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SeismometersGeneral Description

Document revised: July 2011 / February 2009 / June 2008 / June 2007 /...

Latest revision: add new models LE-1DV/5s, LE-3D/BH, LE-3D/BHs. Previously: Added Accessories; updated pole/zero info for 3Dlite/1DV and 5s. Added information on LE-3Dlite MkII. Removed “for further information” page. Updated poles/zeros information for LE-3D/20s, added new countries, updated number of instruments.

ContentsIntroduction .......................................................................................1History .................................................................................................1Track record ........................................................................................ 2Available models .............................................................................. 2

Scientific vs. Engineering models ........................................................................ 2The LE-xD 1 Hz models (surface) .................................................... 3

Technical details for the LE-xD 1 Hz seismometers ........................................ 4The LE-3D/BH downhole seismometers ........................................ 5The LE-xD/5s models......................................................................... 6

LE-3D/5s ........................................................................................................................6LE-1DV/5s ..................................................................................................................... 7Technical details for the LE-xD/5s seismometers ............................................ 7

The LE-3D/20s model ........................................................................ 8Technical details for the LE-3D/20s seismometer ........................................... 9

The LE-xD DIN compliant sensors ................................................ 10Technical details for LE-xD DIN compliant sensors ......................................11

Principle of operation .................................................................... 12The “inverse filter” method ....................................................................................12Feedback sensors .....................................................................................................12The Lippmann method ..........................................................................................12

Connecting with a datalogger .....................................................14Maximum cable lengths ........................................................................................15Extension cables ......................................................................................................15Lightning protection ..............................................................................................15

Frequently Asked Questions (FAQ) .............................................16Calibration coil ......................................................................................................... 16Factory recalibration ...............................................................................................17Huddle test ................................................................................................................ 18Waiting time after power-up .............................................................................. 18No 12 V DC power supply available? ................................................................19Electronic noise ........................................................................................................19

Accessories ....................................................................................... 20CT-EW1 Calibration Table ......................................................................................20STS-1 (and other BB sensors) Warp-Free Baseplate .....................................20STU Shielding: Gabbro Plate and Hood for STS-2 ........................................21

Where are Lennartz sensors in use? ............................................ 22

Lennartz electronic GmbH Bismarckstrasse 136 D-72072 Tübingen GermanyPhone: +49-7071-93550 Fax: +49-7071-935530 [email protected] www.lennartz-electronic.de

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IntroductionThis document gives you a comprehensive overview about the entire Lennartz Seismo meters product range. You will find useful background information and lots of technical detail here! If you have any further questions or comments, please do not hesitate to contact us at [email protected] .

HistoryHow did Lennartz get started in seismometers? Until the early eighties, we would usually resell somebody else’s seismometers (M**k Products, G**tech, K***metrics - you name it). In early 1983, during the annual meeting of the German Geophysical Society in Aachen, we heard the following presentation:

“A simple method for expanding the measure-ment range of electro-dynamic seismo meters”

The method presented was indeed simple, but efficient. We got in touch with Erich Lippmann, and soon ended up with an agreement that would grant us the exclusive right of exploitation of his patent:

There were still some detail problems to be solved, but after a few iterations the first LE-3D seismo-meter, the LE-3D “classic”, was introduced.


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Track recordAt the time of going to print (July 2011), about 11,000 channels of LE-xD seismometers have shipped. Infant mortality rate is very low, long-term stability has proven to be very high (read more about this topic in the FAQ [Frequently Asked Questions] section of this document).

Available modelsThe Lennartz family of seismometers is broadly subdivided into two branches:

• The “scientific” branch (very high resolution, very good low frequency fidelity, limited high frequency response)

• The “engineering” branch (very high clip level, extremely wide high frequency band up to 315 Hz, limited resolution, limited low frequency response)

Within either branch there are several models; some come in 1- and 3-component versions:

Scientific vs. Engineering modelsThe “scientific” instrument line generally puts more emphasis on sensitivity and low-frequency fidel-ity. On the other hand, the “engineering” line has a higher clip level, rendering these instruments more suitable for structural work where higher amplitudes are to be expected. Also, the “engineering” sensors have a switchable upper frequency limit which extends up to 315 Hz, whereas the highest frequency limit for the “scientific” line is 100 Hz.

For the “engineering” product line, there is no choice of eigenfrequency available. This is so because DIN 45669 defines the requirements for a compliant sensor very strictly, and leaves no room for discussion here.

The “scientific” line offers a lot more possibilities in this respect: 1 Hz, 5 seconds, and 20 seconds.


