Land-to-Seafloor Electromagnetic Transmissions in the 0.1 to 10 Hz Band
Abstract
During the summer of 1976, an electromagnetic propagation experiment was performed on the Olympic Peninsula in northwest Washington. The experimental goals were to probe the suboceanic lithosphere by sending signals from land to the seafloor via the so-called down-under-up mode, and to transmit ultra-low- frequency signals to deep receivers via the usual over-down mode. A grounded horizontal electric dipole transmitter, with a peak moment of 160,000 A-m, was used to generate square-wave signals having periods from 10 sec to 1 sec. Strong signals were received on a 100-m-long electrode pair emplaced on the seafloor at a depth of about 1000 ft and at a range of 22 km from the transmitter. The fundamental was received with a 20- to 30-dB signal-to-noise ratio (SNR), and many harmonics--14 in one case-were also clearly detected. Calculations based on the assumption that the above-ground near-fields penetrate downward through the ocean to the receiver give results that agree well with the measurements at 22 km and 1000 ft; viz., theory and experiment agree to within a factor of two for all periods transmitted and harmonics detected. Even if a relatively strong signal had reached the ocean through the suboceanic crust, it would have been masked by this over-down mode.
Document Details
- Document Type
- Technical Report
- Publication Date
- Mar 31, 1977
- Accession Number
- ADA121006
Entities
People
- E. C. Field