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Instrumentation
CDIP has used a wide range of instrumentation over the years. Because
data has been collected from such a diverse assortment of sites -
from inside harbors and basins, from the pilings of piers and oil
platforms, from isolated nearshore and remote offshore locations -
each sensor has to be carefully chosen and prepared for its specific
environment. In addition to enviromental factors, technological
advances have also spurred changes in CDIP's instrumentation. Over the
decades the program has been in operation, sensors have become much
more refined and accurate.
Instrument Types
The focus of CDIP's instrument development and use has naturally been
on wave measurement. Two main approaches have been pursued - using
fixed underwater sensors, which measure the height of waves passing
above, and using surface buoys, which record their own motions with
the sea surface.
Single-point gauges
Single point gauges are pressure sensors which are mounted at a
fixed position underwater. These instruments measure pressure
fluctuations - the changing height of the water column above the
sensor - associated with passing waves. These pressure time series
can be converted to sea surface elevations and wave frequency spectra.
CDIP's very first deployment, at Imperial Beach in 1975, was of
a single-point gauge. A single-point gauge is presently still in use at
Scripps Pier.
Single point gauges are generally used close to shore, where they
can be cabled directly to a land-based field station that
records the data. In areas where there are existing structures
like piers, the single-point gauges can be mounted directly against
the sub-surface portions of pilings or other features. In other
areas, the gauges are deployed atop tripod mounts.
CDIP has used pressure sensors from a number of manufacturers,
such as Paroscientific and Sensotec. For specifications and more
information on the sensors themselves, check the links below.
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Single point tripod, San Francisco, 1998
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Sensotec Gauge Specifications:
Model TJE
Honeywell Sensotec homepage
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Paros Gauge Specifications:
Model 2100AT
Paroscientific homepage
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Arrays
Although single-point gauges can effectively measure wave height
and period, they can not be used to determine the direction waves
are travelling. When multiple underwater pressure sensors are placed
in close proximity, however, comparison of their measurements does
allow direction to be resolved. To take advantage of this fact, CDIP
developed slope arrays which allow directional wave analysis.
The first array was deployed at Santa Cruz in 1977, and
the most recent array, at Kings Bay, Georgia was in service from 1995 to 2006.
CDIP's standard array formation consists of four pressure sensors
placed in a six-by-six meter square. Like the single-point gauges,
arrays are generally used in nearshore waters and cabled directly
to shore. In a few instances, however, CDIP has deployed arrays
on offshore platforms, e.g. on Harvest Platform in California
and at Kings Bay, Georgia. In these cases, the geometry of the
array varies with the platform configuration, and the data is
recorded on the platform and sent via radio link back to shore.
Non-directional buoys
When measuring waves far offshore or at remote locations, buoys
are often a more practical solution than pressure sensors. Riding
on the ocean surface, buoys do not need to be linked by a cable
to shore; with an antenna on top of the buoy, data can be
transmitted via a radio link. They are also easier to deploy,
move, and recover, securely attached to a mooring that anchors
them to the bottom.
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The first buoys used by CDIP were non-directional, measuring wave
energy only. Equipped with one accelerometer, these instruments
measure wave height by recording the vertical acceleration of the
buoy as it rises and falls with passing waves. Over the years from
1978 to 1998, two types of non-directional buoys were used -
Wavecrest buoys and Waverider buoys. The Waverider, a .9-meter
spherical buoy produced by Datawell b.v., was by far the most
frequently used by CDIP.
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A Datawell Waverider buoy
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Buoys follow the movement of the sea surface quite effectively.
While a buoy rises and falls with the waves the force of the
mooring line does in fact change, but this has only a small effect
on the buoy's response. The Waverider, for instance, can effectively
track waves with periods down to 1.6 seconds, recording vertical
displacements with a maximum error of 3%. (Below 1.6 seconds, however,
the Waverider's response quickly decreases.)
Directional buoys
A Datawell Directional Waverider buoy
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During the 1990s, CDIP gradually replaced all of its non-directional
buoys with directional buoys manufactured by Datawell. These buoys
use hippy heave-pitch-roll sensors to measure wave direction as well
as wave energy. They also have a pair of horizontal accelerometers for
measuring north/south and east/west displacements. This translational
system uses pitch-roll to correct the buoy movement to a fixed x-y-z
reference frame; it has a better signal-to-noise ratio than buoys that
use pitch-roll directly to estimate directional wave properties.
