SediMeter™

The SediMeter is a patented instrument for measuring sedimentation, erosion, and sea-bed conditions with high resolution. Individual turbidity detectors are located 1 cm apart in a vertical array, but the measurements are interpolated to give data at a sub-millimeter level.

The SediMeter can be used to get quantitative data on the dynamics of the sea floor with high temporal resolution, such as for monitoring sediment spill from dredging in real-time. SM4 also has dual nephelometric turbidimters (ISO and EPA), plus an accelerometer for tilt, vibration, and conditions based monitoring (CBM).

Imagine an instrument that detects when something unusual happens, such as being run over by a turbidity current, or experiencing an earthquake, and which starts measuring when that happens. The fourth generation SediMeter has that ability. The accelerometer is programmed to trigger an extra measurement for every strong acceleration event, once mounted. The “color” feature, introduced in 2017, measures both straight and oblique backscatter, giving a false color image with 5 mm resolution. It measures a 35 cm long vertical profile of turbidity at 71 levels using 36 near infrared optical backscatter detectors, 1 cm apart. Brown to beige tones denote buried detectors, while those exposed to air become blue.

SM4

SM4P


The sensor consists of 36 near infrared optical backscatter detectors (NIR OBS) spaced 1 cm apart. The active measurement length is 35 cm. Turbidity is measured at 71 levels, every second one as straight backscatter and avery second one as oblique backscatter, and reported in formazin turbidity units (FTU) with no decimals. The level is estimated with 0.01 mm resolution.

About 11 cm higher up there are two turbidity sensors, one ISO style with NIR light, and one EPA style with white light, both reflected 90 degrees. There is also a fluorescence meter emitting UV light and measuring the returned white light. The window of the turbidimeters are protected against fouling by UV LEDs.

The SediMeter SM4 also have a 3D accelerometer for measuring tilt and vibrations. Every measurement it stores either 20 measurements with 12-bit resolution, or 30 measurements with 8-bit resolution. The default is +-2 g range and 10 Hz frequency, although it is capable of up to +-16 g range and from 1 Hz to over 5 kHz (only 8-bit) frequency. Apart from giving information on tilt and vibration, the instrument offers Conditions Based Monitoring (CBM): When the acceleration surpasses a pre-defined level, an extra measurement is taken the next whole second. This means that the data before and after the triggering event is saved. To avoid being triggered all the time there is a 1-second hold-off, plus the threshold is raised after every extra measurement. This means that the largest accelerometer event of every deployment is expected to be captured. It is possible to adjust high-pass filter frequency and minimum duration of the acceleration event so as to target specific types of events (e.g. vibrations caused by strong currents, or earthquakes). A possible application is for capturing the head of a turbidity current.

The SediMeter SM4 have a built-in memory that holds 32,786 measurements. The time interval between measurements is from 1 seconds to 24 hours, although the recommended minimum is 3 s. It can also be used in real-time monitoring, alone or in a network. When used in stand-alone mode it is powered by a built-in battery that is recharged through the same USB cable used to set up the instrument and to download data.

The instrument can be mounted in sediment using the standard anchor and holder tube included if purchasing a kit. This holder is screwed down in mud and sand with the help of a handle.

The instrument is available separately or as a kit in a convenient case, complete with all necessary accessories for stand-alone use in the field (handle, anchor, holder tube, cable, blind plug). The SM4 model has a 15 mm diameter level sensor that goes into the holder tube, and a 20 mm diameter turbidimeter region that stays free of the holder tube. It is powered by a size AA 900 mAh rechargeable Lithium battery with solder tabs, located in a separate compartment. It can be replaced by qualified personnel using a soldering iron. The SM4 can also take a primary battery, such as a AA 2700 mAh 3.6 V Lithium battery. Furthermore, it can be equipped with both a primary and a rechargeable battery at the same time. In such a case, the primary battery acts as a backup to the rechargeable battery, i.e., the instrument prefers to take power from the rechargeable battery but if the voltage drops sufficiently, it starts drawing power from the primary battery. If event more power is desired a custom battery compartment can be made to order.

The SediMeter™ sensor invented by Dr. Ulf Erlingsson in 1985 for detecting incipient sediment motion on the seafloor. It is probably easier to show with a drawing how it works:

The sensor is anchored with the help of a holder tube that is screwed down into the bottom, thus assuring that it is not the instrument, but the bottom that changes level. We now calibrate the response in FTU (Formazin Turbidity Units), or FBU for Formazin Backscatter Units, with the sensor outside of the holder tube. The holder tube adds a significant amount of reflection, but since we are interested in the bottom level it does not matter to us.

Already from the first functional prototype (which used a Z80 NMOS CPU) it became clear that the sensor was able to detect incipient sediment motion: It reacted to the addition to a single grain of sand adjacent to the sensor.

Some users are also interested in the turbidity in the water column, and for that reason there is an extra turbidimeter about 11 cm higher up. In SM3 this extra turbidimeter is identical to the others, measuring NIR backscatter, whereas in SM4 there are two nephelometric turbidimeters, both an ISO style measuring 90 degree reflected NIR light, and an EPA style measuring 90 degree reflected white light.