. In vibro-percussive action, the tube vibration has a lower frequency (~3,000 VPM), but a higher amplitude (3.5 mm), and movement is mainly in the vertical direction. These vibracorers work well in a variety of sediment mixtures. Finally, in vibro-torsional action, the tube oscillates both horizontally and vertically, and the amplitude is greater. These systems are the most effective for firmer sediments, including gravel and cohesive clays, but that usually requires more power and strong metal tubes. Vibracorers vary in scale from small battery powered units to large hydraulic systems. Performance of different core tubes. Vibration energy is conducted best by metal tubes, with harder steel vibrating better than softer aluminum. Softer plastic tubes are poorer conductors than metals, but this disadvantage is offset somewhat by their lower density (less mass to vibrate). Sometimes metal tubes are fitted with plastic liners, the two being joined only at their lower ends. In this case the outer metal tube vibrates efficiently while the inner liner vibrates less. For a given tube, effectiveness may also vary with tube length, shorter tubes vibrating better than longer ones. Retaining core samples during tube withdrawal. Once a core tube has fully penetrated the sediment and the vibrator is turned off, it must be withdrawn without loosing the sample. Helping to counteract suction at the lower end is a one-way core catcher, which collapses and (ideally) prevents sediment from slipping back out. Also, at the upper tube end is a one-way check valve, which seals and maintains suction inside the tube. If the tube was not fully submerged during coring, it contains (besides sediment and water) an elastic column of air, which should be replaced with water and the tube resealed before it is withdrawn. Core sample recoveries. For various reasons the length of a core sample never represents 100 per cent of the sediment column penetrated. First, the core catcher constricts the opening slightly, resulting in a little expansion and shortening of the core inside the tube. Secondly, sediments often contain biogenic gas bubbles which may be lost during coring and/or compressed during withdrawal. Thirdly, some of the plugging or rodding effect (mentioned above) can occur, even in vibrocoring. Finally, some sediment loss can occur through the catcher, especially if sediments are very fluid or oily. In any case, core sample recoveries can be calculated as actual core length divided by the measured depth of penetration. Typically, core recoveries are 90% or more. |
Core sampling basics |
What vibrocoring is. Vibracoring (or vibrocoring) is a technology and a technique for collecting core samples of underwater sediments and wetland soils. The vibrating mechanism of a vibracorer, sometimes called the "vibrahead", operates on hydraulic, pneumatic, mechanical or electrical power from an external source. The attached core tube is driven into sediment by the force of gravity, enhanced by vibration energy. When the insertion is completed, the vibracorer is turned off, and the tube is withdrawn with the aid of hoist equipment. A typical vibracoring rig is shown here. How vibracoring works. A variety of vibrahead types have been developed. In general the frequency of vibrations is in the range of 3,000 to 11,000 vibrations per minute (VPM), and the amplitude of movement is on the order of a few millimeters (mm). The vibrations cause a thin layer of material to mobilize along the inner and outer tube wall, reducing friction and easing penetration into the substrate. The liquid spaces in the matrix allow sediment grains to be displaced by the vibrating tube. Vibracoring works best on unconsolidated, waterlogged, heterogeneous sediments and soils. Silty sediments of mixed grain size are easiest to core. Vibracoring is less effective for relatively dry clays, packed sand or any consolidated (cemented) materials. Advantages over other coring methods. In non-vibratory coring methods a tube is simply pushed, dropped, propelled or hammered into the substrate. One might think that hammer-coring is essentially a very low-frequency, high-amplitude version of vibracoring, but the effect of slow hammering is not the same. In vibracoring, the high frequency vibration transfers more energy to the sediment, greatly reducing wall friction both inside and outside of the tube. The result is longer and more representative cores. In addition, vibracoring is quicker, less labor intensive, and the submersible units can be used over a wide range of water depths. Among the different types of vibracoring systems, the submersible electrical and hydraulic units are less constrained by depth than mechanical or pneumatic systems. The plugging or rodding effect. As a tube penetrates the sediment the material captured inside the descending tube moves upward at the same rate. At some point, however, the increasing wall friction inside the tube may exceed the bearing strength of the sediment. So, even though the tube penetrates further, the material inside stops moving up. Then the tube behaves like a solid rod. No more sediment is collected unless the tube penetrates to a harder layer and the friction inside is exceeded once more. As a result some intermediate layers of sediment may be bypassed -- unknown to the investigator. This effect is called plugging or rodding, and is illustrated in this diagram. Sediment disturbance in vibracoring. In a typical undisturbed sediment column the upper few inches may be a loose watery material that is easily resuspended by any nearby motion. This material will be stirred up by the vibracore tube (or other sampling devices) as it penetrates. In the deeper firmer layers of sediment, only a few millimeters adjacent to the tube wall are significantly disturbed by the low-amplitude vibrations and the movement of the tube. Within the "heart" of the core, details of structure such as layers and gas bubbles are normally still intact. Vibratory action vs. sediment types. Three types of vibratory action have been described for electrical vibracorers engineered by Andre Rossfelder, a pioneer in the field. In resonant action, the vibrohead creates a high frequency (8,000 - 11,000 VPM), low amplitude pattern of vibrations which move along the core tube. Resonant vibration seems to work best in finer grain, more homogeneous sediments. In vibro-percussive action, the tube vibration has a lower frequency (~3,000 VPM), but a higher amplitude (3.5 mm), and movement is mainly in the vertical direction. These vibracorers work well in a variety of sediment mixtures. Finally, in vibro-torsional action, the tube oscillates both horizontally and vertically, and the amplitude is greater. These systems are the most effective for firmer sediments, including gravel and cohesive clays, but that usually requires more power and strong metal tubes. Vibracorers vary in scale from small battery powered units to large hydraulic systems. Performance of different core tubes. Vibration energy is conducted best by metal tubes, with harder steel vibrating better than softer aluminum. Softer plastic tubes are poorer conductors than metals, but this disadvantage is offset somewhat by their lower density (less mass to vibrate). Sometimes metal tubes are fitted with plastic liners, the two being joined only at their lower ends. In this case the outer metal tube vibrates efficiently while the inner liner vibrates less. For a given tube, effectiveness may also vary with tube length, shorter tubes vibrating better than longer ones. Retaining core samples during tube withdrawal. Once a core tube has fully penetrated the sediment and the vibrator is turned off, it must be withdrawn without loosing the sample. Helping to counteract suction at the lower end is a one-way core catcher, which collapses and (ideally) prevents sediment from slipping back out. Also, at the upper tube end is a one-way check valve, which seals and maintains suction inside the tube. If the tube was not fully submerged during coring, it contains (besides sediment and water) an elastic column of air, which should be replaced with water and the tube resealed before it is withdrawn. Core sample recoveries. For various reasons the length of a core sample never represents 100 per cent of the sediment column penetrated. First, the core catcher constricts the opening slightly, resulting in a little expansion and shortening of the core inside the tube. Secondly, sediments often contain biogenic gas bubbles which may be lost during coring and/or compressed during withdrawal. Thirdly, some of the plugging or rodding effect (mentioned above) can occur, even in vibrocoring. Finally, some sediment loss can occur through the catcher, especially if sediments are very fluid or oily. In any case, core sample recoveries can be calculated as actual core length divided by the measured depth of penetration. Typically, core recoveries are 90% or more. |
