Coring methods
Drilling equipment
Drilling equipment used since 1959 was mainly constructed by Eijkelkamp B.V., Nijverheidsstraat 14, 6987 EM Giesbeek, The Netherlands. For the construction of robust extension rods we used a special kind of hollow steel rods (BTR 110), produced by Van Leeuwen precision, Deventer.
The following drilling equipment was used:
- Figure 2 Dachnowski sampler.
- Edelman hand auger (for the upper 1 m, or to the depth of the groundwater table), Figure 1. A movie, illustrating the use of the Edelman-corer, can be downloaded (5.8 MB).
- Figure 1 Edelman hand-augers can be used in peat, clay, sand and gravel.
- Dachnowski-sampler (for 14C sampling of clay and peat), Figure 2. The Dachnowski-sampler is less suitable for sampling in very soft clays and peat. In that case a 7 cm diameter gauge was used.
- Gauge (for clay and peat, up to a depth of 15 m), Figure 3. Standard gauges have a length of 0.8 m or 1.0 m, and a diameter of 3 cm. Greater depths can be attained by adding extension rods of 1 m length. Holocene channel sandbodies can often be penetrated to a considerable depth with the gauge, although sand samples cannot be obtained with the gauge. A movie, illustrating the use of the gauge, can be downloaded (5.6 MB).
- Figure 3 Gauge with peat and a clay layer. Using extension rods, a record borehole depth of 25 m was reached. Picture taken by Geertjan Vis.
- Van der Staay suction corer, Figure 4. This simple, hand-operated and inexpensive, flexible pvc corer was used for coring in sand and fine gravel under the groundwater table, up to a depth of 20 m. Sampling is rapid (~ 1 hour for a 10 m deep core) and of amazing quality. The suction corer cannot be used in floodbasin deposits like clay and peat, but thin layers of clay or peat can be penetrated, if they underlie channel sandbodies. In that case excellent samples can be obtained, because the overlying sand cannot fall out of the pvc tube. The Van der Staay suction corer is home-made. A special type of pvc must be used (hostaliet). Van de Meene et al. (1979) describe the technical details. A movie, illustrating the use of the Van der Staay-corer, can be downloaded (57.1 MB).
- Figure 5 Vibra-corer. This type of euipment is suitable to take relatively undisturbed samples that can be used to study sedimentary structures.
- Figure 4 Record depth reached with the Van der Staay suction corer was 20 m (2002).
- Incidentally, use was made of a percussion corer, and a vibra-corer (Figure 5), to a maximum depth of 12 m below the surface.
Borehole numbering
All boreholes have a unique number, consisting of the year, the group number, and a sequential (borehole) number, Figure 6. Coordinates x and y are taken from the topographic map of the Netherlands, scale 1 : 10,000. Within each kilometer square x and y distances of the borehole location are measured in m relative to the coordinate lines west and south of the borehole. The z-coordinate (elevation relative to Ordnance Datum = NAP ~ mean sealevel) was generally taken from the elevation map of the Netherlands, scale 1 : 10,000. Elevation points on this map (approximately 120 per square kilometer) have an accuracy of ± 5 cm. In some cases (e.g. 14C sampling) elevation was determined by levelling with an accuracy of ± 0.1 cm.
- Figure 6 Coordinates and borehole numbering
Core description
All sediment cores were described in the field at 10 cm intervals (or more detailed, if necessary). This involved a description of texture, organic material content, gravel content, median grainsize, color, iron and calcium carbonate content (using a 5 % HCl solution), occurrence of groundwater, shells and other characteristics. Occasionally, laboratory checks of field descriptions are carried out. The textural units used in describing the cores are given by Berendsen & Stouthamer (2001). An example of a core description form is shown in Figure 7. A sand ruler, used to describe the median grain size of sand, is shown in Figure 8. For clayey samples, the sand content is estimated by biting on the clay (Figure 9).
- Figure 8 Sand ruler, containing various grain size classes of sand.
- Figure 9 The sand content of clayey soil samples is estimated by biting on the clay.
- Figure 7 Core description form, used by Utrecht University in the Rhine-Meuse delta (Berendsen 2005).
A total of almost 100,000 core descriptions is now available in a digital format (Figure 10) in the Laaglandgenese database (Berendsen 2005).
- Figure 10 Detail of the Rhine-Meuse delta, showing the coring density, which varies from 30 to 350 corings per square kilometer. Each square represents an area of 1 square kilometer. Areas without cores are cities, rivers and highways.
During the field course, the program LLG is used to digitize and print the borehole logs (Figure 11). The program BP-PLOTTER is used to print cross sections and maps of selected data. After the field course all the data are incorporated in the Laaglandgenese database. They are subsequently made available for Ph.D. students through a program called LLGinfo, which acts as an interface between the database and GIS-software like ArcMap (ESRI). Figure 10 is a screen grab from LLGinfo.
- Figure 11 The LLG program is used to digitize and print borehole logs.
Literature
- Berendsen, H.J.A. (2005), De Laaglandgenese Databank. CD-ROM, Department of Physical Geography, Faculty of Geosciences, Utrecht University.
- Van de Meene, E.A., J. Van der Staay and Teoh Lay Hock (1979), The Van der Staay suction-corer - a simple apparatus for drilling in sand below groundwater table. Rijks Geologische Dienst, Haarlem: 1-15. Download this article