FORMOST

Instrumentation

Instrumentation and nomenclature used within the project:

• ABS (Acoustic Backscatter system)
• ADCP (Acoustic Doppler Current Profiler)
• ADV
• CDVP (Coherent dopler velocity profilers)
• HF radar
• X-band radar
• LISST Laser In Situ Scattering and Transmissometry
• Bed profilers
• Bed scanners
• STABLE (Sediment Transport And Boundary Layer Equipment)

ABS (Acoustic Backscatter system)
The ABS measures concentration and particle size in the water column by measuring the backscatter signal of particles in suspension. The use of acoustic backscatter (ABS) techniques to measure suspended sediment dates back to the 1980s (Young et al, 1982 and Hay, 1983, Thorne et al 1993). More recently methods to obtain particle size measurements by combining data from a 3-frequency ABS instrument was described (Hay and Sheng, 1992, Thorne and Hardcastle 1997, Thorne and Hanes 2002). Also from the early 1990s, detailed analyses of the acoustic backscatter properties of various suspended sediments were carried out, allowing more accurate approaches to instrument calibration.

ADCP (Acoustic Doppler Current Profiler)
The ADCP (see image below) measures the velocity of water using a physical principle called the Doppler shift. This states that if a source of sound is moving relative to the receiver, the frequency of the sound at the receiver is shifted from the transmit frequency. ADCPs are generally applied over long range of 10-100m, for mean velocity measurements where large bin widths are used. The resulting trade off between resolution and velocity accuracy statistically requires velocity measures to be averaged over several minutes. This system in therefore inappropriate for fine spatial (centimetric) and temporal (subsecond) resolution that is required for turbulent and near-bed flow measurements.

ADV
ADV (see image below) measures the velocity of water using a physical principle called the Doppler effect. If a source of sound is moving relative to the receiver, the frequency of the sound at the receiver is shifted from the transmit frequency. Both transmitter and receiver are constructed to generate very narrow beam patterns. The transmitter generates sound with the majority of the energy concentrated in a narrow cone, and the receiver is sensitive to sound coming from a narrow angular range. The transducers are mounted such that their beams intersect at a volume of water located some distance away. The beam intersection determines the location of the sampling volume (the volume of water in which measurements are made). Each transmitter/receiver pair measures the projection of the water velocity onto its bistatic axis.

Image showing an acoustic doppler current profiler		Image showing an ADV

CDVP (Coherent dopler velocity profilers)
In contrast to conventional ADCP, the coherent Doppler use coherence between pulses in order to obtain "snapshot" of the velocity and the variance is measured over the short pulse-to-pulse time. The CDVP measures the radial component of the velocity profile along the transducer beam axis using the measured rate of change of phase between consecutive backscatter signals (Rolland and Lemmin 1997, Zedal and Hay 1999, Betteridge et al 2006).

HF radar
HF Radar (see image below) is a phased array radar operating in the 10-20MHz band, the technology for which was originally a developed as an 'over-the-horizon' radar for tracking targets during the Cold War. Our system - the WERA - is commercially produced for the oceanographic research community. There are two 150m long antenna arrays, one at Formby, north of Liverpool, and one on the North Wales Coast near Abergele. Each antenna array picks up the Doppler shifted echo from separate transmitters at the same sites. The phased array allows electronic beam stearing and hence a number of directions may be scanned by each antenna. By combining the data from the two stations, both horizontal components of the surface current may be determined from the Doppler data at each point where the beams cross. Each measurement point is effectively an average over a several kilometer wide area, and the range extends in excess of 50km. A further application that is being exploited is the ability to extract 2D wave spectra from the raw data, however these signals are at a lower level than the ones used to determine current and hence the range at which useful wave spectra are able to be calculated is usually lower than that for currents.

A radar station

X-band radar
The X-Band radar operates on a different principle than the HF radar and can be thought of as more akin to an optical system. It is based around a standard, though high specification marine radar with a rotating antenna. A short pulse of 10GHz microwave energy is transmitted in a narrow 1 degree beam and anything capable of reflecting that beam bounces the energy back to the transceiver, including rocks, ships and waves. The range of any particular target is determined by the time it takes the reflected energy to arrive back at the radar at the speed of light. The practical upshot of this is that it produces a 360 degree plan view of the sea surface and anything on it every time the antenna rotates. We record sequences of these images to a range of about 8km from the radar using a Wamos radar recorder and are interested in analysing the behaviour of the waves visible on the image sequences. One of the main interests is to perform depth inversions on the data - this involves accurately determining the behaviour of the waves and inferring the water depth and possibly the current that caused that wave behaviour. This allows the production of bathymetric maps covering large areas of complex sand banks without the need to set foot in a boat, and hence monitor any sand bank movement regularly over a period of years. It is also possible to empirically calibrate the data to determine 2D wave spectra.

LISST Laser In Situ Scattering and Transmissometry
Instrument designed to measure particle size based on the principle of laser diffraction. There are 2 standard LISST models, each designed to measure different size ranges. The type-B and C LISST measure particles in the 1.25 to 250 µm and 2.5 to 500 µm range, respectively. The particle size resolution for both consists of 32 log spaced bins. The standard processing software supplied with the LISST computes the volume concentration in units of microliters per liter (µl/l), which is the measurement unit that will be utilized here. The LISST does not require calibration per se, but the laser optics must be checked on a regular basis to ensure that they remain properly aligned (Styles, 2006).

Bed profilers
The bed ripple profilers have been of three types: a single transducer pointing vertically down towards the bed, which is physically translated along the profile; a multi-transducer linear array (MTA) which provides measurement along a transect and a single transducer which radially rotates to provide a profile of the bed. The transducer allows to find the bed position within a spatial resolution of about 5 mm and with an error of 0.01 m (Williams et al 2004, 2005).

Bed scanners
Ripple scanners are based on sector scanning technology that has been specifically adapted for high resolution images of bedform morphology. They typically have a frequency of around 2 MHz with beam widths of about 1 degree in the azimuth and 30 degrees in elevation. As the pulse is backsacattered from the bed, the envelope of the signal is measured and usually displayed as image intensity.

STABLE (Sediment Transport And Boundary Layer Equipment)
STABLE (see image below) is a platform to Study Near-bed Turbulent Currents and Associated Sediment-Dynamics. The biggest and most complex apparatus designed and built by the Mechanical Engineering sub-group at POL is the STABLE III equipment. STABLE stands for Sediment Transport And Boundary Layer Equipment and STABLE III is the third generation of this apparatus which was conceived originally in 1981. It measures the interactions between turbulent currents and sediments at the sea-bed. It is a tripod standing about 2.5m high and the feet occupy a circle about 3.5m in diameter: it weighs about 2,500kg (Williams et al 2003, Thorne et al 2002).

STABLE being deployed