Geophys. J . Int. (1991) 106, 113-121
Spectra of ocean-bottom seismic noise in the 0.01-10Hz range
Thomas A. Dozorov and Sergei L. Soloviev
P. P. Shirshov Institute of Oceanology, USSR Academy of Sciences, Moscow 117218, USSR
Accepted 1991 January 25. Received 1991 January 2; in original form 1989 October 17
SUMMARY
In 1988, short-term (duration 18 hr each) ocean-bottom seismic noise recordings
were made in the frequency range of 0.01-10 Hz a t t h e depth of 900-1400 m in the
Atlantic ocean. T h e r e were two study areas: in t h e Canary upwelling and on the
Reikjanes ridge. T h e spectral curves obtained for these two areas show t h e existence
of a minimum in t h e frequency range of 0.05-0.10Hz (i.e. in the period range of
10-20 s) together with a maximum reflecting the domain of storm microseisms
(0.1-1 Hz). We recommend t h e use of this minimum for regular recording of strong
remote earthquakes and, especially, for surface wave dispersion studies t o obtain
velocity sections of t h e oceanic crust and upper mantle.
Key words: microseismic noise, seismic noise, ocean-bottom noise.
INTRODUCTION
The Earth’s surface undergoes constant vibration, known as
ismic noise, over a wide frequency range. At the turn of
i century, when seismology was in its infancy, seismic
,oise recordings were mainly in the 0.1-1 Hz range and the
noise came to be known also as microseismic noise, a term
which has since been used to denote noise in other
frequency ranges. Knowing the spectral composition and
level of microseisms is important in itself because it gives a
better insight into microseismic excitation processes, but
much more so because it enables us to select the frequency
range with the lowest noise level in which signals from
earthquakes or explosions can be recorded most effectively.
Probably the first survey of land seismic noise was that
prepared by Brune & Oliver (1959), in connection with the
problem of identification of explosions and earthquakes.
These authors reviewed data in the period range of
0.01-40 s, with the 0.01-10 s data largely derived from the
literature and the 10-40s data obtained through the first
ever experimental long-period observations.
Later, as long-period seismographs were improved and
placed in vaults, it became possible to measure long-period
land seismic noise for the period range 10-2560 s (Haubrich
& MacKenzie 1965; Fix 1972; Savino, McCamy & Hade
1972; Whorf 1972; Murphy & Savino 1975; and others). We
shall be considering as representative only noise data
obtained at the Queen Creek Seismological Station
(QC-AR) located at a depth of 130m in a quartz diorite
mountain in the desert of Arizona (Fix 1972). In structural
terms the long-period noise recorded at other land stations
is not essentially different from the noise of the QC-AR
station.
The first serious ocean-bottom seismological observations
were staged in the early 1960s, also in connection with the
problem of monitoring of underground nuclear tests. They
were carried out both by means of pop-up bottom stations
(Prentiss & Ewing 1963; Bradner & Dodds 1964; Schneider,
Farrell & Brannian 1964) in the frequency range 0.1-15 Hz,
and by means of a long-term bottom station placed at a
depth of 3.9 km WNW of San Francisco in the Pacific and
connected to the shore by a submarine cable. This station
was equipped with three-component sets of short-period and
long-period seismographs (along with many other transducers) and could record seismic events in the period range
approximately from 0.03 to 25 s and acoustic events in the
range up to 250 s (at the level of 0.7). The station functioned
from 1965 to 1971 (with one year’s break) and many of its
observations were published (Sutton et al. 1965, 1970, 1973;
Nowroozi, Sutton & Auld 1966; Nowroozi et al. 1968;
Nowroozi, Kuo & Ewing 1969; Mark & Sutton 1969;
Odegard & Sutton 1969; Odegard et al. 1969; Nowroozi
1973; and others).
