BASIC DATA ANALYSIS

Background information

Infrasound stations & network

Our infrasound stations consists of 3 microphones connected to an amplifier and a low-pass filter card, which is then connected to an A/D-card inside a computer. The microphones have good low frequency response down to DC. The filter cuts of everything above 8 HZ, and the signal is then sampled at 18 HZ.

The data is then collected by a central server for storage and analysis. Here is a schematic over the infrasound network.

Files and file format

The entire day is divided into 48 30-minutes files. Files are written in a binary format (.bin). Within the file data are written in 512 samples blocks, each block containing the timing information and A/D readings from all 3 microphones. The sampling frequency is approximately 18 Hz, so that 32k samples (exactly 32256) is obtained from each microphone during 30 minutes. Time information in all files is the Internet-time (or in some stations GPS-time) with an accuracy of about 0.1 sec.

File names:

For example: k5070830

Means:

k

Kiruna, j=Jämtön, l=Lycksele, u=Uppsala

5

year 2005

07

month July

08

day 8

30

file no., starts at 1500UT

• All data files may be downloaded from http://www.umea.irf.se/ when you select: Infrasound timeseries viewer

• To process a .bin file type, software: crcorr.exe and crview.exe will be needed. The software may be found on the above site under: Locally developed software. The time series are converted into the angle-of-arrival and trace velocity information typing, for example:

  crcorr -c -t1 -s16 k5070830.bin

  The time series analysis is based on a windowed cross-correlation analysis. The crcorr.exe generates files k5070830.az and k5070830.caz. The .az-file is used mostly for viewing - it contains the complete information obtained during the analysis. The image produced by this software is usually not used. In order to view the file type:

  crview -t1 k5070830.az

  The image contains one upper graph (angle-of-arrival, measured clockwise from N) and one lower graph (trace velocity in m/s being a measure of the angle of incidence). Since the entire graph is too large to fit into the screen, it is possible to move it with left/right arrows. The image may be saved as a .png graphic file by pressing P on the keyboard. The time in the bottom of the graph is expressed in UT minutes and seconds. On the left and right side of the azimuth graph there are distributions of observed azimuths, The inner distribution depicts the raw azimuth readings, the outer is a weighted (with the cross-correlation between microphones) distribution. Vertical lines on the graph are the result of an automatic signal detector at its default settings. It may be adjusted. Information about all available switches and functions may obtained typing the software name without any switches. All software must be executed in a DOS-window.

• .caz-file may be used for further data processing. Columns in the file are:

  1. Time UT in minutes and decimals
  2. Angle-of-arrival in degrees, measured clockwise from North.
  3. Trace velocity in m/sec (velocities < speed of sound correspond to local pressure fluctuations and should be discarded).
  4. A, the average amplitude in A/D readings for all 3 microphones.
  5. Cr, the product of all 3 cross-correlation coefficients between 3 microphone pairs. If Cr<0.008, all 3 correlation coefficients are, on average, <0.2 and the reading, most likely, represents noise.
  6. A*Cr, the product of the amplitude and the cross-correlation coefficients product
  
  

  Results of analysis showing both the azimuth and the trace velocity may be stored in .png files by pressing the key "P".

Additional software:

il2tmseries.exe may be used to convert .bin files into ASCII time series files. That software is also available at the above html-address.

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