Managing Waveform Data and Related Metadata for Seismic Networks Cairo, Egypt, Nov 8 – 17, 2009 Modern Data Acquisition Systems – Reinoud Sleeman Modern data acquisition systems: digitizers and dynamic range Reinoud Sleeman ORFEUS Data Center Royal Netherlands Meteorological Insitute (KNMI) knmi.nl IRIS - ORFEUS Workshop Managing Waveform Data and Related Metadata for Seismic Networks Helwan, Cairo, Egypt 8 – 17 November 2009

Managing Waveform Data and Related Metadata for Seismic Networks Cairo, Egypt, Nov 8 – 17, 2009 Modern Data Acquisition Systems – Reinoud Sleeman Layout introduction digitizing theory (dynamic range, oversampling) ADC - delta sigma modulator decimation (SEED) measuring and representing instrumental noise instrumental noise of today’s dataloggers instrumental noise of the STS-2

Managing Waveform Data and Related Metadata for Seismic Networks Cairo, Egypt, Nov 8 – 17, 2009 Modern Data Acquisition Systems – Reinoud Sleeman N-SE-W Z STS-2 STS-1 seismic station: Heimansgroeve (HGN), Netherlands sensors: STS-1, STS-2 time windows: 2002 ( ) and 2003 (029 – 043) Introduction and motivation seismic background noise (m/s 2 ) power spectral density

Managing Waveform Data and Related Metadata for Seismic Networks Cairo, Egypt, Nov 8 – 17, 2009 Modern Data Acquisition Systems – Reinoud Sleeman from STS-2 manual Low noise model STS-2 noise level Introduction and motivation

STS-2 noise (Wielandt, 1991) Johnson-Mathiesen seismometer > 1 Hz STS-1 < 0.01 Hz STS-1

Managing Waveform Data and Related Metadata for Seismic Networks Cairo, Egypt, Nov 8 – 17, 2009 Modern Data Acquisition Systems – Reinoud Sleeman … … analog signal digital representation How do we make a digital (bit stream) representation from an analog signal ? Introduction and motivation

Managing Waveform Data and Related Metadata for Seismic Networks Cairo, Egypt, Nov 8 – 17, 2009 Modern Data Acquisition Systems – Reinoud Sleeman … …… How do we get a digital (bit stream) representation from an analog signal ? How accurate is the representation ? Does the digitizing system bias the digital data ? What does ‘dynamic range’ mean and how must we interpret these numbers (e.g. 145 dB) given by vendors ? How can we measure the noise level (dynamic range) ? What information about the recording system is useful or important for seismologists and needs to be stored as (SEED) metadata ? Introduction and motivation

Managing Waveform Data and Related Metadata for Seismic Networks Cairo, Egypt, Nov 8 – 17, 2009 Modern Data Acquisition Systems – Reinoud Sleeman Quantization: Variance of error: Dynamic range of a N-bit digitizer ∆ : smallest discrete step (LSB) Digitizing theory + ∆ / 2 - ∆ / 2 x, q(x) t x q(x) e

Managing Waveform Data and Related Metadata for Seismic Networks Cairo, Egypt, Nov 8 – 17, 2009 Modern Data Acquisition Systems – Reinoud Sleeman ∆ : smallest discrete step (LSB) 2A: full scale input Quantization: Variance of error: Dynamic range of a N-bit digitizer Quantization levels in a n-bit ADC: Digitizing theory

Managing Waveform Data and Related Metadata for Seismic Networks Cairo, Egypt, Nov 8 – 17, 2009 Modern Data Acquisition Systems – Reinoud Sleeman ∆ : smallest discrete step (LSB) 2A: full scale input Quantization: Variance of error: Dynamic range of a N-bit digitizer Quantization levels in a n-bit ADC: Sine wave (amp A): Digitizing theory

Managing Waveform Data and Related Metadata for Seismic Networks Cairo, Egypt, Nov 8 – 17, 2009 Modern Data Acquisition Systems – Reinoud Sleeman ∆ : smallest discrete step (LSB) 2A: full scale input Quantization: Variance of error: Dynamic range of a N-bit digitizer Quantization levels in a n-bit ADC: Sine wave (amp A): Dynamic range: (Bennett, 1948) Digitizing theory

Managing Waveform Data and Related Metadata for Seismic Networks Cairo, Egypt, Nov 8 – 17, 2009 Modern Data Acquisition Systems – Reinoud Sleeman ∆ : smallest discrete step (LSB) 2A: full scale input Quantization: Variance of error: Dynamic range of a N-bit digitizer Quantization levels in a n-bit ADC: Sine wave (amp A): Dynamic range: (Bennett, 1948) Dynamic range digitizer: 6 dB per bit Digitizing theory