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The LE-xD 1 Hz models (surface)These are the workhorses of the scientific product line. Originally available in a blue rectangular housing (known for a long time as the “LE-3D classic”, now out of production, shown below left), hundreds of these sensors are in daily duty all over the world. The later version of the same model (shown below center) comes in a cylindrical stainless steel housing and is called LE-3Dlite. Addition-ally, a vertical single component version is available, aptly named LE-1DV. The original LE-3Dlite design as shown in the small photo below has recently gone out of production (after more than 1,000 units) and has been replaced by LE-3Dlite MkII which has a female KPTC connector instead of the permanently attached cable. The same goes for LE-1DV MkII (introduced early 2008).

Users are attracted by these unique features that set these sensors miles apart from their mechanical predecessors:

• Extremely compact and lightweight

• Rugged

• Sheer simplicity; true plug-and-play operation

• All sensors calibrated to identical output voltages; no need to keep records of which sensor has been connected to which datalogger

• Low noise, low power (typically 3 mA per component @ 12 V DC)

• Dynamic range > 120 dB

• Comparatively insensitive to improper leveling

• No mass lock, mass center, control box, or other contraption required; can be transported in any orientation

• Proven long-term stability; over fifteen years of field experience

top: LE-3D “classic“

right: LE-3Dlite, LE-1DV

top: LE-3Dlite MkII


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Technical details for the LE-xD 1 Hz seismometersThe following table shows the relevant technical parameters for both the single and the three com-ponent versions of Lennartz 1 Hz surface seismometers:

The “Upper corner frequency” row denotes the frequency up to which the sensors have been tested on a shake table. Users should be aware that there is an inherent low pass filter in the sensor’s circuitry which will gradually damp away frequencies higher than the upper corner frequency.

LE-1DV MkII LE-3Dlite MkII

LE-3D/BH(s)

Power Supply 10…16 V DC, unstabilized

Power Consumption 3 mA @ 12 V DC 8 mA @ 12 V DC

Output Voltage 400 V/m/s, precisely adjusted 400 V/m/s, precisely adjusted on all components

Damping 0.707 critical (internal damping; independent of datalogger input resistance)

Dimensions 85 mm diameter 55 mm height

95 mm diameter 65 mm height

Borehole: 58 mm diameter, 1000 mm overall length

Weight 1.1 kg 1.6 kg Borehole: 4.9 kg (without

cable)

Temperature Range -15 … +60 °C

Housing Stainless steel, matted sur-face, splash proof, with level

adjustment feet and water bubble level control

Stainless steel, matted surface, splash proof, with level adjustment feet and

water bubble level control

Borehole: completely water-proof stainless steel

Eigenfrequency 1 Hz

Upper Corner Frequency 100 Hz

RMS Noise @ 1 Hz < 3 nm/s

Dynamic Range (typical) 136 dB

Poles 3 poles: –4.44 / +4.44j –4.44 / –4.44j –1.083 / 0.0j

Zeroes Triple zero at the origin


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The LE-3D/BH downhole seismometersAn exact copy of the LE-3Dlite, but in a sleek downhole package, intended for use at depths up to 500 meters (see limitations below). The borehole sensor comes in two versions:

• LE-3D/BH including upper clamping mechanism, azimuthal adjustment dial, spear tip, and hole lock (shown in the left photo, just prior to deployment)

• LE-3D/BHs for installation in sand – simple cylindrical housing without clamping mechanism, no hole lock supplied, slightly angled conical tip (photo right)

Photo courtesy of Kyushu University

The LE-3D/BHs sensor is comparatively undemanding in terms of installation requirements. Since there is no hole lock to install, there is no absolute need for a cased borehole (although an uncased borehole may make it somewhat difficult to calculate the amount of sand required for filling up the space between the borehole wall and the sensor housing.

Also, since there is no hole lock to install and, consequently, no downhole orientation to be determined, the actual sensor installation is almost trivial, and does not require the presence of Lennartz staff.

For the LE-3D/BH sensor, on the other hand, there are a number of prerequisites that need to be fulfilled for successful installation and operation:

• cased borehole, preferably PVC casing (see below)• if hole lock cannot be installed beforehand (i.e. before the casing is deployed), depth less than

200 meters, and presence of Lennartz staff required• if azimuthal orientation is to be determined by downhole measurement (as opposed to surface

measurement), casing needs to be non-magnetic so that a magnetic compass can be used

For more information, please contact us, providing as much detail as possible about your existing or planned borehole installation.


