SEDIMENT CORE SAMPLING USING A GRAVITY CORE

A gravity core is a marine sediment corer that works by utilizing gravitational force. This tool is shaped like a long tube that is dropped several meters above the seabed so that it penetrates the sediment and takes a sample in the form of a sediment core.

The need to collect undisturbed sediment cores in various fields such as geological and environmental research, submarine cable and pipeline planning, offshore construction site investigations, and resource exploration.

This method is often chosen because it has the advantage of providing vertically undisturbed sediment samples, allowing for the examination of sediment layers, compared to grab samples, which only obtain sediment samples from the surface.

In terms of cost, the gravity core method is cheaper than vibrocore/drilling.

There are limitations to the process of collecting sediment cores using this method if the work site has a very hard seabed, preventing the tool from penetrating it.

Where can the gravity core be applied? In principle, it can be used in almost all aquatic environments with sufficient depth and not covered by hard layers/rocks/reefs. The optimal time for sediment core sampling using the gravity core is when there are no extreme waves or strong currents.

Parts of the Gravity Core

In general, the gravity core consists of the following parts:

1. Drive Motor/Winch – Used to pull the sampling tool. This is a drive motor that uses diesel power or another type of pulling motor. The winch is used to lower and pull the Gravity Core connected to a sling cable or steel wire.
2. Head Weight – A weight attached to the top of the tool. Adding weight (mass) to the gravity corer increases the total weight of the tool. This increase in weight provides greater thrust, allowing the corer to penetrate deeper into the sediment.
3. Core Barrel – A steel tube in which the liner is placed.
4. PVC Liner – A PVC tube placed inside the barrel, where samples are stored.
5. Nose Core – The tip of the sediment cutting core that allows it to penetrate the sediment. The tapered section at the bottom of the core facilitates penetration into the sediment.
6. Core Catcher – A device that prevents samples from escaping after entering the liner during recovery.
7. Tail Fin / Stabilizer – Serves to keep the device vertical when dropped.
8. Wire & Shackle – Fastens the core and connects it to the drive machine (winch).

Working Principle

The working principle of this method is broadly as follows:

• The gravity core tube is lowered into the sea in a vertical position.
• The weight (from the head weight) adds gravitational force and causes the tool to fall freely to the seabed.
• The tube penetrates the sediment and takes a sediment core, which enters the PVC liner.
• The “core catcher” valve at the bottom holds the sample in place so that it does not come out.

• The tool is pulled back onto the ship and the recovery core.
• The PVC liner is removed from the core barrel, the top and bottom are closed, and labeled.
• The PVC liner is transferred to a storage area (core box) for further testing and analysis in the laboratory.

Specifications of Gravity Core Equipment:

Barrel Length	1 / 1,5 / 3 meter
Barrel Diameter	3 inch (78mm)
Head Weight	Large 40kg – Small 23.30kg
Liner Diameter	2½” (73 mm)
Barrel Material	Galvanized
Core Catcher	Stainless Spring Type
Operating Depth	10 – 80 meter
Core Output
Length	~ 3 meters (depending on depth and core penetration)
Diameter	± 3 inches (approximately 78 mm)

Qyudos Geosurvey Indonesia has experience in sediment core sampling in the Larantuka Strait, Kuta Sawangan Beach, Saguling Reservoir, and the Riau Islands.