TEMA V SISMOMETRÍA Sismómetros y ecuaciones constitutivas

TEMA V
SISMOMETRÍA
Sismómetros y ecuaciones constitutivas
Ruido de fondo
Redes Sísmicas
Localización de temblores
Elementos de la Teoría Generalizada de Inversión
Seismic noise measurement (Manual of Seismic Observatory Practice, 1979)
It is usual to include regular continuous seismic disturbances in the range of period from
0.01 to 10 seconds, although microseisms with longer periods have been observed. Long
period tilts, due to moving loads or the pressure of wind on the surface may sometimes
interfere seriously with recording.
• Seismic noise with periods <=0.1 s is mainly of local origin, and of limited extent. It is
caused by wind, traffic, machinery, surf, waterfalls, running water, volcanic activity, etc. Its
intensity and periods are influenced by the characteristics of the ground. Often a dominant
frequency is observed; it corresponds probably to the minimum group velocity of surface
waves. The amplitude ratio between daytime and night sometimes fluctuates in the range
2:1 to 10:1 and over, according to the source. In Japan, it has been found that short-period
seismic noise has a higher frequency on granite than on soft ground.
• Seismic noise having periods between 0.2 and 2.0 s belongs to a second characteristic
group. The most prominent is the component with T = 0.5 s, which has been observed in
Europe, the USSR and the USA. The cause of the prevailing occurrence of this maximum
has not yet been explained except that the larger amplitudes have been found near towns
and industrial centers. No distinct tendency for a daily or seasonal variation has been
observed.
• The third group of seismic noise, the typical microseisms, are most widely recorded and
most discussed. On the station records they appear as groups, in each of which the
amplitude increases and then fails off, suggesting some kind of beats. The periods range
from 3 to 10 s. During the winter season microseismic storms lasting often 1 to 2 days are
strong enough (ground amplitudes on the coast 10-20 µm, inland 1-2 µm) to make some
seismograms useless. Microseisms of this sort are caused by cyclonic storms over
oceans, and are propagated with gradual loss of energy into the central areas of
continents. Sometimes shorter periods (1.5-2 s) of microseisms of similar character are
observed near large lakes.
Microseismic noise at island coastal and continental interior
sites, 29 May 2002 06:00:00 3600 sec. Island of Hawaii,
POHA BHZ, northern California, SAO BHZ, continental
interior, Colorado ISCO. Note increasing microseism noise
from Colorado to California to Hawaii.
AHID car traffic noise
PSD and time series.
Car noise record 21
May 2002 18:00:00.
Quiet record 30 May
2002 06:00:00. Note the
increased power at 5-10
Hz for the car noise
record.
Histograms of powers, in 1-dB bins, at four separate period bands for
station AHID BHZ.
Time series and PSDs for recording system transients and earthquakes.
PDF for station AHID BHZ, constructed using 19,432 PSDs during
the period from September 2000 to September 2003.
Diurnal variations for
(a) BINY BHZ and
(b) ANMO BHZ.
Seasonal variations
for (a) DWPF BHZ
and (b) EYMN BHZ.
PDF mode noise
levels above the
NLNM mapped
across the United
States in three
separate period
bands.
NLNM: new low
noise model
NHNM: new high
noise model
Seismicity of hurricane Charley, before and after crossing Florida.
(Peters, Mercer University Physics, June 2005). The variation of `noise‘ was at
a higher level either side of that time when the eye of the storm was centered
on the state. This is seen from following figure to result largely from changes
in the magnitude (and frequency characteristics) of the microseisms
generated by the hurricane.
Spectra showing the microseism activity.
NETWORKS
Main types of stations
First-order stations
The first-order conventional stations provide the main coverage of the earth's
surface for the observation of long-period and shortperiod earthquake waves
over teleseismic distances. The sources of the seismic waves are distributed
over the whole earth, the interest in the data is also world-wide
Second-order stations
Second-order stations are usually established for short range observation, and for this purpose
are frequently grouped into regional networks, reporting to a single interpretation center.
Temporary stations
Mobility of equipment does not exclude an,y of the advantages of permanent establishment.
Microearthquake networks
Some microearthquake networks are intended to study seismicity and active tectonic
processes in relatively large regions. Others are set up for a single objective (e.g., to
evaluate particular sites for emplacement of critical structures such as dams or
nuclear power plants). In any case, the expected function of a microearthquake
network should be the primary concern in its design and operation.
Arrays.
An array consists of a number of seismometers spaced out on the ground, and
connected by cable or radio links to a central recording system. The overall
performance of such a system depends on the shape and size of the array, on the
number of elements in it, and on the methods that are used in processing the data.
Basically, however, the object is to increase the sensitivity to a particular seismic
signal in comparison with other signals or random noise, and thereby, in the most
fully developed installations, to produce a 'telescope' which can look into the earth in
a specified direction.
Examples of Array Types
•Summation in small clusters. The seismometers are distributed over an area which
extends over a fairly small fraction of a wavelength of the seismic signal that the system is
designed to detect. The outputs of all the seismometers are added together, and the sum is
recorded on a single information channel. Under such circumstances, the desired signal
arrives approximately in phase at all the detectors, and the output is approximately the
arithmetic sum of the individual elements. In contrast to the coherent signals, noise which
arises from small incoherent sources near the detectors will produce a sum which is more
nearly proportional to SQRT(n), where n is the number of elements. The net result of this
difference in summation properties is that the signal-noise ratio increases approximately in
proportion to SQRT(n).
•Extended circular arrays. If the seismometers are evenly spaced around the
circumference of a circle, and we consider the response to a sinusoidal train of seismic
waves, it is found that the sum of the individual outputs has the same frequency and phase
as that of a single seismometer at the centre point. The amplitude of the summed output
depends only on the wavelength of the incoming signal, being independent of azimuth. By
subtracting an appropriate fraction of the ring-output sum from the output of a central
element, it is possible to obtain a null in the total output for a particular signal wavelength. By
using a number of concentric rings it becomes possible to construct a band-rejection
wavelength filter, and by passing the output of such a system through a frequency filter we
can produce a system which rejects a band of wave velocities.
•Directional resolution. The principle of wavelength filtering can be
extended to provide directional resolution if the outputs of the different
channels are recorded separately, or if appropriate on-line processing facilities
are provided. The procedure is to introduce time lags during final processing
which compensate for the propagation time of the incoming wave, and then to
add the outputs together. In this way, the amplitude of the chosen wave is
enhanced in comparison with that of random noise, or with that of other waves
for which the wavelength or azimuth of approach differs from that of the
selected signal.
Referencias
Manual of Seismological Observatory Practice (1979 Edition). P. L.
Willmore, Editor, Institute of Geological Sciences, Edinburgh,
Scotland, September 1979, Reprinted September 1982
Published by World Data Center A for Solid Earth Geophysics
IASPEI , New Manual of Seismological Observatory Practice,
(NMSOP), Peter Bormann, Editor, GeoForschungsZentrum
Potsdam, 2002, ISBN 3-9808780-0-7Volume 1&2
Daniel E. McNamara and Raymond P. Bulan, Time series and
PSDs for recording system transients and earthquakes. Bulletin
of the Seismological Society of America; August 2004; v. 94; no.
4; p. 1517-1527