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The LE-xD/5s modelsLE-3D/5sThis instrument is the runner-up in terms of popularity, providing flat velocity response down to 5 seconds while still preserving basic fieldworthy qualities such as ruggedness and portability. It quickly gained acceptance among those requiring a little bit of extra low frequency response. Recently, a

single-component vertical version has been requested by GFZ Potsdam to be used in site evaluation studies for the GITEWS Tsunami Early Warning System (see next page for details on the 1D version).

LE-3D/5s is the sensor of choice for a wide variety of field and observatory tasks – being still portable, it will allow you to go places where a broadband sensor just won’t do the job. If space and weight constraints prevail, LE-3D/5s will help you meet them. It is not only the dimensions and weight of the sensor proper that count – do not forget to take power requirements into account. In this department, the LE-3D/5s leaves broadband seismometers in the dust. At only 100 mW, LE-3D/5s won’t require its own truck battery even for a long-term deployment. As an example, a 20 Ah battery (which is still easily handled) will last up to 100 days!

But do not believe that this noteworthy power consciousness will incur a significant performance penalty – the noise level of this sensor (approx. 1 nm/s RMS at 1 Hz) is such that it can be used in most low-noise sites without problems.

• Compact and lightweight, rugged construction• Sheer simplicity; true plug-and-play operation• All sensors calibrated to identical output voltages; no need to keep records of which sensor

has been connected to which datalogger• Low noise, low power (typically 3 mA per component @ 12 V DC)• Dynamic range > 120 dB• Comparatively insensitive to improper leveling; no mass lock, mass center, control box, or

other contraption required; can be transported in any orientation• Proven long-term stability; over twenty years of field experience

Thinking Nakamura? LE-3D/5s is the ideal sensor:»The accelerometers were in general very poor, and in some cases not sensitive enough. The epis**sor, which should have been very good, was unstable and therefore very poor at low frequencies. The Lennartz 5sec sensors were the best performing in terms of the frequency range and sensitivity.«Quoted from: INFLUENCE OF INSTRUMENTS ON H/V SPECTRAL RATIO OF AMBIENT NOISE (SCF-3-01-P, Poster presented at XXVIII ESC General Assembly, Genova, 2002)B. Guillier (1), K. Atakan (2), A-M. Duval (3), M. Ohrnberger (4), R. Azzara (5), F. Cara (5), J. Havskov (2), G. Alguacil (6), P. Teves-Costa (7), Nikos Theodulidis (8) and the SESAME Project WP02-Team.(1) LGIT, Observatoire de Grenoble, BP 53 - 38041 Grenoble Cedex - France, (2) UiB, Bergen, Norway, (3) CETE, Nice, France, (4) IGUP, Potsdam, Germany, (5) INGV, Rome, Italy, (6) UG, Granada, Spain, (7) CGUL, Lisbon, Portugal, (8) ITSAK, Thessaloniki, Greece


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Technical details for the LE-xD/5s seismometers


LE-1DV/5s LE-3D/5s


Power Consumption 3 mA @ 12 V DC 10 mA @ 12 V DC

Output Voltage 400 V/m/s 400 V/m/s, precisely adjusted on all components

Damping 0.707 critical (internal damping; independent of datalog-ger input resistance)


195 mm diameter 165 mm height

Weight 2.5 kg 6.5 kg


Housing Matted stainless steel, splash proof, with level


Painted alumin(i)um, splash proof, with level


Eigenfrequency 0.2 Hz

Upper Corner Fre-quency

50 Hz


Dynamic Range (typical)

140 dB

Poles 3 poles: –0.888 / +0.888j –0.888 / –0.888j –0.290 / 0.000j


LE-1DV/5sThe LE-1DV/5s seismometer comes in a stainless steel hous-ing similar to LE-1DV (see photo). Other than the different housing and having just one vertical component it is for all practical purposes identical to the LE-3D/5s.


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The LE-3D/20s modelThis is the latest addition to the product line, further boosting low frequency response down to 20 seconds. For those interested in “classical” regional earthquake seismology, or in the low frequency grumblings of volcanoes, this instrument fills a niche between short period and “real broadband”.