Ocean-bottom seismic noise was discussed in one of these
publications (Latham & Nowroozi 1968), but its authors
concentrated on the problem of the formation of the most
intensive microseisms with periods of 6-7 s by storms which
had passed through the station site, and produced no
spectral curves for ultra-low-frequency noise. They did
mention, however, that seismograms from the long-period
horizontal components showed background noise in the
period range of 100-200s. In general the component that
had its sensitive axis parallel to the coast showed larger
background noise than the component perpendicular to the
coast. This ultra-long-period noise appeared to be correlated
with water current amplitude.
[A short note on spectra of ocean-bottom ULF seismic
noise obtained as a result of the above-mentioned
113
114
T. A . Dozorov and S. L. Soloviev
Columbia-Point Arena experiment has appeared recently
(Sutton, Barstow & Carter 1988), but was not available to
us during the preparation of this paper.]
Subsequent observations in the 1970s and the 1980s using
pop-up bottom seismographs were mostly in the frequency
band ranging from 1-3 to 20-100Hz, with some
high-frequency instruments designed chiefly for deep seismic
sounding (Soloviev 1985, 1986). Only one of the California
University instruments had a pass-band extended as low as
O.lHz, as well as instruments used by the Institute of
Oceanographical Sciences, Blacknest, England, and by the
Geological Observatory of Lamont-Doherty, USA, as far
as their descriptions go (Carmichael et al. 1973; Francis &
Porter 1977) but there were no published data at ultra-low
frequencies. The evidence that has been accumulated about
seismic noise in the 0.5-20Hz band has been analysed and
described by Ostrovskiy (1982) who has obtained the
highest, lowest and average levels of the square root from
the power of the spectra. All the curves show a flat
minimum at lOHz and a maximum at 0.2Hz and are
characterized by an increase in the level at a rate of
10-18 dB per octave as the frequency falls from minimum to
maximum.
A capsule contained a Block-Moore quartz fiber
seismometer which responded to subtidal frequencies was
deployed in about 3.000 m of water 250 miles southwest of
San Diego, California, probably in the early 1970s (Prothero
& Munk 1974). We did not see the original of this report but
received some information on it by correspondence.
There have been few attempts to study ultra-lowfrequency ocean-bottom noise. The authors of the present
paper have succeeded in finding only three publications on
this matter (Prothero & Schaecher 1984; Webb & Cox 1984,
1986). The instrument used by Prothero & Schaecher was
based on the one developed by Prothero (1979) earlier,
which was essentially a pop-up ocean-bottom seismograph
fitted with a microprocessor, in which the Mark Products
seismic sensors with a natural frequency of 4.5 Hz had been
replaced by sensors from the same manufacturer having a
natural frequency of 1Hz (Ld-Cl). The sensitivity of these
sensors falls at a rate of 18dB per octave at periods longer
than 1s. To cope with this problem, special amplifiers with
frequency-dependent gain were developed to allow oscillations with periods approximately up to 20-30s to be
observed. The electrical gain at such periods was required to
be 100 times higher than at a period of 1 s, and it is no
wonder that the level of the input noise of the input
recording channel was considerable. This OBS was used in
1981 for two 1 month long experiments at depths of 180 and
3660 m off the coast of California (with the instruments
working in start-stop recording mode) but was later lost.
On the plotted bottom noise spectra, maxima of storm
microseisms at 5-7 s period and minima at 10 s period stand
out conspicuously. The noise level grows towards much
lower periods. Webb & Cox (1984, 1986) attempted to get
an idea of the spectrum of bottom seismic noise in the
0.003-20 Hz band by measuring fluctuations in the
near-bottom hydrostatic pressure and in the electric voltage
induced by the telluric currents flowing through the long
(300-600 m) isolated bottom cable. In 1983-1984, three
experiments from 3 to 5 days long were carried out at depths
of 1650, 3700 and 3850m off the coast of California. The
spectrum plot of the hydrostatic pressure shows conspicuously a band of 0.003-0.04Hz with a high level of
noise, a pronounced minimum in the 0.05-0.1 Hz band, a
maximum of storm microseisms in the 0.1-0.4 Hz band and
a rapid drop in the level above 0.4Hz. The spectrum of
fluctuations in the electric volta4e resembles the spectrum of
variation in the hydrostatic pressure, but its extrema are less
pronounced.
