Dipartimento di Georisorse e Territorio - Università di Udine

A three-component data acquisition system for monitoring the seismic activity at Stromboli volcano

ABSTRACT

An automatic seismic station, which was installed on Stromboli volcano in 1989, has been upgraded in May 1992, with regard to both hardware and software, the major improvement being the presence of three components instead of one. The summit station transmits data analogically to a receiving station located in the Stromboli village, where digitization takes place and data are processed automatically by a MS-DOS based Personal Computer, which activity can be remotely monitored via a modem connection without interfering with the acquisition. The trigger algorithm consists in a combined amplitude- frequency threshold test followed by the computation of the STA/LTA ratio. 60 seconds events are recorded together with activity reports and periodic tremor samplings. A number of off-line processing programs was also developed, together with data conversion routines which allow the use of several commercial software packages.

1. Introduction

Since October 1989 an automatic station on Stromboli volcano is functioning for the control of the seismicity due to explosive activity (Beinat et al., 1989)(Beinat et al., 1990). These two years of operation have been useful for the verification of the project choices and the consistency of the data collected by such a system (Riuscetti, 1994).

The positioning of electronic instrumentation in such logistic and environmental situations, the choice of telemetry data transmission, the complete automatization of the event discrimination and preprocessing system necessarily included several unknowns: the initial project has shown its reliability and only small modifications have been required.

Fig. 1 - Sketch map of the station

Successive software upgrades have been done in order to improve the quality of the data acquired. We remind here the main ones:

October 1989: installation;

May 1990: software upgrade to allow hourly tremor sampling together with the creation of reports regarding both volcano and recording system activity;

October 1990: summit station power augmenting and software upgrading concerning the trigger method, with the introduction of the STA/LTA ratio algorithm, and the carrier wave check via RS-232 interface;

May 1991: construction of a concrete basement to place seismometers and substitution of the Mark L4C geophone with a Willmore MKIII/A; modem connection between Stromboli station and our laboratory in Udine.

During May 1992 we have proceeded to a substantial developing although the design followed during the first realization and the configuration (i.e. an acquiring and transmitting remote station, sited close to the craters, and a receiving and preprocessing station, sited in the Stromboli village, connected with Udine via modem) have not been changed (see Fig. 1).

2. Main features of the system

The new seismic station on Stromboli volcano differs from the previous one mainly for the presence of three components instead of one, thanks to the installation of two additional horizontal geophones, and to the substitution of the old 8086-based personal computer with a new PC with industrial standards and a 80386 main processor; this was needed to fulfil the new computing requirements and to prevent possible problems arising during "h24" continuous functioning in severe environmental conditions such as high humidity, salinity and temperature ranging from 0 to 50 C .

Seismic data are always treated analogically until the A/D conversion board that constitutes the interface with the computer (Kulhánek, 1976). The need of an acquisition system being simple, reliable and upgradable at the same time, has pointed the choice to an analogical instrumentation made by Lennartz firm.

Due to the short distance of transmission and to the absence of electromagnetic disturbances the signals can be handled analogically while keeping a good signal-to-noise ratio.

The calibration of the sensors and of the transmission line allows the exact knowledge of the amplitude ratio between the seismometer output and the values read by the computer; the dynamic range of the system is equal to 68 dB.

A fourth channel has been introduced in order to acquire the BCD time code delivered by a clock synchronized with the OMEGA signal.

The recording time window has been raised from 40 to 60 seconds while the digital sampling rate was lowered from 100 to 80 samples per second for each channel; in order to prevent aliasing four low-pass filters with cut- off frequency of 25 Hz have been provided.

3. Transmitting station

The summit station is located at 800 m a.m.s.l., near the path leading to "Pizzo sopra la Fossa", at about 300 m from the craters. The instrumentation, wholly buried except for solar panels and antenna, is housed in stainless steel boxes in order to prevent possible problems arising from severely corrosive conditions and dust abrasion: panels are protected by shock-proof glasses (see Fig. 2 and Fig. 3).

Fig. 2 - Configuration of the summit station

Fig. 3 - Block diagram of the transmitting station

Sensors: three Willmore MKIII/A seismometers adjusted to a period of 2 seconds, along the three components Z-R-T, positioned on a concrete basement. Seismometers have been calibrated at the SIT centre of ISMES by an ECCI1 excitement system, following the PT-DSS-229 procedures, by comparison with a standard accelerometric line. The response curve in the range from 1 to 100 Hz has been derived for each seismometer together with the sensitivity by inserting an external damping resistance in order to make the geophone work at the critical damping.