While physically identical to LE-3D/5s (except for the different paint colour), it is a fundamentally different instrument under the hood. In order to bring a physical 2 Hz electrodynamic geophone to behave like a 20 sec seismometer (remember those clumsy yet delicate beasts? You barely touched them, and the mass would shift out of whack!), considerable electronic wizardry needs to be applied. To some extent, this comes at a price – increased power consumption (but still within very reasonable limits). However, many of the qualities that have made Lennartz seismometers the practitioner’s favourite for more than a decade are present in this instrument. Compared to “true broadband” sensors, the most salient advantages are much faster startup time (the instrument will output meaningful results in less than a minute after power-up!) and drasti-cally reduced sensitivity to temperature or pressure fluctuations. Since there is no physical long-period element inside, nothing will respond to daily tempera-ture drift, or to the sudden air draught of a door being shut.

For the record, if anyone is interested in learning what a “real broadband” installation entails, this is a very valuable resource of information:

http://www.seismo.berkeley.edu/seismo/bdsn/instrumentation/guidelines.html

If you don’t want to read the whole article, here is one paragraph that sums it up:

☞ “At minimum, it takes 500 man-hours to search, plan, travel, prefabricate, and install a site. If the distances are great, or construction is complex, the time requirement can easily go to 2000 man-hours. For example, we spent approximately 1500 man-hours on KCC (sited in an existing tunnel), 2000 man-hours on HOPS (because of vault construction), and we spent 1300 man-hours on FARB (our most inaccessible station).“

Basically, this means that unless you have that kind of time and resources to invest (and who does, especially in a mobile scenario??), you won’t get anywhere near the full data quality that a broadband sensor can theoretically deliver. So, the advantages of having a simple, easy to use, quickly up-and-running instrument may very well compensate for the fact that it isn’t a “true broadband” sensor on paper.


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Technical details for the LE-3D/20s seismometer


LE-3D/20s


Power Consumption Average 50 mA @ 12 V DC; max. 100 mA (for full scale output)

Output Voltage 1000 V/m/s, precisely adjusted on all components

Damping 0.707 critical (internal damping; independent of datalogger input

resistance)


Weight 6.5 kg


Housing Painted alumin(i)um, splash proof, with level adjustment feet

and water bubble level control

Eigenfrequency 0.05 Hz

Upper Corner Frequency > 40 Hz


Dynamic Range (typical) 136 dB

Poles 3 poles:–0.222 / +0.222j–0.222 / –0.222j–0.23 / 0.000j



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The LE-xD DIN compliant sensorsAs explained previously, Lennartz makes a line of sensors used predominantly in a typical engineering scenario. For vibration measurement equipment to be used in buildings, there is a DIN (Deutsches Institut für Normung; German Norm Institute) regulation called DIN 45669. Unfortunately, copy-right restrictions do not allow us to reproduce the DIN 45669 specifications here. In a nutshell, though, these requirements deal more with high frequency issues than with low frequency and low noise problems. In other words, the standard “scientific” Lennartz sensors would simply not comply.

DIN compliant sensors are available in 3D and 1D vertical versions. They all come in the same rectangular housing depicted below. The housing can be bolted to the structure under test with M6 screws. An optional base plate with adjustable hardened steel tip feet, manufactured in compliance with DIN 45669, is also available.

In order to fulfill the DIN 45669 requirements, these sensors have a switch-selectable upper frequency limit of 80 or 315 Hz; eigenfrequency is 1 Hz. All sensors are guaranteed to be free of spurious resonances up to the selected upper frequency limit.

As you will readily see from the table specifying the technical details in the next chapter, the DIN sensors do not really compete with the “scientific” product line. Their sensitivity is much lower (rendering them much less suitable for earthquake work), but they have a higher clip level and a higher upper frequency limit.

In accordance with DIN 45669, each sensor undergoes a rigorous shake table test. A Calibration Certificate is included. Factory recalibration service is available (see the FAQ [Frequently Asked Questions] chapter of this document for details).


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Technical details for LE-xD DIN compliant sensors

LE-1DV/DIN, LE-3D/DIN


Power Consumption 5 mA @ 12 V DC (1D model), 15 mA (3D model)

Output Voltage 100 V/m/s (certified at 16 Hz), precisely adjusted on all compo-nents

Maximum unclipped ground velocity

± 50 mm/s

Minimum resolvable ground velocity

Significantly less than ±0.05 mm/s (±0.05 mm/s is the threshold stipulated by DIN 45669)

Damping 0.707 critical (internal damping; independent of datalogger input resistance)

Dimensions (identi-cal for 1D and 3D models)

75 mm width (+20 mm for the connector) 55 mm height (+12 mm for the bubble level)

120 mm depth

Weight 930 g (without optional base plate)


Housing Painted alumin(i)um, protection class IP 65, water bubble level control

Eigenfrequency 1 Hz

Upper Corner Fre-quency

switch selectable: 80 or 315 Hz


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Principle of operationThis description applies to all Lennartz seismometers except the DIN compliant sensors, i.e. the whole “scientific line”.

