About

Controlled-source EM (CSEM) is an active-source electromagnetic technique that employs a deep or surface-towed horizontal electric dipole to generate electric and magnetic fields at low frequencies. The transmitted CSEM fields diffuse through the seafloor whereas the returning secondary fields are recorded by either ocean-bottom or fixed-towed electric receivers. The CSEM method is primarily sensitive to resistive structures at crustal depths.

Porpoise Array is a surface-towed CSEM system that comprises an onboard EM transmitter, horizontal electric dipole (HED) antenna, and an array of electric field receivers. This marine project utilized the Porpoise array to produce a resistivity tomography of the sub-seafloor, down to a depth of ~500 mbsf. The Porpoise CSEM system was developed and owned by the EM laboratory at the Scripps Institution of Oceanography.

Schematic illustration of the Porpoise surface-towed CSEM system. The system includes a 40 m horizontal electric dipole (HED) source that emits a current of 100 amps, and 4 Porpoise receivers at increment distance ~250 m to form a 1 km array. Each Porpoise includes a 2 m inline electric dipole, a data logger, and a GPS unit. At the end of the array, a Dorsal towed unit consists of an altimeter, conductivity/temperature measuring device and a GPS unit.

Additional marine geophysical datasets have been acquired using a high-resolution R2Sonic2024 multi-beam system and a G-882 marine magnetometer. These additional datasets helped us in mapping the seabed bathymetry and detect strong magnetic features such as dike systems. Additionally, the backscatter data provided information regarding the seafloor texture, whereas multi-beam data derived from the water column detected localized regions of prominent freshwater influx to the ocean.

The CSEM survey encompassed the entire offshore region that is parallel to the Hualalai terrestrial aquifer, acquiring data using 5 parallel survey lines (48 km each), and 5 perpendicular survey lines (1-3 km each). The total survey length is approximately  250 km.

The marine CSEM survey layout: The red lines denote the parallel survey towlines, whereas the black lines represent the survey crosslines. Right inset: Bathymetry/elevation map of the island of Hawaiʻi. The survey area is indicated by black rectangular.

The CSEM results derived from this study have been analyzed in a joint interpretation framework that included additional marine geophysical datasets (e.g., multi-beam, backscatter, and magnetic). Such an integrative approach has led to a broad understanding of the groundwater system in western Hawai‘i island, both onshore and offshore submarine regions. The resultant submarine resistivity models has been published in Science Advances (Attias et al., 2020) and are now available to the Hawai‘ian community, the board of water supply, and local stakeholders via the ‘Ike Wai project web-based Gateway platform.

A short video describes our recent discovery of the transport mechanism of freshwater from onshore to offshore Hawai’i:

https://youtu.be/6dbKQbnpfoc

Link to the paper in Science Advances: https://advances.sciencemag.org/content/6/48/eabd4866

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