Introduction to seismic sensors subject 3 2 Peter

Introduction to seismic sensors (subject 3. 2) Peter Novotny PACMAN meeting, CERN, 7 October 2014

What is seismic sensor? Instrument which measures ground motion. (Not only earthquakes!)

Ground Motion in frequency domain Power Spectral Density – PSD of ground motion Cultural noise Micro seismic peak RMS of ground motion

How does seismic sensor work? Relative motion (mass to ground) Ground acceleration ↕ Absolute motion (ground) Dynamic described by 2 nd Newton’s law => Forced damped harmonic oscillator.

How does seismic sensor work? Transfer functions: Laplace transform Natural frequency !! Information about absolute motion is in relative motion!!

How we measure relative motion? Using different types of transducers: • Pen (mechanical seismometers - obsolete) • Resistive • Capacitive • Piezoelectric • Optical (Interferometers, encoders) • Inductive – coil + magnet • Eddy current • LVDT - Linear variable differential transformer • …

Seismic sensor parameters • Bandwidth – fmin - fmax • Sensitivity S – conversion factor from D, V, A to Volts • Self noise N – determine resolution • Resolution R = S/N – smallest measureable value • Dynamic range – Max/Min measureable value • Active/passive – need of power supply • …

requirements on sensor • Bandwidth = (0. 1 – 100) Hz • Resolution = ↑ Sensitivity and ↓ Self-noise • for R = 0. 1 um • for NANO STABILISATION R = 0. 1 nm • Stray magnetic fields resistance • Weight < few kg & compact size • Temperature stability • “Radiation resistance” => only for stabilisation • Price ≈ 4000 sensors for CLIC • …

Geophones • Output voltage proportional to ground velocity above natural frequency. • Simple and compact solution requirements ü Low noise ü Weight and size ü Price × Natural freq. > 10 Hz × Magnetic resistance

Geophones with feedback => seismometers • Utilise the feedback control of mass position to extend bandwidth to low frequencies. Seismic mass Leaf spring sensing & force coils

Geophones with feedback => seismometers • Output voltage is proportional to ground velocity in certain frequency bandwidth. requirements ü ü ü • × × Very low noise Resolution Broad bandwidth Weight and size Magnetic resistance Price

Accelerometers • Output voltage proportional to ground acceleration below natural frequency. • Different types: • Piezoelectric, piezoresistive, capacitive, Force Balanced (FBA) • Possibility of utilizing MEMS Piezoelectric MEMS capacitive acc. Principle + size and price reduction - high self noise requirements ü ü • × × Bandwidth Magnetic resistance Price Higher noise at lower freq. Resolution MEMS realisation Final product

Noise in seismic sensors Interferometer • Sources of noise: • Thermomechanical (Brownian) noise Model Noise [um*s-2/Hz 1/2] 87 0. 9 @1 Hz 0. 25 @10 Hz 0. 1 @100 Hz bb rms <4 um*s-2 Wilcoxon 731 A 0. 3 @2 Hz 0. 1 @10 Hz 0. 04 @100 Hz Brüel & Kjær 8340 14. 7 @0. 1 Hz 14. 7 @55 Hz 393 B 31 0. 6 @1 Hz 0. 1 @10 Hz 0. 04 @100 Hz SF 1600 S. A 3 in 0. 1 -100 Hz BW TSA-100 S rms 0. 22@1 Hz rms 0. 2@10 Hz rms 0. 7@10 Hz Producer Endevco • Molecular collisions with mass • Thermoelectrical (Johnson) noise • Random thermal excitation of charge carriers • Schottky noise Capacitive sensor • Random movement of charge across potential barriers • Flicker 1/f noise • Impurities in semiconductors • Discretisation noise • ADC converters PCB COLIBRYS Metrozet

Classification of seismic sensors Output voltage proportional to: • Displacement • Velocity: • geophones • feedback geophones = seismometers • Acceleration: • Accelerometers • Force Balanced Accelerometers (FBA) By type of sensing transducer: • • • Capacitive Inductive Piezoelectric Optical … Other classifications: • Bandwidth • Short period, Long period, Broad band, Very BB • Signal intensity • Strong motion, High sensitivity

Few sensors and their producers Producer GURALP ion (Sensor Nederland B. V. ) Sercel (Mark Products) Kinemetrics (Streckeisen) Nanometrics REFTEK Lennartz electronic Metrozet Eentec SINUS Geo. SIG Endevco Wilcoxon DYTRAN DJB Instruments MMF Brüel & Kjær PCB COLIBRYS R-sensors METTECH GEOTECH INSTRUMENTS type of sensors Seismometers geophones Seismometers + FBA Seismometers + FBA FDBCK MET geophones + FBA Piezo-velocity sensor geophones + FBA accelerometers accelerometers MET geophones + acc MET Seismometers

Secondments • Development of the seismic sensor for stabilisation of the Final Focus (of CLIC) • Equivalent of 7 months • High precision mechanic manufacturer (already started) • Their measuring and control system • 3 months starting in March 2015 • Project: • Measurement and adjustment techniques for precision applications

Thank you for your attention!