The literature (Goodman et al. 1989; Trevorrow et al.
1989 a,b) also gives a description of a Profiler equipped with
accelerometers with a pass band above 0.01 Hz and shows a
number of measurements made by it of spectra of bottom
noise at shallow depths (at depths from 3-4 to 70 m) off the
Atlantic coast of the USA. However, the level of noise in
shallow waters turned out to be so great (of the order of 100 p)
that it seems unreasonable to apply the results to the bottom of a
deep ocean.
Of the two borehole subbottom seismographs installed in
the Pacific, the southwestern one, developed by TeledyneGeotech, which worked for 5 days in February 1983, had a
working frequency pass-band of 0.1-20 Hz (Adair, Orcutt &
Jordan 1986, 1987). The northwestern OSS was developed
in the Hawaiian Institute of Geophysics. Installed in
September 1982 to work at least till May 1983, it had a
recording pass-band from 0.03 to 40Hz (Byrne et al. 1987;
Harris et al. 1988).
The spectrum plots of noise recorded by the two borehole
seismographs have been published, along with those from
ocean-bottom seismographs which worked near the mouths of
the boreholes, for oscillations with frequencies above 0.05 Hz.
Almost all the spectrum plots show maxima of storm
microseisms at 0.2 Hz and minima at 0.1 Hz (Adair et al. 1986,
1987; Duennebier et al. 1987).
The above summary gives the impression that bottom
seismic noise in the ultra-low-frequency pass-band of
0.01-0.1 Hz has been studied insufficiently until recently.
OCEAN-BOTTOM SEISMOLOGICAL
EXPERIMENTS A N D EQUIPMENT U S E D
The present paper describes some results of experiments
performed during the 49th cruise of the R/V ‘Akademik
Kurchatov’ in 1988. Instruments were installed on the floor
of the Atlantic Ocean in two areas (see Table 1). The
positions of the instruments were determined by the ship’s
satellite navigation system, and the error in estimating the
coordinates of the OBSs is taken to be 250 m (Soloviev et al.
1988).
The object of the experiments was to study ocean-bottom
seismic ultra-low-frequency noise and the dynamics of the
bottom boundary layer (Soloviev 1990). For this reason, two
(Canary upwelling) or three (Reykjanes ridge) stations (Fig.
1) were installed at a time on relatively flat sections of ocean
bottom. The distance between the extreme stations did not
exceed 1km. Station A on the Reykjanes ridge was
equipped with a three-component set of low-frequency
piezoelectric seismic sensors supported on gimbals, and a
self-contained digital package, Potok, suspended 2.5 m from
the bottom, for measuring water temperature and the
magnitude and velocity of benthic currents. The burp-out
(deployed) sensor container was dropped to the bottom 2 m
from the expandable anchor of the station. Pop-up station B
Spectra of ocean-bottom seismic noise
115
Table 1. Locations and recording periods for ocean-bottom seismographs used in the experiments.
Coordinates of OBS
Study area
N . Lat.
W. Long.
Depth (m)
Canary
upwelling
21O49.7‘
17O43.5’
1350
Reykjanes
ridge
60O49.5’
27O47.7’
960
Recording
period
(ships time)
27-28
1988 July
26-27
1988 Septembex
consisted of a string of four Potok meters deployed at 5, 10,
15 and 25m from the bottom. Tethered station C was a
frame firmly placed on the bottom, with two spherical
containers. The frame was separated from the anchor by an
auxiliary anti-jerk anchor. The lesser container, 350 mm in
diameter, held a three-component set of low-frequency
electrochemical seismic transducers supported on gimbals,
and the bigger container, 450mm in diameter, housed a
digital-analogue recorder. Both seismic stations were also
fitted with low-frequency hydrophones of different construction which were screwed into the recorder containers.
