ERDC/CHL CETN II-43
June 2000
Four primary checks must be performed on the EMCM data to ensure that the currents are
measured appropriately. These checks include instrument position and orientation, continuous
instrument submersion, EMCM offset checks, and checks on electronic noise. Some of these
quality-assurance checks may be performed in the field, as data are collected, others may be
performed during post-processing of the data. Quality assurance checks during data collection
are recommended as these checks may identify an improperly deployed or malfunctioning
instrument. Methods for checking EMCM data quality include checks on instrument position and
orientation, instrument submersion, offset checks, and electronic noise.
Instrument Position and Orientation: Instrument position and orientation must be verified
primarily during data collection. Regular checks of instrument position and orientation during
data collection will ensure that the data can be applied appropriately for transport estimates.
During post processing, the measured cross-shore current may be compared to the pressure
signal to verify the proper instrument position. Assuring that the current direction is properly
associated with the wave phase is a fundamental check on proper instrument orientation.
Ensuring that longshore currents are consistent through the water column is another useful check
on proper instrument orientation that may be performed during post-processing of the data.
Instrument Submersion: To properly operate, the EMCM must be continuously submerged.
Intermittent submersion of the instrument (by varying water surface of waves) may cause
recorded signals from the instrument to spike erratically and become unreliable, even after the
instrument becomes submerged again. Recorded signals of EMCMs that are above the surface or
become intermittently wet and dry during data collection are relatively easy to identify. Erratic
spikes, uncharacteristic of currents produced by water waves, easily identify the signals of
intermittently submerged EMCMs. Instruments located above the water surface typically
produce a relatively flat signal that is uncharacteristic of the orbital velocities associated with
waves. Signals that appear to be out-of-water or intermittently submerged should be omitted
from sediment transport estimates.
Offset Checks: Water velocity can be estimated from a quadratic equation describing the electro-
magnetic response of an EMCM to water flowing past its sensors. The response function in the
form of V=mU+b, relates the signal output (V) to current flowing past an EMCM (U) times the
instrument's gain (m) and the instrument's offset (b). The EMCM offset may change with
damage to the EMCM nodes, bio-fouling, or other changes to the instrument's configuration.
EMCM offsets should be tested periodically by performing an offset check.
Many methods are available to perform offset checks. One method is to deploy the EMCMs into
the longshore current and collect data several minutes. After collecting the initial data set, the
EMCMs are rotated by 90, 180, and 270 degrees and additional data sets collected. These four
data sets are collected over short enough duration to assume that the mean longshore current is
relatively constant. Verification that no changes in EMCM offset have occurred is accomplished
by comparing the longshore velocities measured by independent channels of the EMCM.
During post-processing, changes in EMCM offset may be evident by comparing the longshore
current estimated from EMCMs at various elevations in the water column. An indication that an
EMCM offset has changed is if one sensor estimates radically different currents from the other
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