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The M24 Modular Digital Seismograph System White Paper


This document describes actual and planned features of the new M24 Modular Digital Seismograph System. Technical details given in this document are preliminary and are subject to change without notice.

Why a modular digital seismograph?

Traditional digital seismographs attempt to cover the whole gamut of users’ requirements in an "all-in-one" approach, packing a huge amount of functionality into one box. While this appears a very attractive proposition at first sight it does have its drawbacks. Users expect system lifetimes in excess of five years (and have every right to do so); even the best attempts of anticipating technological advance as well as users’ growing needs will have to fail over such spans of time. A few years down the road, users will find themselves partially stuck with outdated technologies (e.g. in terms of mass storage) whereas other components of their systems are still perfectly viable.

A modular approach allows users to get the maximum return out of their investment over a long period of time. It is much easier to keep "in sync" with the radically different paces of innovation in various fields of technology. Case in point: Over the last five years (1994 through 1998), "standard hard disk size" (the typical size of a mainstream PC’s mass storage) has increased by a factor of well over ten. On the other hand, Crystal’s current family of 24-bit A/D converters was already available in 1994, and has not changed a great deal since. You may argue that it is not a big deal to swap out a small hard disk and replace it by a bigger one - true, but the changes in technology to be expected over the system’s lifetime may be much more deep-rooted than that. Case in point: The 8-bit SCSI interface is being phased out, high-speed serial interfaces such as USB 11 and FireWire (IEEE 1394) are going to replace it. Grafting such a change of paradigm into an all-in-one system is anywhere from ridiculously expensive to completely impossible. Now imagine a modular system where the digitiser(s) and the field recorders) are loosely coupled. You can preserve your investment in the digitiser and just replace the recorder, thus bringing your system back in line with the latest technological developments at a moderate cost

Recent publications by large scale users (such as PASSCAL) point in the same direction. Modularity is a key element in future instrument designs.

Based on these considerations we have decided to split up the M24 system into a number of reasonably autonomous functional groups. In the next chapter we outline these elements in more detail.

M24: System concept

M24 implements a three-tiered architecture, with multiple choices for interconnecting elements from different layers. For example, to telemeter data to a central site, a dedicated one-way digital radio line may be used, or a field PC could be used to connect the M24 Digitiser(s) to wide-area networking services. The choice is up to the user. Pick whatever is most appropriate for your particular scenario!

Interconnection options are generally non-exclusive; for example, an M24 Digitiser may be connected to a digital telemetry line in parallel to an M24 Field Recorder.

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The Elements of the M24 System

A modular system has the advantage of offering not only a choice of configurations in terms of functionality, but also in terms of quality. For example, a digitiser is expected to have low power consumption as well as high resolution and dynamic range. These requirements are to some extent mutually exclusive. Hence, it is perfectly well imaginable to have a high-quality digitiser which draws comparatively more power, and a low-power digitiser providing a bit less data quality.

A three-channel low-power digitiser is the first available M24 component, and will soon be followed by a field recorder which can accomodate up to two digitisers, thus offering 3- or 6-channel recording capacity.

Further planned additions include a one-way telemetry system and a software package for a Unix-based PC for data acquisition and transmission over a variety of communication channels such as dial-up phone lines, ISDN, and Internet connections.

It is important to understand that (as briefly mentioned before) these components are not mutually exclusive! The serial data stream coming from the digitiser(s) may be fed into several subsequent components simultaneously. For example, it is possible to use the field recorder box for continuous recording, and at the same time to hook up a Unix PC to communicate the data across the Internet.

The M24 Low Power Digitiser

The first and most obvious element in an M24 system is the digitiser, responsible for converting the seismometer’s analog output voltage into a series of digital words.

As shown in the sketch, there are three preamplifiers (the triangle symbols), three A/D converters (made by Crystal Semiconductor), and a Digital Signal Processor (DSP). The latter is responsible for downsampling the fast data stream coming from the A/D converters, applying a digital anti-alias lowpass filter along the way. Finally, data is formatted for stream output through a serial interface. The serial interface can be switched between RS-232 mode (useful for direct connection to a PC serial port) and RS-422. The latter mode of operation is very useful to drive long cables. RS-422 is a much more robust solution than RS-232 in this case.

