Modern seismic instruments

presented at the seminar in honour of

Peter Bormann

Potsdam, September 2004

by

Jens Havskov, Department of Earth Science

University of Bergen

Norway

Before:

- Seismographs were specially made

- Few standard components were used

- Very specialized software

Now:

- Stations and networks are mainly made with standard industrial components

- Digital technology used throughout

- More standardized software

- Sensors currently the most specialized element

- Now possible to build a seismic station with mainly off the shelf products

SENSORS

-Trend is to use more broad band sensors (BB), even when overkill, however BB sensors now have a similar price as 1 Hz sensors

- 1 Hz sensors will go out except when used with feedback technique

-4.5 Hz geophones the cheapest sensor, now used by several, either directly or with a feedback technique

- FBA based sensors will probably dominate the market in the future

Typical geophone

M=5.5 quake at 300 km distance recorded with a 4.5Hz sensor

Signal filtered 0.01 – 0.1 Hz

Displacement transducer

Spring

Volt out ~ AccelerationForce coil R

C

Mass

Simplified principle behind Force Balanced Accelerometer. The displacement transducer normally uses a capacitor C, whose capacitance varies with the displacement of the mass. A current, proportional to the displacement

transducer output, will force the mass to remain stationary relative to the frame.

The FBA can have the digitizer integrated in feedback loop

13 cm

The Kinemetrics 3-component Episensor, an FBA accelerometer

Kinemetrics Episensor internals

Left: The internals of the Güralp CMG-3T BB sensor. Right: Sensor with

digitizer. Photo’s supplied by Nathan Pearce, Güralp.

Principal elements of a MEMS (micro electro mechanical systems) accelerometer with capacitive transducer. The mass is the upper mobile capacitor plate which can rotate around the torsion bars. The displacement, proportional to acceleration, is sensed with the variance in the capacitance. For high sensitive applications, a feedback circuit is added which controls a restoring electrostatic force, thus we have a FBA.

The size of the sensor above is about 2 mm. Figure from www.silicondesigns.com/tech.html.

---------------- 2 mm ------------

meshelectrodes

microporousspacers

electrolytechannel

sensitiveaxis

signaloutput

A simplified schematic of the electrochemical motion transducer used in MET seismometers (modified from R. Leugoud –PMD Scientific, Inc.-, 2003, personal communication). The electrolyte fluid is free to move in a channel. A set of platinum electrodes creates an ion concentration gradient by a small DC voltage. When the fluid moves due to acceleration, an additional current proportional to fluid velocity flows past the electrodes. The symmetric arrangement improves the linearity. The seismometer may operate with feedback to shape the response and increase linearity and dynamic range.

Different sensors at Univeristy of Bergen vault

A small window of the common traces for Z-channels. The numbers above the traces to the right are max amplitude in counts and the numbers to the left, the DC offset in counts.

Raw traces for different sensors

A small window of the common traces for the Z-channels. The traces have been corrected for instrument response and show displacement in the frequency band 1-20 Hz. The numbers above the traces to the right are max amplitude in nm and the numbers to the left, the DC offset in nm. Notice that the last 3 traces are not

from the same time window.

Displacement 1-20 Hz

A small window of the common traces for the Z-channels. The traces have been corrected for instrument response and show displacement in the frequency band 0.2-1.0 Hz. The numbers above the traces to the right are max amplitude in nm and the the numbers to the left, the DC offset in nm

Displacement 0.2-1.0 Hz

Acceleration and displacement. The seismograms in the figure show the first few seconds of a P-wave of a small earthquake. On the site there is also an accelerometer installed (A) next to the seismometer (S). The top traces show the original records in counts. The signal from the seismometer is similar to the accelerometer signal, but with higher frequency contents for this later, and the amplitudes are different. The middle traces show the two signals converted to accelerations and the bottom traces, converted to displacement (frequency band 1-20 Hz). The signals are now very similar and of the same amplitude.

Instrument sensitivity of several Geotech seismometers ranging from the short period S-13 to very broad band 54000 seismometer. The curves show input acceleration equivalent to sensor internal noise in dB relative to 1(ms-2)2/Hz . NLNM is the Peterson New Low Noise Model (Peterson, 1993). Slightly modified from Geotech home page, www.geoinst.com.

Predicted total noise equivalent acceleration of a standard electronic circuit with a 4.5 Hz geophone. The contribution from each element is shown and the Peterson noise curves are shown for reference. Thermal noise is the noise due to Brownian thermal motion of the mass, Johnson noise is caused by thermal fluctuation of the electrons within any dissipative element in the electronic circuit, voltage and current noises are generated within the amplifier. Figure from Barzilai et al (1998).

The equivalent ground acceleration noise spectrum for a digitizer with its input shorted, using a low-sensitivity virtual sensor of 4.5 Hz and G=30 V/(m/s). The record of the digitizer noise has been reduced to the equivalent ground motion using its response combined with an imaginary low-sensitivity sensor. The spectrum then shows the worst-case sensitivity for ground motion that can be achieved with this digitizer and sensor. Of course, a more sensitive sensor would give a lower equivalent ground noise. The smooth curves on top and bottom are the Peterson New Low-Noise and New High-Noise models, for reference.

Noise spectrum from instrument using 4.5 Hz sensor, 16 bit digitizer located in Tibet. Soil site

Computer Data storageAnalog to

Digitalconverter

Powersupply

GPS

Communication

Sensor

input

Main units of a seismic recorder. There are no flow arrows between the units since all can have 2 way communication. The GPS can be connected to the digitizer or the recorder. The power supply may be common for all elements or each may have its own regulator, but usually the power source is unique (e.g. a battery).

Jan Mayen station, Liberg

The current trend in the development of the different elements of the portable recorders is:

Computer: Based on a standard computer and operating system:

-Linux seems to be the favorite operating system, but Windows NT/2000 is also used.

-Single board PC’s with low power consumption.

Communication and data transfer:

-RS232

-Ethernet/TCP/IP

-USB + others

Sample rate, dynamic range and sensitivity:

-Sample rates from 1-1000 Hz,

-Dynamic range of at least 22 bit

- LSB (least significant bit) resolution of 0.1 μV.

Standard Data acquisition software

Power consumption: Below 2 W.

PC104 computer

8 cm

Field equipment made by UiB

Mobile phone with internet, TCP/IP (56 kb), operating system and 0.5 gb memory

13cm x 18cm x 34cm

Nanometrics Taurus, the next generation handheld recorder (25 x 15 x 6 cm). Power consumption 0.8 W. From Nanomtrics home page, www.nanometrics.ca.

 

Use of a small seismic array

Improved detection of weak signals

Automatic detection of P and S-wave arrivals

Determination of azimuth

Automatic location

Location of weak emergent arrivals like volcanic tremor

Building a regional location capability in a small area

Determination of azimuth and apparent velocity with Chiriqui array

Simple seismic station for stations and arrays. The station uses 18 bit converter, 4,5 Hz geophone, ost EU 1000. Recording with PC.

ba

c d

-2 0 2 4 6 8 1 0

eas t (km )

-2

0

2

4

no

rth

(km

)

-8-6-4-20

depth (km )

-8

-6

-4

-2

0

de

pth

(km

)

Symbols of the earthquakes indicates the focal depth of the events: dots smaller than 1 km, crosses between 1 and 3 km and squares focal depth greater than 3 km. Note that most events are very close to the array.

Deception Island