Amplifiers: Lennartz mod. 7064 with a selectable power of 2 amplification factor (which was set to 4 for the vertical component, to 3 for the horizontal ones), passing band from 0 to 20 Hz and noise rerred to input approx. 0.1 mV (from 0.5 to 12 Hz)

Modulators: Lennartz mod. 7048 with carrier frequencies respectively of 860 Hz, 2.1 KHz and 4.4 KHz, with typical stability of 25 ppm.

Mixer: Lennartz mod. 7059.

Transmitter: Lennartz mod. 4001 operating in frequency modulation with carrier wave at 433 MHz (stability +/- 3.5 KHz), output power 0.1 W, modulation bandwidth from 10 Hz to 5 KHz, modulation sensitivity 5 Vpp for maximum deviation and spurious emission - 65 dB u.C.

Power supply: Two solar panels with power supply of 40 W, charging limitator to 12 V and batteries with total capacity of about 140 A/h.

An important aspect concerns the protection from lightnings and ground potentials; all the instrumentation has been electrically connected to a steel net and pipes that serve as ground electrodes.

4. Receiving station

The receiving station (see Fig. 4) is located in the Stromboli village, in the office of the volcano guides, who cooperate to the control and handling of the system and change the back-up magnetic media.

Fig. 4 - Block diagram of the receiving station

Receiver: Lennartz mod. 4010, with minimum sensitivity threshold of 1 mV and input spurious level less than 70 dB.

Demodulator: Lennartz mod. 7222 with three demodulating boards for the frequencies 860 Hz, 2.1 KHz and 4.4 KHz, signal to noise ratio 72 dB, maximum output signal 7 Vpp with passband from 0 to 100 Hz, output amplification ratio equal to 10.

Time signal: Omegarec, a receiver for the OMEGA international signal transmitted in the VLF band (in the range 10 - 13 KHz), interfaced to Omegaface, quartz clock and BCD code generator.

Filter: four channels anti-alias AAF-1 low noise eight order Cauer Elliptic low-pass filter, continuously set- able with 68 dB minimum damping for frequencies greater than 1.5 times the cut-off frequency.

Analog to digital conversion: DAS8 board by Keithley Ins., configured to sample 4 channels at 80 samples per second each, maximum input signal amplitude 10 V, 12 bit resolution for a dynamic range of 68 dB.

Computer: MS-DOS PC, 80386 processor at 25 MHz, 4MB RAM, Hard Disk 60 MB; the computer is housed in a cabinet according to IP 54 standard to prevent damages due to dust and humidity.

Back-up: streamer TEAC, with storage capacity of 150 MB per cartridge.

Modem: Hayes Multiway mod. 2400MNP5, 2400 baud, with automatic call and answer with MNP5 protocol.

Power supply: Uninterruptible Power System IREM US60, 600 VA, with typical backup time at nominal power greater than 10 minutes; protection against overvoltages, voltage fluctuations, blackouts, transients and line noises.

5. The software

With respect to the old configuration, the main improvement made to the acquisition system in the new station consists in the raising of the number of sampled channels from 1 to 4; this, being that the acquisition frequency has been reduced to 80 Hz, fixes the number of items to be processed to 320 per second.

The other major upgrade, i.e. the widening of the temporal window associated to each event from 40 to 60 seconds, lets now make use of the information obtainable from the code of the BCD time signal.

The noteworthy rise of the computing needs, both in terms of speed and in terms of storage capacity, that comes from the upgrade, faced to the intrinsic limitations of DOS and to the capabilities of the QuickBasic compiler, has suggested a substantial revision of the project of the software component of the system, depicted in Fig. 5.

Fig. 5 - Software modules overview

The module which has suffered the most from the upgrade is certainly the one which constitutes the heart of the on line system, i.e. the acquisition, triggering and preprocessing module. Its main characteristics are shown in Fig. 6.

Fig. 6 - Acquisition module: main data structures (a) and flowchart (b)

The main data structures used by the algorithm are only three, as can be seen in Fig. 6a. The first one, DAS_WA, is the Work Area of the Data Acquisition System board; this, after initialization, keeps on filling the vector DAS_WA, which is virtually seen as a circular buffer, accessed via Direct Memory Access (DMA). The minimum number of elements in DAS_WA is given by the number of items in a record, i.e. seconds * frequency * channels, but, as to allow a number of extraseconds for the transfer of each record in the program work area after its completion, it is necessary to reserve additional extraseconds * frequency * channels memory locations. In any case, due to DAS-8 software driver limitations, this array can't exceed 64 kbytes.