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Note that, while the digitiser does provide a synchronisation input, it does not enforce the use of a time signal receiver. We will describe the reasons for this choice in the next chapter.

Why does the Digitiser not include a time signal receiver?

This choice has been made quite deliberately. In order to keep the structure of the Digitiser simple, and to provide a flexible choice of connection options, it was decided that the Digitiser should provide streaming serial output as opposed to packetized, time-stamped output.

The Digitiser does not buffer data in any way. Except for a fixed delay incurred by digital signal processing, data is output in (very near) real time.

Not providing any buffering or time delay makes it a lot easier, for example, to interface the Digitiser to off-the-shelf digital telemetry systems with a serial interface.

In order to keep several independent digitisers synchronised to a common time base, there is a SYNC IN port which can optionally be used.

When and where does time stamping occur, then?

Time stamping and packetizing occurs when data is formatted for recording. This could be in the Field Recorder, or, in the case of a telemetry system, at the time of writing data to a PC’s hard disk. Further details will be given later in this document, when the individual options will be discussed.

What’s under the hood

When we started the design cycle for M24 we basically started from a clean slate. While compatibility with existing instrumentation (e.g. the MARS-88 and MRSlite family) is certainly a goal worth considering, it was not to be achieved at the cost of using yesterday’s technology. Thus, the hardware has been designed around the latest and most up-to-date processor components and technologies. Care for a few appetisers? No more separation between "classic" CPU and DSP (Digital Signal Processor). One single ARM processor provides enough muscle to do all the DSP functionality (digital filtering, decimation, etc.) required. On the side, it runs the operating system, does data formatting and all the other chores normally assigned to a "usual’ CPU. And it does all that at a remarkably low power consumption.

If you’ve been thinking: "ARM?? Never heard of that! Must be an utterly exotic processor... - rest assured. We haven’t picked a lemon here. ARM (which stands for Advanced RISC Machines) processors are found in a wide variety of high technology products: Portable GSM phones (Sony, Kyocera, Ericsson, NEC, Panasonic .... ), handheld PCs (PSION, HP Jornada), consumer set-top boxes, and even game consoles (SEGA Dreamcast)! So, chances are you’ve been in touch with an ARM powered product lately. ARM processors are known to provide the best power-to-MIPS ratio, i.e. they offer more processing throughput per milliwatt than competing processor designs.

But the processor is not the only area of innovation. All circuitry uses modern 3.3 V technology - the same technology you’re likely to find on today’s PC motherboards.

The M24 Field Recorder

This is an autonomous instrument designed to interface with one or two M24 Digitisers (to form a three- or six-channel unit respectively) and to provide timestamping and recording services.

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Those familiar with the current-generation MARSlite instrument can consider the M24 Field Recorder a stripped-down MARSlite; there is obviously no analog input section, nor is there a lot in the way of a user interface.

M24 Field PC

To drastically expand the potential of the M24 system, strong PC orientation has been designed in. A great deal of the functionality that had to be hardcoded and hardwired into previous generations of seismological field instruments is already provided in a standard PC - only that an ordinary PC is not too suitable for typical field use.)

Single-board PCs have been around for quite a while now, but until very recently they lacked the resources required to run ‘real operating systems" (DOS doesn’t quite fall into that category).

That situation has changed, though. You may have heard about the "matchbox PC’ - the world’s smallest web server. This is a 486 with 16 MB RAM, running Red Hat Linux and a stripped-down Apache web server:

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N.B. Lennartz can not be held responsible for the spelling on this fine Russian collector’s item :-)

So, the technology for providing on-site computer resources is in place. Putting a small, low-power PC next to the Digitiser will of course provide a wealth of telecommunication options. For example, such a setup provides Internet connectivity ‘for the rest of us’. While previously only large-scale institutions could afford to make their data available on the Web, an M24 Digitiser and a small PC can now do the job.

Some example M24 configurations

This chaper shows some possibilities for configuring M24 systems. We will use the following symbols to denote M24 elements.