The “inverse filter” methodThere have been many attempts in the past to compensate for the well-known low frequency rolloff of typical electrodynamic seismometers. Early attempts would try to achieve this by a so-called inverse filter, i.e. an amplifier that selectively enhances the low-frequency part of the spectrum, providing progressively higher amplification for lower frequencies. As a matter of fact, this is how the “engi-neering line” works – there is nothing inherently wrong with this principle, but it does have some drawbacks that get in the way for high precision, low noise work.

Feedback sensorsAnother possibility for enhancing low frequency performance is to apply feedback to the suspended mass. This method exploits a well known measurement method that is also used in many other dis-ciplines: rather than measuring the entity itself, measure an ancillary entity that is required to keep the system in a steady state. In the case of feedback sensors, a force is generated that keeps the mass steady. To apply this force, we can use either the normal geophone coil (in which case we need a second element, e.g. a capacitive sensor, that tells us the exact position of the mass), or we need to use a sensor with a normal pickup coil and a calibration coil. In the latter case, the calibration coil can be used to generate an EMF (electromotoric force) that keeps the mass steady.

Most broadband sensors operate on this principle. It has got a number of advantages; most promi-nently, the fact that the mass remains steady means that (at least in a first-order approximation) all nonlinearities of the suspension spring are irrelevant – after all, the spring should theoretically never be stressed and strained.

One obvious disadvantage of the feedback method is that it requires certain sensor properties that are not easily found in standard, off-the-shelf products such as geophones used for exploration seismo-logy. Sensors that can be used with the feedback method tend to be largish or delicate or expensive or any combination thereof :-)

The Lippmann methodLennartz sensors operate on a “kind of” feedback principle, keeping the advantages (avoid the non-linearities) while circumventing the disadvantages (it does not require special or modified sensors).

To explain the principle, let us start from a standard electrodynamic moving coil geophone. Such sensors are in fact used in all Lennartz seismometers. It is textbook knowledge that the transfer func-tion of such a geophone depends on the damping resistor applied across its output. With no damp-ing at all (i.e. an infinite resistance), it will show a more or less pronounced resonance peak. With a damping resistor applied, the transfer function will approach the desired shape (denoted 0.707 in the graph below) with a flat response above the eigenfrequency (f0), and a rolloff proportional to f 2 below f0 (note that we use frequency ( f ) instead of angular frequency (ω) in the text).

When the value of the damping resistor is further decreased we approach critical damping (damp-ing constant of 1.0). For higher damping constants, the response curve becomes even flatter. We


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can see in the graph that for damping factor values > 1, the transfer function will eventually reach a completely different shape, namely proportional to f. Such high damping constants, however, can not be achieved by “real” damping resistors; even a damping resistor of zero (a shorted sensor output) will not achieve this. Only a negative resistor will do! This is exactly what happens inside the seismometer. One could say that the negative resistor feeds back the sensor through the same pins that are used for signal output – a feedback sensor without an extra feedback coil.

After this first step we arrive at the orange curve, proportional to f. Since the graph shows the response with respect to ground velocity, the orange straight line effectively describes a response that is flat to acceleration. While this may be a merit in its own right, it is definitely not what we want. What we need to do now is to convert the acceleration propor-tional output back to velocity, shifting the eigenfrequency in the process. Since our transfer function after this first step

is proportional to frequency, we now need an element which is proportional to f below the new f0 , and proportional to 1/f above it.

This is easily achieved by implementing a simple RC-type bandpass filter whose center frequency becomes the new “virtual” f0 of the enhanced sensor:

In the example shown here, the “virtual” f0 is one fifth of the original f0. This is not too far from reality; for the Lennartz 1 Hz sensors, the original f0 is 4.5 Hz.

Summing up the characteristic traits of the Lippmann method, we can see the following points in its favour:

• Can be used with inexpensive and, even more importantly, ROBUST exploration grade sen-sors

• Suppresses nonlinearities of sensor springs equally well as “conventional” feedback princip-les

• Extremely power conscious• Low sensitivity to environmental effects (temperature drift, air pressure changes)

0.7070.999

> 1.0

Eigenfrequency of mechanical sensor

0.01 0.1 1 10 ω/ω0

∼ω1

∼ω0

∼ω2

of transfer function

Asymptotic

al behavio

ur

Result of step 1

∼ω+1

∼ω+1∼ω −1

Eigenfrequency of electronically enhanced sensor

0.01 0.1 1 10 ω/ω0

∼ω0

∼ω+2

Result of step 1

Bandpass filter

Final result


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Connecting with a dataloggerThis chapter deals with the various cabling options available for connecting a Lennartz sensor to a third-party data acquisition system. Needless to say, there are cables available that fit all Lennartz dataloggers perfectly. However, the sensors can be used with pretty much any type of datalogger available. Even if your datalogger does not provide 12 V DC output, there is a solution available.

