LP Sensors3/24/041 Sensors Developed in the US for Low Frequency Seismology W. E. Farrell Science Applications International Corp

LP Sensors3/24/042 Topics Summary Motivation –Nuclear test detection –Physics and geophysics Geotech KS series Other Geotech sensors Academic projects

LP Sensors3/24/043 Summary The principal driver for 1/2 century of development of LP sensors has been nuclear test monitoring –The heyday lasted a long time, –Currently here seems to be little interest within DoD or DoE for R&D in LP sensors Much innovation came from University programs –Academics partly rode on the shirt tails of the test detection funding, but were focused on fundamental problems –NSF and private foundation money was key to these programs AFOSR was a supporter in the early years –Sensor R&D in universities tapered off through the 70s Perception that available technology good enough?

LP Sensors3/24/044 Motivation - Nuclear Test Detection The problem of nuclear test detection has generated the most money for R&D in long period sensors –Berkner Report 1959 Mb:Ms discriminant requires good SNR in the.1-.03Hz range –Lower frequency data required for estimating shallow earth structure, but ultra low frequencies were not a concern –Borehole packaging deemed critical DARPA & Vela Uniform –See Farrell, 1985, Sensors, Systems & Arrays AFTAC –Sponsored sensor R&D related to its monitoring mission DOE

LP Sensors3/24/045 Motivation - Physics and Geophysics Fundamental research in physics & geophysics has been an equally important driver of progress –Dicke Theories of gravity Lock-in amplifiers, capacitor pickoff & electrostatic feedback –IDA Systematic observation of free oscillations for splitting and Q(r) Inherited and applied Dicke approach –Superconducting gravity meter “Zero” drift would lead to data in the tectonic frequency band A few in service as seismometers –Strain meters

LP Sensors3/24/046 Geotech KS series InstrumentYearsDescriptionDeployment KS component, capacitive sensing, magnetic force feedback, borehole package & remote leveling Seismic Research Observatory (SRO, 13 stations) KS component, capacitive sensing, electrostatic feedback Experimental system, not deployed KS present 3 component, capacitive sensing, magnetic force feedback, borehole package & remote leveling Global Seismic Network (GSN, many stations)

LP Sensors3/24/047 Other Geotech sensors InstrumentYearsDescriptionDeployment S-11, S ? Vertical sensor packaged in sealed container High-gain, long-period (HGLP) array, upgraded to ASRO, 5 stations Strain- Inertial Quartz-bar strain sensor and conventional inertial seismometer Not deployed Open-loop EM sensor with 5 kg mass, 20 second period in a tetrahedral configuration Alaska long-period array (ALPA) SL210, SL Open-loop EM sensor with 20 second period for surface vaults

LP Sensors3/24/048 Academic projects InstrumentYearsDescriptionDeployment Block Moore Quartz cantilever, Dicke feedback (capacitive sensing, electrostatic forcing). High Q, 1 Hz resonance (electronic refrigeration) Diax failed to commercialize. Never deployed LaCoste-Romberg gravity meter Ca 1950’s vintage underwater gravity meter with Dicke feedback International Deployment of Accelerometers (IDA) 20 stations Superconducting gravity meter present Levitated Nb sphereCommercialized by GWR. 20 in operation Strain meters1968- present 1 km Michelson interferometet, volumetric Not conducive to global network