Finding the correct sampling tool for collecting sediment may be surprisingly more complicated than most people expect. With hundreds of options and new technology every year, finding the right tool for the job can be a tricky question to answer. In this brief article, we will discuss surface sampling tools. We will follow with another article on subsurface sampling tools.
Sediment – also commonly referred to as mud or uncommonly as seafloor soils, is that layer of material we find on the surface of the seabed. This layer ranges from hard to very soft material and is often mixed in harbors and developed areas with wood, refuse, and contaminants. This critical surface layer down to 15 cm holds much of our industrial history as well as the bulk of the biological activity in the sediment.
Key Factors for Selecting a Sediment Sampling Tool
- Undisturbed samples – always opt for a sampler that minimizes the disturbance of sediments. For many projects, it is critical to collect samples with overlying water particularly for chemistry sampling. In soft material, it is often better to use a lightweight system to ensure the pressure or weight of the sampler does not push or blow away surface sample. In more challenging substrates, you may need to compromise with a sampler that does disturb material (examples such as the Hamon grab)
- Fit for vessel – does the selected grab work with your vessel and lifting system. Many of the larger power and hamon grabs can weigh more than 500 lbs or more when full of sediments
- Penetration depth – how deep do you need to go? Most of the smaller samplers only penetrate 10 cm with the larger grabs 20-40 cm.
- Presence of debris or rocks – most of the bottom closure samples do not fare well with rocks and debris as the sampler can not close and fine sediments are washed out. If the goal is a sample even if it is disturbed utilizing samplers such as the Hamon or Shipek may be better options
Summary of Surface samplers
This is a great little sampler designed for soft muds and higher water content substrates. The sampler spring closes with about 15 lbs of force. It cannot be used where any sands, rocks, wood debris, etc may come in contact. This unit is nice because it is small and light and can be connected to a pole for shallow water. Grab is closed with a messenger. Sample volume is usually very small = 0.025 cubic meters so plan on many drops to collect a practical volume for analytical testing.
Another easy to handle grab that can be used in deeper water off of a small boat with no winch. As the name suggests it is small, but it does have much more closing force than an Ekman by utilizing lifting bars that close the jaws. This unit is great for rivers and shallow waters. The low volume issue still comes into effect like the Ekman for analytical testing and the closing force is relatively low.
Van Veen Grab
This is the industry standard tool for getting that top 15 cm of material. This grab weighs about 90 lbs and has the same leveraged bars for lifting as the ponar. It has good closing force and works well for sampling in areas with depositional material present. The Van Veen is popular for contaminant programs because you can get plenty of volume for sediment analytical characterizations. A double Van Veen set-up is also available for collecting extra sediments for bioassay testing or for replicate treatment. The Van Veen does have a few negatives including a funky release chain mechanism that does not always work and sometimes pinches fingers. It also does not work well on slopes and fails completely if cobble or wood debris are present.
This is similar to a Van Veen with the addition of a frame to help level it on the seafloor. The day grab also benefits from its added weight and the ability to control penetration depth. The Ocean Instruments Salish Grab is a version of the day grab with an additional frame feature that holds the trigger mechanism for easier rigging and reduced hazard of pinching.
This British built sampler uses a bucket very similar to the knuckle movement on a backhoe to scoop through the sediment. This is similar to a Shipek grab sampler (below) and works well in unsorted cobble beds. The Hamon usually uses lots of weight to hold the sampler in place while a single leveraged arm rotates the sampler through the sediment. This sampler is not often accepted for chemistry as it does disturb the sediments. Many sizes are available along with power options using a closing cylinder.
This grab uses a similar rotating scoop design as the Hamon but utilizes a spring for closure. The sampler is used often for offshore geotechnical work and is compact. Sample volume is low with this unit and the material is significantly disturbed during collection making it not ideal for chemistry.
If you are not concerned about disturbing your sample and just want volume or screened material the dredge sampler may be your simplest option. Dredge samplers are often used for selectively collecting certain grain sizes of material for geotechnical or mineralogical testing. The dredge is dragged for a period of time behind a vessel and uses guiding teeth and a metal bag to collect material.
Smith McIntyre grab
The Smith Mac is a deep ocean grab sampler that uses a very powerful spring to close its jaws. This device is great in consistent bottom substrates but does not work well if it encounters cobbles, wood debris, or other material because it depends on a heavy spring only to close. Many of the Smith Macs do have a bottom frame which helps with taking samples on slopes. Many consider this sampler to be a little dangerous – so don’t allow novices to operate it!
Power Grab Samplers
The power grab sampler uses the same jaw closure as a Van Veen although it is often much larger. The power grab uses a bottom frame and cylinder powered by either air or hydraulic pressure to close the grab. The power grab system is good because the bottom frame keeps it level on the seabed and the sample is taken by forcing the jaws down while not lifting (the Van Veen lifts as it closes). This often gives you the most undisturbed sample and the highest sample penetration success. Power grabs have closing forces from 150 to 1500 ft/lbs. Power grabs are also used to collect deeper than standard sediment samples and to collect grabs in rocky environments. The clear downside to the power grabs is that they are heavy and need at minimum a 300 lb davit to deploy and retrieve.
Ocean Instruments sells versions of most of the listed samplers providing stock versions of the Day Grab (Salish Grab), double Van Veen grab, and the smaller ponar grab. OI also custom builds the Hamon grabs and power grabs to customers’ specifications.