The following sketch shows the standard cables available for connecting Lennartz sensors to a Lennartz datalogger (MARS-88, MARSlite, or M24), to older Reftek loggers (using a PU77/U connector), and finally to an arbitrary “brand XYZ” datalogger.

We can also supply custom-made cables for Reftek 130 and Nanometrics Taurus dataloggers (not shown in the drawing). Please inquire.

All “third party” cables have an open end which you can use to fit the respective connector. At the other end there is a Cannon KPT connector; this connector family is used on all current Lennartz sensors. The LE-1DV (not MkII) and LE-3Dlite (not MkII) are the only sensors that have a short length of cable permanently attached (but both models are out of production now); all other sensors require an extra cable in any case.

The cables and reference numbers shown below are not exhaustive. Different types and lengths of cable can be made to order. When in doubt, please do not hesitate to ask. Please do also refer to the following subchapter that deals with maximum cable lengths.

LE-3Dlite MkIILE-1DV MkIILE-1DV/5sLE-3D/5s

LE-3D/20sAll DIN compliant

models

LE-3Dlite (not MkII)LE-1DV (not MkII)

MARS-88MARSlite

M24

Brand XYZ(fit your own connector)

dire

ct c

onne

ctio

n po

ssib

le (a

ppro

x. 1

.5 m

)

352-

0020

(5 m

)35

2-00

58 (1

0 m

)35

2-00

65 (3

0 m

) 352-0042 (5 m)352-0064 (10 m)

390-0056 (5 m)

390-0057

352-0025

390-

0058

b (5

m)

390-

0058

(10

m)

390-

0058

a (5

0 m

)

352-0097b (10 m)

RefTek50 m

278-0008a


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Maximum cable lengthsSince all Lennartz sensors are active sensors with a built-in preamplifier and low-impedance output, driving long cables is far less of a problem than with conventional mechanical sensors. Cable lengths up to 200 m have been tested with no ill effects; longer lengths may be possible but are currently not offered as a standard.

If you intend to use cable lengths in excess of 10 m, please be sure to read the subchapter on light-ning protection below!

Extension cablesWe make cable drums containing 50 m of fully shielded cable with suitable Cannon KPT connectors at either end. Multiple cable drums can be chained together to obtain longer cables.

Since 50 m is the maximum length of cable that can be conveniently fit on a hand-carried drum, longer lengths are not available on drums. A 100 m ring of cable (again with KPT connectors) is available. If more than 100 m are desired, you should use more than one ring. In excess of 100 m, the cable tends to become quite unwieldy.

A special cable version that can be used ONLY with LE-1DV or LE-1DV/5s is also available. Due to the reduced number of wires, this cable has a smaller diameter and is more flexible. Hence, about double the amount of the standard 3D cable can be fit onto a given cable reel.

Lightning protectionAs you certainly are aware, long seismometer cables make excellent antennae to pick up lightning induced overvoltage. It is therefore strongly recommended to put a lightning protection box at either end of a long cable (just how long is “long”? That depends on many factors, but more than 10 m is a safe bet!). Lennartz makes such boxes, providing plug compatibility with the connectors used on seismometers and cables. Of course, for the lightning protection to be efficient, the user will need to provide a good solid earth connection.

The perceived savings from not using a lightning protection box will literally go up in smoke sooner or later... you have been warned!


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Frequently Asked Questions (FAQ)Over the years, certain questions tend to be asked again and again – these are the most popular ones, given here in no particular order.

Calibration coil

Q What about calibration? You mention elsewhere that the sensors do not have a calibration coil. How can I be sure that the sensor works at all, and that its characteristics

have not changed over time?

A It is true that the sensors do not have a calibration coil. Instead, an electronic facility is provided. While not functionally equivalent to a calibration coil, this facil-

ity closely reproduces the classical “weight lift” test in that it applies two pulse signals so that the mass will perform a significant excursion both in the positive and in the negative direction. Besides being useful as a simple “go / no-go” test, the response of the sensor to this pulse train could be used for long-term monitoring of the sensor’s behaviour. However, classical methods (such as trying to deduce the sensor’s damping constant from the output signal) should not be applied. The following graph shows the response of an LE-3D/20s to the calibration pulse sequence (which can be initiated by simply pulling the -CAL pin to ground). The two pulses are spaced far enough apart to allow the sensor to completely settle back to baseline in between. For the 5 sec and 1 Hz seismometers, the two pulses are less distant.