Managing Waveform Data and Related Metadata for Seismic Networks Cairo, Egypt, Nov 8 – 17, 2009 Modern Data Acquisition Systems – Reinoud Sleeman PSD vs. sampling rate In an ideal digitizer (assuming white digitizer noise) the quantization noise power is uniformly distributed between [0 – f NYQ ] Hz. The noise power does not depend on the sampling rate. For higher sampling rates the power spreads over a wider frequency range, so decreasing the power spectral density (and thus decreasing the quantization error) ! Higher sampling rate improves the accuracy of the estimate of the (analog) input signal. Oversampling

Managing Waveform Data and Related Metadata for Seismic Networks Cairo, Egypt, Nov 8 – 17, 2009 Modern Data Acquisition Systems – Reinoud Sleeman Oversampling T s = sampling interval (s)

Managing Waveform Data and Related Metadata for Seismic Networks Cairo, Egypt, Nov 8 – 17, 2009 Modern Data Acquisition Systems – Reinoud Sleeman Oversampling T s = sampling interval (s) noise power

Managing Waveform Data and Related Metadata for Seismic Networks Cairo, Egypt, Nov 8 – 17, 2009 Modern Data Acquisition Systems – Reinoud Sleeman Oversampling T s = sampling interval (s) PSD of quantization noise depends on the (initial) sampling rate, and so does the dynamic range ! Theoretical expression for the (one-sided) PSD of quantization noise in a n-bit digitizer: noise power

Managing Waveform Data and Related Metadata for Seismic Networks Cairo, Egypt, Nov 8 – 17, 2009 Modern Data Acquisition Systems – Reinoud Sleeman oversampling factor 4 leads to increase of SNR of ~ 6 dB (or 1 bit) Oversampling

Managing Waveform Data and Related Metadata for Seismic Networks Cairo, Egypt, Nov 8 – 17, 2009 Modern Data Acquisition Systems – Reinoud Sleeman oversampling factor 4 leads to increase of SNR of ~ 6 dB (or 1 bit) 1-bit ADC with 256x oversampling achieves a resolution of 4 bits Oversampling

Managing Waveform Data and Related Metadata for Seismic Networks Cairo, Egypt, Nov 8 – 17, 2009 Modern Data Acquisition Systems – Reinoud Sleeman oversampling factor 4 leads to increase of SNR of ~ 6 dB (or 1 bit) 1-bit ADC with 256x oversampling achieves a resolution of 4 bits to achieve 16 bits resolution (96 dB) you must oversample with factor 4^16 (~ ), and for 24 bits resolution this factor is 4^24 (~ ); both can not be realized ! Oversampling

Managing Waveform Data and Related Metadata for Seismic Networks Cairo, Egypt, Nov 8 – 17, 2009 Modern Data Acquisition Systems – Reinoud Sleeman oversampling factor 4 leads to increase of SNR of ~ 6 dB (or 1 bit) 1-bit ADC with 256x oversampling achieves a resolution of 4 bits to achieve 16 bits resolution (96 dB) you must oversample with factor 4^16 (~ ), and for 24 bits resolution this factor is 4^24 (~ ); both can not be realized ! this problem is overcome by the delta-sigma modulator with the property of noise shaping, to enable a gain of more than 6 dB for each factor of 4x oversampling. Oversampling

Managing Waveform Data and Related Metadata for Seismic Networks Cairo, Egypt, Nov 8 – 17, 2009 Modern Data Acquisition Systems – Reinoud Sleeman Delta-Sigma Analog-Digital (A/D) Modulator (one-bit noise shaping converter) Comperator: ADC or quantizer Feedback: average of y follows the average of x Integrator: accumulates the quantization error e over time Pulse train: pulse density representation of x Inose and Yasuda, University of Tokyo, 1946

Managing Waveform Data and Related Metadata for Seismic Networks Cairo, Egypt, Nov 8 – 17, 2009 Modern Data Acquisition Systems – Reinoud Sleeman xixi sisi uiui eiei q(u i ) Delta-Sigma Modulator IRIS Workshop, Oct 2007, Kuala Lumpur - R. Sleeman

Managing Waveform Data and Related Metadata for Seismic Networks Cairo, Egypt, Nov 8 – 17, 2009 Modern Data Acquisition Systems – Reinoud Sleeman noise shaping xixi sisi uiui eiei q(u i ) 2-nd order: Delta-Sigma Modulator IRIS Workshop, Oct 2007, Kuala Lumpur - R. Sleeman