Temperature sensors were provided inside the seismic
sensor containers.
The results relating to the dynamics of the bottom
boundary layer are not discussed in this paper, but it is
useful to note that the use of current meters enabled us to
choose for analysis seismogram sections when strong
currents had no effect on the ocean-bottom seismographs.
As far as the structure of ultra-low-frequency bottom
noise is concerned, the main results were obtained by means
of electrochemical transducers whose design is described
elsewhere (Abramov et al. 1978). The transducers are
parameter-type inertia-action instruments with electrolyte as
liquid inertial mass. Physically, each transducer is
dumbbell-shaped glass vessel with electrodes soldered inside
its connecting bar. The transducer, along with a
pre-amplifier, is housed in a metallic cylinder 170mm long
and 65mm in diameter. The electrolyte and the electrodes
form a reversible oxidation-reduction system. When
subjected to motion, the body of the transducer together
with the electrodes oscillates relative to the electrolyte.
These oscillations are converted by the electrochemical
reaction into an alternating electric signal. These electrochemical transducers are simple and robust in service.
The authors of this paper first used such transducers as
part of their seismic stations in 1983 when data about
ocean-bottom seismic noise in the Brasil basin, Atlantic
Ocean, in the 0.2-30 Hz band was obtained (Soloviev et al.
IQ
Q-,-,4
6
Q B
A
T
ICh.
4i
4‘
I
iji 1
4‘
4-i
1
C
Li.1
Figure 1. Scheme of the Reykjanes ridge experiment: A-pop-up OBS; B-pop-up station for studying the bottom boundary layer;
C-tethered OBS; 1-recorder container; 2 s e i s m i c sensor container; -elf-contained
current and temperature meters ‘Potok‘;
4deep-water float; 5-radiobeacon; &flash beacon; 7-radar reflector; &hydroacoustic beacon and release mechanism; Mxpandable
anchor; 10-anchor; 11-auxiliary (anti-jerk) anchor.
116
T. A . Dozorov and S. L. Soloviev
Figure 2. Block-diagram of digital recorder: ADC-analogue-to-digital
converter; ATR-analogue
tape recorder; C-connector;
ChA-channel-to-channel adaptor; CI-communications interface; CO-crystal oscillator; CPU-central processor unit; FDC-floppy disc
storage; H-hydrophone; I-interface; LPF-low-pass filter; M-monitor; MM-memory manager; MP-microprocessor; MPRAMmicroprocessor’s random access memory; OA-overall amplifier; PSSM-processor of the solid-state memory; RAM-random acces
memory; RAMSrandom access memory storage; S k e i s m i c sensor; SSMS-solid-state memory storage; TE-time encoder; TS-tape
storage. TU-time unit (that is CO and TE).
1988). During the 1988 expedition, we also used three
low-frequency piezoelectric transducers manufactured in the
Moldavian Academy of Sciences in an orthogonal set-up.
These transducers have not been described in the literature,
but they had been on long-term trial in the tunnel of the
‘Kishinev’ seismic station, working alongside with conventional long-period electrodynamic seismographs.
Signals from the electrochemical transducers were
recorded by special analogue-digital hardware (Akhsakhalyan et al. 1989). Its block-diagram is shown in Fig. 2. The
hardware consists of a solid-state memory digital unit and an
analogue tape recorder unit. The digital unit incorporates a
central processor unit (CPU), a random access memory
(RAM), solid-state memory storage (SSMS), a processor of
the solid-state memory (PSSM), memory manager (MM),
communications interface (CI) , analogue-to-digital converter (ADC), four low-pass filters (LPF) with a
pre-amplifier, and a time unit (TU).