As can be seen in the lower trace (a magnified rendition of one of the horizontal channels whereas the top trace is the vertical channel), the calibration pulses are superimposed on the normal output signal (in this case, plain noise in a busy laboratory).


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Factory recalibration

Q If I can’t recalibrate the sensor myself, what is the factory recommendation for a reasonable recalibration interval? How is it done? What is the approximate cost?

A This is not a simple question to answer. For the DIN compliant sensors, the answer is easy: Factory recalibration requirements are dictated by the DIN regulations. At the

time of this writing, DIN 45669 requires periodic recalibration every three years or after any major incident (repair / damage). For the “scientific” sensors, the (somewhat unsatisfactory) answer is “it depends”. Before shipping, each sensor undergoes an individual test and calibration cycle on Lennartz’s own shake table. Shown below: One horizontal 5 sec sensor attached to the shake table harness, and the round printed circuit board containing the support circuitry. Once this initial calibration is in place, the sensors will remain highly stable over time – with more than twenty years’ worth of ex perience, we ought to know by now!

It is certainly not a bad idea to have your sensors recalibrated after, say, two years of field duty, but there is no strict recalibra-tion scheme that we need to enforce. Of course, if your institution adheres to a quality standard such as ISO 9xxx, special reca-libration interval require-ments may apply.

A recalibration service is available for a flat fee (at the time of going to print, 290 €, subject to change without notice). The flat fee applies as long as there are no additional services (beyond recalibration) to be provided. Apart from “cosmetic flaws” that the sensor will undoubtedly have acquired over time, your sensor will be equivalent to a new instrument after the procedure. A Sensor Calibration Sheet will be issued.


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Huddle test

Q In your Sensor Inspection Sheet you mention that each sensor has been submitted to a so-called huddle test. What exactly does that mean, and what is its relevance?

Isn’t the shake table test good enough?

A Huddle testing means that we put a number of identical sensors (typically, five or six) physically very close to each other, and record ground noise during a few hours in the

dead of night. Traces are then compared using various statistical methods to ensure good coherence. Why do we go to these extra lengths? The shake table test is fine, but since fairly large amplitudes are used here, it does not completely test the sensor’s behaviour. The sub-micrometer sensitivity of the huddle test is able to reveal flaws that the shake table test can not show.

Waiting time after power-up

Q You ment ion quick up-and-running t ime as a par t icu lar advantage of your sensors. What exactly does that mean? I am used to having to wait a couple of minutes

at the very least...

A Obviously, the lower the sensor’s eigenfrequency, the longer one will have to wait until the system has reached a steady state, and hence delivers stable results. One of

the unique advantages of Lennartz sensors is that this time has been cut down quite drastically. The example shown below is for the LE-3D/20s; it shows the sensor output after power-up (the datalogger has been up and running before; the sensor was powered up separately). As you can see, after the initial overshoot the sensor signal will quickly settle down, and a mere 30 second wait is enough! Obviously, the shorter period sensors will perform equally well or better.


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No 12 V DC power supply available?

Q What if I need to connect the sensor to an instrument that does not have 12 V DC output?

A We can offer a breakout box with a built-in 12 V battery (1.3 Ah). A wall plug AC recharger is included. Recharging during normal sensor operation is possible. Sensor

output is normally provided on BNC connectors (one per channel); other connector types are available upon request. The 1.3 Ah battery will typically last six to seven days with a 1 Hz or 5 sec 3-component sensor.

Electronic noise

Q The sensors being electronic, I imagine there is some noise present. What is the noise level, and how does it depend on frequency?

A Obviously, this assumption is correct – there is no such thing as a noise free electronic circuit. However, the noise level is so low that there are only few sites with low

enough ground noise for you to realise there is electronic noise at all, not to mention the data-logger as a source of electronic noise. The graph below shows noise power spectral density, measured in a 1/6 decade band-width, and compared to USGS High and Low Noise Models. Translated to equiva-lent velocity, around 1 Hz we are in the sub- to low-nano meters/sec range.