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Datawell's Directional Waverider buoy is now the primary instrument
deployed and operated by CDIP. Like the non-directional Waverider, it
effectively measures waves with periods from 1.6 to 30 seconds, with
an error of no more than 3%. It also measures wave direction and
sea surface temperature. The temperature sensor is located at the
base of the .9 meter spehrical buoy, approximately 45 cm or 18 inches
below the surface. In accordance with IALA standards, the buoys are
equipped with a light that flashes 5 times in a 20-second cycle
(5 flashes 2 seconds apart, followed by a 12-second pause).
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Datawell b.v.
homepage
Buoy Specifications:
Directional Waverider
Full Documentation (pdf):
Directional Waverider
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Buoy Calibration
CDIP constructed a Calibration Facility based on Datawell's own facility design in order to calibrate its directional wave buoys before deployment. The facility allows CDIP to test buoys with variable frequencies which imitate the ocean's different wave conditions.
Wind, air pressure, and temperature sensors
Although CDIP focuses on the collection of wave data, at some
sites wind, pressure, and temperature data are collected to complement
our wave measurements. In addition to sea surface temperatures for
directional buoys, CDIP has collected mid-column, sea bottom, and air
temperatures at Scripps Pier. At Kings Bay, Georgia, air pressure
measurements have been made. And winds - so important for the
generation of waves - have been measured at a number of sites.
For the most part, CDIP's wind, temperature, and air pressure
measurements have been made with standard instrumentation. One
exception to this is the CDIP wind buoy, an instrument specially
designed for deployment in proximity to a wave buoy. Built from the
hull of a Waverider buoy, the wind buoy is powered by solar panels
and batteries, and collects wind data with an
R.M. Young anemometer
positioned 2.8 meters above the sea surface. Wind direction is
calculated by comparing the direction recorded by the anemometer
with that of a digital compass in the hull of the buoy.
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The CDIP wind buoy
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Deployment & Use
Given the dynamic nature of the ocean environment, deploying and
maintaining wave sensors can be a very demanding undertaking. As
CDIP has shifted emphasis from pressure sensors to buoys, the
nature of this work has changed, but always remained challenging.
Up through the
early 1990s, when CDIP was primarily using pressure sensors and
arrays, much of the challenge stemmed from the fact that the
instruments were being deployed close to the surf zone and had
to be connected by cable to a shore station. While installations on
existing structures such as piers or harbor pilings were relatively
straightforward, other installations - especially on high-energy
beaches - could be very involved. In some cases, CDIP had to rely
on amphibious vehicles (LARC) or helicopters to transport
instruments outside the surf zone, lower them to the bottom, and
lay cable back to shore.
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A helicopter deployment
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Buoys present a different set of challenges. Although they do not need
to be cabled to shore, they do need a secure anchor and mooring line.
In most cases, the 400-lb directional buoys are anchored with about
1200 pounds of ballast chain. This anchor is attached to a mooring line
approximately twice the water depth at the buoys location. For a buoy
like those at Point
Conception or Point Reyes, in over 500 meters of water, a mooring system
of over 1000m may be used. Laying out all of this mooring and then
dropping the anchor and buoy at precisely defined coordinates in the
open ocean is not an easy task.
In Southern California waters, CDIP uses its own work boat, a 26' Wilson
named the SeaDipper, for deployments and recoveries. Further afield, the
Coast Guard and other organizations often lend assistance.
Preparing for a dive
Cleaning undesirable growth from buoy
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Once the instruments have been deployed, service and maintenance
needs are generally minimal. For single-point sensors and arrays,
uninterrupted data acquisition commonly proceeds for years at
a time. There is no limit to their life, since they are powered
through the cable that attaches them to shore. Buoys, on the
other hand, require more attention since they are powered
by batteries in the buoy hull. Approximately every 2 to 3 years, buoys
are swapped out and replaced with newly-calibrated buoys filled with
fresh batteries. Despite the longevity of the sensors, CDIP staffers
enjoy getting in the water, and periodically dive to work on
instruments and their moorings.
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