Managing Waveform Data and Related Metadata for Seismic Networks Cairo, Egypt, Nov 8 – 17, 2009 Modern Data Acquisition Systems – Reinoud Sleeman (Hz) PSD noise Assumption: quantization noise is white noise without feedback with feedback The feedback loop in the quantizer shapes (differentiates) the quantization noise, with the result of smaller quantization noise at lower frequencies at the price of larger quantization noise at higher frequencies. Noise shaping does not change the total noise power, but its distribution. The downsampling (decimation) process uses digital anti-alias filters (FIR) which characteristics must be known by seismologists. Therefore the FIR coefficients must be part of the metadata. IRIS Workshop, Oct 2007, Kuala Lumpur - R. Sleeman initial sample rateoutput sample rate Improved resolution (reduced quantization error) at a lower effective sampling rate

Managing Waveform Data and Related Metadata for Seismic Networks Cairo, Egypt, Nov 8 – 17, 2009 Modern Data Acquisition Systems – Reinoud Sleeman Delta-Sigma Modulator Delta-Sigma Modulator / Decimation SEED

Managing Waveform Data and Related Metadata for Seismic Networks Cairo, Egypt, Nov 8 – 17, 2009 Modern Data Acquisition Systems – Reinoud Sleeman Layout introduction digitizing theory (dynamic range, oversampling) ADC - delta sigma modulator decimation (SEED) measuring instrumental noise instrumental noise of today’s dataloggers instrumental noise of the STS-2

Managing Waveform Data and Related Metadata for Seismic Networks Cairo, Egypt, Nov 8 – 17, 2009 Modern Data Acquisition Systems – Reinoud Sleeman Measurement of instrumental noise (dynamic range ) 50 ohm shortened input recording (digitizer) common input recording (digitizer or sensor)  coherency analysis of 2 channels (Holcomb, 1989)  coherency analysis of 3 channels (Sleeman et. al., 2006) - triplet method Holcomb, L. G., A direct method for calculating instrument noise levels in side-by-side seismometer evaluations. U.S. Geol. Surv., Open-File Report (1989) Sleeman, R., A. van Wettum and J. Trampert. Three-channel correlation analysis: a new technique to measure instrumental noise of digitizers and seismic sensors. Bull. Seism. Soc. Am., 96, 1, (2006)

Managing Waveform Data and Related Metadata for Seismic Networks Cairo, Egypt, Nov 8 – 17, 2009 Modern Data Acquisition Systems – Reinoud Sleeman 2-channel vs. 3-channel model

Managing Waveform Data and Related Metadata for Seismic Networks Cairo, Egypt, Nov 8 – 17, 2009 Modern Data Acquisition Systems – Reinoud Sleeman Three-channel technique:  direct method for estimating instrumental noise and relative transfer functions (relative calibration!) based on the recordings only  no a-priori information required about transfer functions  method not sensitive for errors in gain IRIS Workshop, Oct 2007, Kuala Lumpur - R. Sleeman

Managing Waveform Data and Related Metadata for Seismic Networks Cairo, Egypt, Nov 8 – 17, 2009 Modern Data Acquisition Systems – Reinoud Sleeman The digitizer experiment STS-2 Q4120 IRIS Workshop, Oct 2007, Kuala Lumpur - R. Sleeman

Managing Waveform Data and Related Metadata for Seismic Networks Cairo, Egypt, Nov 8 – 17, 2009 Modern Data Acquisition Systems – Reinoud Sleeman PSD of self-noise Q4120 measured with common STS-2 vertical signal 20 sps) IRIS Workshop, Oct 2007, Kuala Lumpur - R. Sleeman Power Spectral Density graph 143 dB

Managing Waveform Data and Related Metadata for Seismic Networks Cairo, Egypt, Nov 8 – 17, 2009 Modern Data Acquisition Systems – Reinoud Sleeman Quanterra Q4120 NARS datalogger Does the quantization error increases during quantizing a seismic signal ? 139 dB

Managing Waveform Data and Related Metadata for Seismic Networks Cairo, Egypt, Nov 8 – 17, 2009 Modern Data Acquisition Systems – Reinoud Sleeman

Q330-HR pre-amp enabled STS-2 noise (Wielandt) STS-2 self noise measurement needs Q330HR with pre-amp enabled !

Managing Waveform Data and Related Metadata for Seismic Networks Cairo, Egypt, Nov 8 – 17, 2009 Modern Data Acquisition Systems – Reinoud Sleeman The sensor experiment Q330-HR; pre-amp

NERIES framework (TA5) (funding) Conrad Observatory (infrastructure, local conditions) 4 STS-2 (same generation) 4 Q330-HR, enabled pre-amplifier Antelope ® acquisition The Conrad Observatory Experiment

Q330-HR

12 dB ! Background noise LNM Wielandt (STS-2) Triplet (STS-2) Without thermal isolation

Background noise LNM Wielandt (STS-2) Triplet (STS-2) With thermal isolation BH LH

Background noise LNM Wielandt (STS-2) Triplet (STS-2) thermal isolation no isolation

Seismic PSD Triplet (STS-2) julian day (2008)