ACOUSTIC GEODESY
The acoustic geodesy system was designed to monitor centimeter-scale
horizontal movements of the seafloor over a region of a few
kilometers. In this system, transponders—devices that receive
an acoustic signal and automatically re-transmit the received
signal—were placed on the seafloor to provide fixed reference
points, and nodes—small probes that incorporate their own
power source, computer, and clock (to allow them to act as
an autonomous recording unit)—are buoyed off the seabed.
Their exact location at any one time is determined by triangulation
using the distance inferred from the acoustic travel time.
To measure seafloor movement Dave Chadwell’s team combined the information about
the two-way travel time and the temperature profile to calculate
the straight-line distances, or geometric ranges, between each
node and transponder.
2007
The 2007 Acoustic Geodesy project will include the continued analysis of
a four-month-long data set collected in 2006 offshore San Diego, the recovery
of data from nodes at Gaviota (including the replacement of node batteries
and redeployment of the nodes), the analysis of the data from the Gaviota
deployment, the addition of rapid interrogation mode to nodes to support
AUV navigation, and the preparation of a report and publication(s).
AUV-BASED REPEAT BATHYMETRY MAPPING
2003-2006
The goal of repeat mapping using AUVs is to monitor seafloor movements on the
10 cm scale over larger areas. The method involves using an AUV equipped
with a seafloor imaging system, or multi-beam sonar, to map an area of the
seafloor. By repeatedly mapping exactly the same area – a process that requires
extremely accurate location information and precision navigation – then carrying
out calculations to overlay and compare the maps, it is possible to see if
movement has taken place. Starting with a basic, commercially available mid-size
AUV with no payload, the efforts over the past few years in this program
have been to acquire, integrate, and test the high precision navigation systems
and acoustic imaging systems into the AUV and to modify the AUV for acoustically
silent operation.
2007
The 2007 AUV-Based Repeat Bathymetry Mapping project will
- Conduct a 5-day engineering test west of San Diego.
- Analyze the data collected in this AUV engineering trip to measure the navigation accuracy of the AUV using data from a variety of instruments.
- Collect Imagenex multibeam sonar data at various altitudes above the bottom, and examine the quality of the data using the processing software acquired in 2006.
- Measure the radiated acoustic and vibration noise of the AUV propulsion system using a hull-mounted acoustic array.
- Analyze and publish the results.
If the results from this AUV engineering test indicate that no major issues
need to be resolved, the 2007 project will conduct a second AUV trip at the
Gaviota Slide site; however, if serious problems are observed in the first
AUV survey data, then the second AUV trip will take place off San Diego at
the same site as the first survey trip.
If the second survey is conducted at Gaviota, these data collected will act
both as the baseline survey of the site and as a test of the extended LBL navigation
range enabled by use of the Acoustic Geodesy nodes and a PXP transponder integrated
into the AUV payload midsection.
The purpose of a second AUV survey off the San Diego coast, if this option
is taken, is to
- Collect those data originally intended to be collected in the first AUV survey.
- Collect simultaneous acoustic imaging and optical imaging data and examine the ability to space-align and mosaic these image types together.
- Determine the ability to repeat map a region of the ocean bottom mapped in the first AUV survey trip.
- Investigate novel ways of locating the AUV with the transponder replies, including the use of vector and tensor sensors as receivers on the AUV.
FIBER OPTIC SEAFLOOR STRAINMETER
2003-2006
As part of a program to measure and model horizontal strain on the sea floor,
Mark Zumberge’s team developed a sensor to accurately and stably record displacements.
The sensor consisted of an optical fiber, encased in a thin-walled stainless
steel tube, stretched from one sea floor anchor to another (placed several
hundred meters away). An electronic distance meter was connected to the fiber
to measure the optical length with a precision of about 1 mm. An acoustic modem
provided a means to periodically upload the data (sampled once per hour) after
deployment.
In the first year of this proposed three-year effort, most of the instrument
details were studied and significant issues were resolved. In the second year,
the team’s work focused on the methodology for deploying the fibers without
the use of a remotely operated vehicle. Zumberge’s team constructed a deployment
sled and tested it extensively on land and at sea. Unfortunately, during the
full implementation deployment cruise to the Gaviota Slide, the crew was unable
to successfully deploy the device. Although this setback did permit the team
to identify the weakness in the deployment system, the BP review team determined
that further support for this research topic would not promote their goals
within the Collaborative Program, and the program was not funded for 2007.
MARINE EM RESEARCH
2003-2006
Since 2004, the SIO-BP Collaboration permitted Steve Constable to support the
work of Yuguo Li, an expert in geophysical EM modeling. Yuguo’s skills provided
a good complement to the experimental expertise of our Marine EM Research group.
He has, in fact, worked with Kerry Key, to produce a series of papers and computer
codes on 2D modeling of marine EM data. These codes were very well received by
the exploration community and are possibly the best of their kind.
As part of the SIO-BP Collaboration, the Marine EM Research group also carried
out training sessions in London and Houston, and SIO student James Behrens worked
as a summer intern for BP.
2007
In 2007, the marine EM modeling program will continue to develop tools, including
2D inversion schemes and a 3D finite element forward model code, while also moving
towards a 3D inversion code.
BOUNDING AEROSOL UNCERTAINTIES
FOR CLIMATE
A new area of research for the SIO-BP Collaboration, the 2007 bounding aerosol
study, will assess aerosol uncertainties for climate change using existing
measurements of the size, composition, and morphology of ambient aerosol
particles. To characterize the current spatial and temporal coverage
of aerosol measurements globally, we will examine existing data from
worldwide aerosol measurement campaigns and promulgate the use of these
data to the modeling community by summarizing and quantifying the existing
observations and their shortcomings.
The scope of this program is targeted at a key missing aspect of most programs funded by national agencies: compilation and quantification of existing atmospheric measurements of chemical size, composition, and properties of aerosol particles in order to assess the remaining uncertainties and global gaps in information. To the extent possible, the project will assess the record of changes in this composition over time. This effort will be guided by sensitivity studies from global aerosol models to identify the types of information needed to assess both direct and indirect aerosol effects.
The overall goals of the Physical Oceanography effort is to understand
the dynamics of the Loop Current (LC) in the region of interest
in order to make improved predicitions of its strength and location.
We will apply new observational systems (HF radar and gliders)
to the problem of observing the current in real time, and use modern
data-assimilating models to gather together the data and make hindcasts
and forecasts of the LC structure and movement.
• Glider Monitoring of the Loop Current
• HF Radar Monitoring of the Loop Current
• Ocean Data Assimilation
• Wave Measurements and Prediction