ERDC/CHL CETN II-43
June 2000
EMCMs. Indications that the offset of an EMCM has changed may require either the estimation
of the new offset from offset checks in the field or omission of the instrument from transport
estimates.
Electronic Noise: As the case with many electronic sensors, EMCMs are susceptible to electronic
noise. The EMCM is particularly sensitive because the sensor's operating principle requires the
measurement of changes in electromagnetic fields. Precautions such as buffer distances between
sensors, other instruments, and ferrous metals; shielding of cables; and electronic filters
significantly reduce electronic noise recorded by EMCMs. As effective as these precautionary
measures are in reducing noise, they are not fail-safe. Under certain conditions, electronic noise
may be significant enough to require additional measures in processing the current data.
During the data collection period, monitoring of the real-time signals of the EMCMs may
indicate a problem with electronic noise. Signals from the EMCM should be monitored for
electronic noise, and if necessary the source of electronic noise should be identified and action
taken to reduce the noise if possible.
As part of post-processing of the current data from EMCMs, the analyst should monitor the
recorded signals, assess the significance of electronic noise present in the data, and take
appropriate action. The source of electronic noise is often a 60 Hz signal from AC power. The
electronic noise will not appear as 60 Hz in the collected data if the sampling rate is significantly
less than 60 Hz. Instead the electronic noise will appear wrapped into a lower frequency. For
example, 60 Hz noise for 16 Hz sampling rate will appear as a peak at 4 Hz. Appropriate action
may be in the form of discarding the data or applying filters to the data to reduce the electronic
noise and obtain a reasonable signal. Filters will work only in a portion of the spectrum where
significant wave energy is absent.
Determining bed location: Estimating the position of sensors above the bed is a fundamental and
key action for estimating sediment transport. The sensitivity of transport estimates to the location
of the lowest instruments in the water column is related to the fact that the highest suspended
sediment concentrations occur near the bed. Bed elevation for the STORM experiments was
determined primarily from sonic altimeter. These devices emit acoustic signals that are reflected
from the change in the density of the transmitting medium at the sediment bed/water interface.
The acoustic sonar performs well when sounding off a hard bottom (with a distinct difference in
density at the sediment-water interface) and few bubbles in the water column. (Bubbles tend to
scatter the emitted and reflected signal and result in a less distinct reflected signal.)
When the sonar receives a strong reflected signal, a "lock" in the recorded signal is achieved.
The locked signal is an indication that the sonar is repeatedly receiving reflected signals from the
bottom at a certain distance from the sonar head. By relating the relative position of the sonar
head to the other sensors, the distance from each sensor to the bottom is obtained. Interface
location data should be analyzed both visually and statistically to accurately measure location of
instruments. When estimating transport, errors of only a few centimeters for interface location
may result in significant changes in transport rate estimates because the concentrations and
transport near bottom will change orders of magnitude within a few centimeters.
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