Once completed, as we already mentioned, the record is transferred into the ProGraM Work Area, which we shall call PGM_WA, suitably dimensioned (again seconds * frequency * channels). An additional data structure keeps the pretrigger, i.e. the preseconds tail of the record previously acquired; its length is given by preseconds * frequency * channels and the transfer of the tail is done immediately before the transfer of the new record from the DAS_WA to the PGM_WA, as depicted in Fig. 6b.

After the transfer of the current record from the DAS_WA to the PGM_WA, which is done every 60 seconds, the algorithm tests if there is the need to complete an event triggered during the previous cycle, condition which is signalled by the activity of the continued_event flag: in this case a number of items sufficient to complete the 60 seconds record is saved.

After having reset the continued_event flag, the algorithm proceeds with the verification of the trigger conditions; if these are fulfilled, the program stores on a hard disk file the event together with its pretrigger section and, if the current data is not sufficient to complete a 60 seconds record, sets the continued_event flag.

The program then simply waits for the next record in the DAS_WA to complete. At regular intervals, which duration is configurable by the user and is typically of the order of 1 hour, a 60 seconds long tremor sampling is also stored.

6. The trigger

The trigger procedure examines only 1 channel out of 4, the choice of which can be easily changed by modifying an external configuration file, which also contains all other parameters which are likely to be subject to modifications, without the need of recompiling any part of the program.

The procedure is based on the verification of two distinct conditions which are checked on consecutive time windows of 1 second each (i.e. 80 samples).

The first condition, which uses a combined amplitude-frequency threshold, requires the signal to exceed a positive and negative level (amplitude window) for a certain number of times (frequency window), see Fig. 7.

Fig. 7 - Trigger conditions

The second condition is based on the classical STA/LTA algorithm (Allen, 1982), i.e. the ratio between the integral of the wave within the selected time window and the integral of a corresponding tremor sampling. This last value is constantly updated as the execution proceeds, after each tremor sampling, in order to follow the fluctuations of the noise level. To avoid instant effects, the LTA reference value is smoothed by averaging the last tremor wave integral and the previous LTA reference value. A further improvement could be achieved by differently weighing each of the two terms.

The efficiency of the algorithm has been tested in our laboratory on about 200 analogical events recorded during previous campaigns, resulting in about 95%.

All of the trigger parameters (thresholds, frequency window, STA/LTA ratio) are read by the external configuration file.

7. Reports

The acquisition module also keeps some information regarding the seismic activity and the state of the recording station hardware and software in two distinct files. In the first one the program saves the name of each event or tremor file stored, together with some useful parameters which characterize the event, i.e. date, Greenwich time, peak amplitude and wave integral.

The second file maintains the history of the recording station, keeping track of all powering on and off of the hardware, normal and abnormal starting and ending of the acquisition program, back-up information, status of the carrier wave, which is checked by means of the RS-232 interface. Tab. I shows a sample extract from each of these files. These reports are essential for a proper interpretation of the data when analyzing the time series of the events.

B2019175.EVN 20/02/1992 19:18:36 2.46 7.65

B2018392.TRM 20/02/1992 18:39:58 0.47 6.45

B2018503.EVN 20/02/1992 18:51:18 2.18 7.59

B2019051.EVN 20/02/1992 19:05:59 1.61 7.20

B2019095.EVN 20/02/1992 19:10:42 5.51 10.07

Program start 31/12/1991 14:16:23

Missing carrier wave 15/01/1992 06:36:10

Active carrier wave 16/01/1992 01:46:57

Backup n. 8 done 17/01/1992 18:11:14

Normal program end 17/01/1992 18:11:14

Tab. I - Sample report files

8. Data collection

Both files are saved with the sampled data into the streaming tapes used as back-up storage but, being their size quite limited, they are also transferred regularly to the off line computer, sited at the Seismological Laboratory of our Department in Udine, via a modem connection. That makes a prompt intervention possible in the presence of faults which would compromise the sampling continuity.

The modem connection, which can take place with the acquisition module remaining completely unaware, gives also access to the video memory of the remote PC and to all of its hard disk directories and files.