-180

-170

-160

-150

-140

-130

-120

-110

-100

0.01 0.1 1 10 100

LNMLE-3DliteLE-3D/5sLE-3D/20sHNM

dB r

elat

ive

to 1

m/s

2 rm

s in

1/6

dec

ade

Period [seconds]

Self noise of Lennartz seismometers(as determined by Prof. E. Wielandt)

compared with USGS High and Low Noise Models


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AccessoriesCT-EW1 Calibration TableDesigned by none other but Professor Erhard Wielandt, the inventor of broadband seismometry as we know it today, the CT-EW1 is a high precision electromechanical device used for the absolute calibration of broadband, intermediate- and even short-period seismic sensors.

The sole purpose of the CT-EW1 is to determine the absolute transduction factor (typically expressed as X V/m/s) as precisely as possible. While it is generally not a big problem to determine the relative transfer function of modern seismometers (these are often given by the manufacturer as poles and zeroes), the absolute transduction factor is much more tricky. CT-EW1 does this for you, typically with better than ±1% precision.

Compared to sending your sensor back to the manufacturer for periodic recalibration (for example, if your institution adheres to ISO 9xxx qual-ity control standards), the CT-EW1 has the major advantage of being able to get the job done in situ, verifiying your whole data acquisition chain as a side effect (for example, you will quickly learn about inverted polarity if it exists).

While the stock CT-EW1 will handle normal STS-2 (1,500 V/m/s) and Güralp sensors without problems, high-sensitivity versions of these sensors may exhibit clipping when used on the CT-EW1 even in “low gear”. An optional “high sensitivity

kit” will mechanically and electrically modify the CT-EW1’s table motion in such a manner as to not overload a high-sensitivity sensor.

STS-1 (and other BB sensors) Warp-Free BaseplateAnother brilliant design by Erhard Wielandt. Glass hood not supplied. Made from meticulously CNC’ed anodized aluminum, with a Fischer connector for the sensor signals and a high-grade

vacuum stopcock.

Glass hoods come with, or without, a vacuum valve. For use with our Baseplate, a vacuum valve on top of the glass hood is not required.

The sandwich-type baseplate keeps warping and tilting motion induced by air pressure variations away from the sensor.

The coverplate and manometer shown here are not part of the setup. This photo was taken during a long-term leakage test at approx. -1 bar. The coverplate substitutes the glass hood.

CT-EW1 and STS-2 in the MOXA valut during an international workshop sponsored by BGR


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STU Shielding: Gabbro Plate and Hood for STS-2Every broadband seismologist knows that proper shielding of the sensor from environmental influ-ences is the key ingredient to obtaining useful recordings. The highest quality sensor can be severely degraded by an improper and insufficient setup, with shielding often being the most crucial part.

Lennartz now offers a “carbon copy” of the famous STU (Stuttgart) design (again - you guessed it - conceived by Erhard Wielandt). For details, please see

http://klops.geophys.uni-stuttgart.de/~widmer/DGG2006poster.pdf

Like the Warp-Free Baseplate, the Gabbro Plate comes equipped with a Fischer through-connector for easy plug-and-play installation. No modification to existing cabling infrastructure is required. Described as “hermetically sealed pressure vessel with rigid gabbro base plate and compliant stainless steel pot”, this setup achieves an “attenuation of ambient pressure variations are attenuated by 40dB”. In addition, the huge thermal mass of the massive slab of gabbro provides excellent attenuation of ambient temperature variations.


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Where are Lennartz sensors in use?Rather than naming individual institutions, suffice it to give a list of countries where Lennartz sen-sors are in permanent and regular use. This list contains only the “home sites” of sensors; in many cases, users will literally take them around the world. We know of Lennartz sensors that have been around the world several times over...

LuxembourgMalaysia Mexico Monaco Mongolia MoroccoNew Zealand Nicaragua NorwayPanama Peru Poland PortugalRussiaSlovak Republic Slovenia Spain Sweden SwitzerlandTaiwan Trinidad & TobagoUnited Arab Emirates United Kingdom United StatesVenezuela Viet NamYemen Yugoslavia (former)Zambia

Antarctica Australia

Belgium Bolivia

Bosnia-Herzegowina Bulgaria

Cameroon China

Colombia Costa Rica

Croatia Cuba

Czech RepublicDenmarkEcuador EthiopiaFinland France

Germany Greece

Honduras Hongkong

HungaryIceland

India Indonesia

Iran Israel

ItalyJapan

Kazakhstan Kenya

The content of this document has been carefully edited and is believed to be correct at the time of going to print (2008-06). However, Lennartz electronic GmbH can not be held responsible for any errors contained herein. Subject to change without notice.

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