Projects

A geophysical survey using electrical resistivity tomography (ERT) is a key component of the GSP. The ERT survey will be used to build upon the foundation of information acquired through the recently completed Sentinel Well project. The ERT survey will help provide three dimensional characterization of basin stratigraphy and water quality conditions within the CGB in the vicinity of the Sentinel Well site and the Carpinteria Salt Marsh, and will be important in developing the hydrogeologic conceptual model for the GSP.

ERT is a geophysical technique for imaging the distribution of subsurface electrical resistivity in a cross-sectional format. Resistance data are collected through rolling linear arrays of electrodes, coupled to a DC resistivity transmitter and a receiver. Current is injected over specified time intervals between two electrodes. During each injection interval, voltages are measured between reception electrodes. The electrical resistivity of a given geological unit is related to the pore-fluid conductivity, clay content, liquid saturation, temperature, and matrix composition, and is used to map the extent of units with similar electrical properties when bounded by units with contrasting electrical properties. The final product of each line of survey is a 2-D cross-section plotting electrical resistivity versus depth. Raw geophysical and positional data is post-processed, and cross sections with interpretations of the hydrostratigraphy along each survey line will be generated.

A field crew of approximately seven individuals will handle the survey equipment and collect the geophysical data. The survey will involve laying out insulated copper wire cables on the ground along three survey lines; one along the beach (1.42 miles), and two within the Salt Marsh Reserve (0.34 miles and 1.16 miles). The locations of the survey lines are shown on Figure 1. Stainless steel electrodes are spaced along and attached to the cables at intervals of between 30 and 80 feet. A 12 volt car battery will be used as a power source to the ERT transmitter. Small amounts of electrical current will be injected through the cables to two electrodes at any one time. The current injected into the ground is then measured at receiver electrodes. The electrical current injected is relatively low and safe, and field personnel will monitor the entire length of each survey line to ensure that no contact is made with the electrodes while the current is induced.

Equipment will be transported along the beach and through the sensitive areas within the Salt Marsh Reserve using hand-pulled utility wagons equipped with large inflatable tires to minimize disturbance to the ground. All equipment will be removed from the site at the end of each day. It is estimated that the survey will take four to five days to complete. Figure 2 shows photographs of surveying equipment that will be used for this project.

The field work associated with the geophysical survey is scheduled for the week starting Monday, April 5, 2021.