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M24 Digitiser (three analog inputs)

M24 Filed Recorder (two digital inputs)

PC running a Unix variant (FreeBSD or Linux)

All other elements will be annotated. Sensors will not be shown in the sketches.

Field station, local recording

Extremely simple: One or two Digitsers coupled to a Field Recorder

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The time signal receiver is not shown here, but would be connected to the Field Recorder. Such a setup is functionally equivalent to a MARSlite system; unlike MARSlite, however, it also supports a six-channel configuration.

A simple telemetry setup

While two-way digital telemetry is certainly a technically superior alternative, and is the preferable choice for long-term deployment networks, good old one-way continuous transmission telemetry offers one advantage that may be the "make or break’ point in an aftershock recording scenario: Speed. Speed in getting a network up and running. Plus speed in transferring data. Not requiring a back channel, one-way telemetry can squeeze the last bit of transmission capacity out of a given amount of bandwidth. For example, modern spread spectrum transceivers operate at 9,600 baud in full-duplex mode whereas in half-duplex mode, 14,400 baud are achieved (in our setup, we do not even require half duplex; simplex is enough).

With the recent advent of very powerful laptop PCs, and the M24 system’s support for PC-based Unix variants, the central station of a telemetry network can actually reside in your briefcase...

Off-the-shelf telemetry systems offering an M24 compatible RS-232 interface are available in several varieties (fixed frequency or spread spectrum). So, here is an example for a three-station network with a PC-based central station:

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Of course, owing to the modular nature of the M24 system, the two examples given so far can also be combined into one, i.e. into a system providing local recording as well as digital radio telemetry, as shown in the next sketch.

Local recording plus one-way radio telemetry

This system is functionally equivalent to the PCM 5800 system which, in its day, used to be a very popular choice for stationary and mobile setups of this kind. Of course, the storage capacity afforded by PCM 5800 recording media (a four-track NAGRA recorder with 18 cm reels would hold the equivalent of a whopping 64 megabytes!) is no match for today’s multi-gigabyte capacity.. And of course, the data quality of M24 is vastly superior to the 5800 system while maintaining a similar, equally compact, transmission and recording format.

Local recording adds redundancy to the whole process, in case of telemetry outages, data is still preserved on local recording media.

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We have reduced the previous example to a single-station configuration for clarity’s sake, but of course the number of stations is limited only by the number of serial ports available at the central site, and the number of telemetry frequencies that can be allotted to the network.

Note that the serial data stream coming from the M24 Digitiser can be simply split up in two; this is only possible because it’s a simple one-way communication path.

Versatile Field Station with Local PC

This setup offers the utmost flexibility. Making use of industry-standard components to the largest extent possible, this configuration is powerful enough to satisfy today’s requirements and provides enough expansion capabilities to carry it well into the new millennium.

A particularly attractive prospect is the use of GSM and satellite-based transmission capabilites. Several companies offer GSM-based ‘modems". Since GSM bandwidth limits data transfer speed to 9,600 baud, only selected waveform segments can be transmitted. On the other hand, GSM’s Short Message Service (SMS) offers fantastic possibilities. One attractive scenario involves the use of one or two M24 Digitisers, a small PC (see page 6 for an example) with a local hard disk used as a ring buffer for several day’s worth of data, and a GSM modem. If the PC detects an event it would transmit a message to that effect to a central station via SMS. A few descriptive parameters of the waveform (e.g. maximum amplitude, onset time, predominant frequency) can be included in the message.

The central station can then implement a coincidence trigger mechanism and request specific waveform segments to be transmitted, using the full-bandwidth GSM data channel. This would be an ideal configuration for a strong motion network. A near-real-time alert channel is provided by SMS; the limited bandwidth of the GSM data channel is no real obstacle because the expected overall data rate is very low.

Similar configurations can be built around the PC with other communication media, e.g. satellite-base transmission. Or, you could set up the PC to provide basic Internet server capabilities, making your station’s waveforms accessible from anywhere in the world. Since the PC ‘shields" the Digitiser hardware and modem software from whatever communication backend you decide to use, changes and/or improvements in communication technology can be very quickly and conveniently implemented on the PC platform.

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The M24 system, in conjunction with industry-standard hardware and software, offers you access to the latest achievements in global communication.