Fig 1.
Overview of the Trans Bay Cable route and survey area.
The Trans Bay Cable (dark blue line) conducts electricity from the city of Pittsburg, CA to San Francisco. Magnetic field surveys were conducted over an area with a width of 1 km (pink survey lines) at the following sites: 1) Benicia Bridge, 2) San Pablo Bay, 3) Richmond Bridge, and 4) Bay Bridge. The inset shows both a picture of the cable and its construction.
Fig 2.
Magnetic survey transects conducted at Richmond Bridge.
The survey transects (see horizontal red lines) extended one kilometer to either side of bridge. The path of the Trans Bay Cable (vertical red line) passes under the bridge in a north-south orientation. Insert: transverse gradiometer (G-882 TVG, Geometrics, Inc, San Jose, California) featuring two magnetometers mounted side by side 1.5 m apart (Photo: Geometrics).
Fig 3.
The local magnetic field anomalies existing around the Trans Bay Cable and Richmond Bridge.
Note that the anomaly in the field produced by the cable is evident in the dark blue and red points along the line indicating the path of the cable that passes through the bridge along a north-south axis. The color scale uses non-linear color mapping based on the data distribution. Using this color equalization technique, each color occupies the same area on screen as any other color, ultimately increasing map resolution and visualization of smaller magnetic anomalies.
Fig 4.
The gradient map of local magnetic fields existing around the Trans Bay Cable and Richmond Bridge.
This quasi-analytic signal map illustrates the rate of change in local magnetic field anomalies, denoted as nT/m. Note that the rate of change in the local magnetic fields associated with the bridge is much greater than near the cable, and the distortion in the field extends farther from the bridge than the cable.
Fig 5.
The magnetic anomaly induced by electrical current passing through the Trans Bay Cable.
Profile plots of the cable’s measured magnetic anomaly are illustrated for both shallow, ≤ 1 m below surface, and deep tows, ≤ 3 m above bottom, along survey transects far away from the bridge. The anomaly is shown as the gradiometer was towed from east to west over the cable along transects orientated parallel to the bridge and perpendicular to the path of the cable. The anomaly recorded at the surface was 94 nT while the anomaly near the bottom was 245 nT. The increase in magnetic intensity in the latter profile was due to the gradiometer’s increased proximity to the cable. At ±80 meters from its centerline, the cable’s calculated contribution to the background field is about ±2 nT (~0.0042% of background) regardless of measurement depth when the cable carries its rated load of 1,000 amps. The anomaly exists over a distance of 80 meters, and consists of a negative and positive increment to the main field.
Table 1.
Summary of magnetic field anomalies associated with the Trans Bay Cable (TBC) and bridges in the San Francisco Estuary.
These are deviations from the earth’s natural background magnetic field. The local magnetic fields (in nanotesla, nT) of the bridges and cable were measured with both surface and deep tows during transects that ran perpendicular to the object of measure. The cable was surveyed at bridge site locations as well as a non-bridge location in San Pablo Bay. Cable data is only presented from transects where the cable anomaly was clearly identifiable. Measurements from the two magnetometers were averaged for this study.
Fig 6.
The magnetic anomaly produced by the Richmond Bridge as measured by perpendicular survey lines.
Profile plots for surface and deep tows are illustrated along transects (black vertical lines) travelling perpendicular to the bridge. Note that the surface anomaly from the bridge is 728 nT and sub-surface anomaly is 726 nT, exceeding that of the cable in magnitude by a factors of 7.7 and 3.0, respectively. The anomaly occurs over a distance of 1,200 m, and does not consist of a positive and negative excursion but only a negative excursion.
Fig 7.
The magnetic anomaly produced by the Richmond Bridge as measured by parallel survey lines.
Profile plots for surface and deep tows are shown for a survey transect located parallel to Richmond Bridge, approximately 50 m south of the bridge. Successive maxima and minima along the profile are separated by a distance of 100 m, the approximate distance between bridge supports. These observations indicate that the supports are also a strong source of anomalies, adding to the background field at each structure, and subtracting from between each structure.
Table 2.
Passage of Chinook salmon smolts and adult green sturgeon through the magnetic anomalies produced by the Richmond, Benicia, and Golden Gate Bridges.
These are deviations from the earth’s natural background magnetic field. Data represents the number of individual fish detected at each bridge in each year as well as the percentage of total fish detected at the first array for subsequent locations along each migration route.