Obviously the dimension (38400 Byte) and the number (about 6 per hour) of the event and tremor files do not allow their transfer via modem to the off line computer at a reasonable cost. Fig. 5 shows the module which provides a back-up of those files to a streaming tape unit; the tapes are then shipped to our Department via regular mail. The capacity of the streaming tapes (150 MByte) allows for an autonomy of about 3 weeks between two successive changes.

9. Implementation

For what concerns the implementation, the language QuickBasic 4.5 (Microsoft, 1988) was extensively used, for which the library of routines to deal with the DAS board was already available (Metrabyte, 1988).

In order to eliminate the bottle-neck given by the extremely slow I/O Basic routines, which was in particular noted during the storing of the events onto the hard disk (in spite of the choice of the binary format, faster and more compact), new fast routines for reading and writing whole vectors have been written in Assembly language; only the interface was written in the C language, so as to assure an easy calling. The routines, which do not use any particular library, are then compatible with all programming languages in the MS-DOS environment, so leaving a complete freedom of choice in the developing of successive reading and processing programs. Moreover, the routines ask for a really minimal extra memory need, characteristic highly desirable in our context.

Due to the I/O optimization, a 80386 - 25 MHz computer now remains idle more than 40 seconds out of 60. This makes theoretically acceptable the lose of performance unavoidably suffered when passing from a monotasking environment such as MS-DOS to a multitasking one like MS-Windows. This environment in fact would allow us to run other concurrent processes, such as:

- modem handling

- back-up handling

- monitoring

- simultaneous triggering procedures

- particular analyses of tremor

Processes which need them can in fact access the 60 seconds records coming from the acquisition module PGM_WA thanks to a virtual (RAM) disk interface, being a real sharing of data between DOS and Windows processes actually not allowed.

The problems which we are now observing when the acquisition module is put in the background seem to be connected to the sampling frequency, equal to 320 Hz; in fact, such problems disappear at lower sampling frequencies.

The Windows environment results on the contrary completely satisfactory for what concerns the off line processing. The easy way of constructing visual interfaces offered by a language like Visual Basic, together with the efficiency of routines written in different Windows languages such as Turbo Pascal for Windows or the latest versions of Microsoft Fortran and C, has allowed the implementation of the following off line programs:

EVENTS_PLOT: the program allows a complete graphical presentation of the signals (both events and tremors) acquired by the seismic station, featuring zoom and pan operations on the three components and the time signal; data in the currently active window can be saved in a number of different formats (binary, ASCII, PlotCall) or examined by the computation of some additional parameters.

PRE-EXCEL: the program is used to create daily reports, in which files corresponding to events are separated from periodic tremor samplings; its output is directly importable into Microsoft Excel, which execution can also be handled by the program itself.

EVENTS_STAT: computes simple statistics regarding the time intervals separating successive events on the basis of the data stored in the report files.

10. Conclusions

With this upgrade the seismic station has reached its optimal functionality; therefore no additional major modifications to the hardware configuration are planned. For what concerns software, being the acquisition computer idle most of the time, we plan to add preprocessing routines to increase the number of computed statistical parameters.

The presence of three calibrated components together with an increased reliability allow for a greater significance of data which supports a better statistical characterization of the seismic volcanic activity of Stromboli (Beinat et al., 1988).

Acknowledgements

We thank C.N.R. - G.N.V. for the financial support and the volcano guides for their collaboration.

References

Allen, R. (1982): Automatic phase pickers: their present use and future prospects, Bull. Seism. Soc. of Amer., 72, 6, S225-S242

Beinat, A., Iacop, F., Riuscetti, M., Salemi, G. (1988): Caratterizzazione statistica dell'attività sismica di Stromboli, Atti VII Convegno CNR - GNGTS, Roma, Vol. III, 1471-1482

Beinat, A., Iacop, F., Riuscetti, M., Salemi, G. (1989): Studio della sismicità legata all'attività esplosiva di Stromboli, Boll. CNR - GNV, Vol. I, 47-53

Beinat, A., Iacop, F., Riuscetti, M., Salemi, G. (1990): Integrated PC-Workstation for geophysics monitoring: a seismic application on Stromboli volcano (Aeolian Islands, Italy), Eur. Seism. Comm. XXII General Assembly, Barcelona

Kulhánek, O. (1976): Introduction to digital filtering in geophysics, Elsevier

Metrabyte Co. (1988): DAS-8 and DAS-8 PGA user's manual

Microsoft Co. (1988): Microsoft QuickBASIC - Programming in BASIC

Riuscetti, M. (1994): Seismic activity of Stromboli as monitored by the summit station, Acta Vulc., 5, 207-210, 1994