Each of the four LPFs is basically a sixth-order
Butterworth filter having a cut-off frequency of 0.2 Hz and a
slope of 36 dB/oct. A low-noise pre-amplifier with a variable
gain programmed by the CPU depending on microseismic
noise level is provided at the input of the LPF. Signals from
the output of the LPF go to the four-channel multiplexer of
the ADC, at the output of which there is a non-linear
element whose purpose is to expand the dynamic range. The
TU is a frequency-standard crystal oscillator with a
frequency divider and a time encoder which generates serial
and parallel codes of absolute time.
The CPU is plug-compatible with the Elektronika
microcomputer installed on the ship for processing
recorded information.
The RAM has a capacity of 4 K of 16 bit words and is
used for storing the processor operation program and for
buffering the signals. The SSMS has a capacity of 256 K of
16 bit words and consists of 64 memory banks each with a
capacity of 4 K of 16 bit words.
The PSSM generates signals needed to operate the SSMS
and in this way establishes communication with the bus
system of the microprocessor (MP). The capacity of the
SSMS exceeds the maximum addressable memory space of
the CPU and therefore the communication between the
SSMS and the CPU is by means of the MM, which allows
programmed switching of the memory banks to the bus of
the MP. The MM also controls the RAM banks when the
station is being used in conjunction with the Elektronika-60
microcomputer. The communication of the ADC and the
TU with the MP bus system is by means of the CI.
The MP, like the Elektronika-60 microcomputer, uses
memory bank zero. Therefore, to avoid duplicating zero
bank when the MM is connected to the microcomputer, the
RAM bank of the instrument changes over from zero to
bank three automatically when connected to the channel-tochannel adapter.
The digital unit works in three modes: continuous,
start-stop triggered by input signal level and timed
start-stop. The CPU works as input signal analyser and as
timer. It also automatically sets the zero of the signal
channel and connects the seismometers to the inputs of the
LPF. The digital recording bandwidth is from 0.01 to
0.3 Hz.
The analogue recorder unit uses direct tape recording and
Spectra of ocean-bottom seismic noise
consists of three amplifiers with filters which shape the
amplitude-frequency characteristic of the tape recording
channels, a bias oscillator and a tape speed stabilizer.
The record amplifiers are essentially operational amplifiers, each with two outputs with different gains to allow
recording at two sensitivity levels. Seismic signals from the
outputs of the transducers are amplified by a three-channel
record amplifier and then is supplied to the first six channels
of the magnetic head. The 7th channel is used for a 16Hz
signal generated by the crystal oscillator to identify analogue
and digital records and the eighth channel is used for the
serial code of absolute time.
The analogue recorder has the following technical
characteristics:
recording mode
bandwidth
dynamic range
minimum input signal
signal channels
service channels
continuous
0.1-10 Hz
37 dB
2 pV
6 (2 per seismometer).
2
The digital recorder unit has the following characteristics:
LPF cut-off slope
information channels
sampling frequency
minimum input signal
dynamic range
relative instability of crystal
oscillator
intermediate memory length
36 dB/oct
4
1Hz X 4 channels
10 pv
66 dB
lo-’
1-8 min
solid-state storage capacity
signal averaging time
noise averaging time
signal-to-noise threshold ratio
threshold trigger ratio
record length after threshold
ratio excess
maximum record length in
continuous duty
power consumption
10
0
2 -10
c
rl
+=
2 -20
.d
0‘
.A
r i
n
3 -30
-40
- 60
- 60
F.requency, f[z
Overall response curve of broad-band analogue-digital OBS.
256 K words
1-120 s
600-7200 s
1-4
1-4
1-200 min
18 hr
1.5 w
As the upper frequency of the digital unit and the lower
frequency of the analogue unit overlap in the 0.1-0.3Hz
band, it was possible to combine low and high-frequency
noise data with adequate reliability. At the same time, by
dividing the frequency band into two sections, it was
possible to make the most of the memory capacity of the
digital recorder and increase the whole length of operation
of the bottom seismograph.
The response curve of the vertical sensor and recording
channel of an OBS with an electrochemical transducer is
shown in Fig. 3.
At each study area, the long-period seismic noise was
recorded in programmed runs for a total of 18hr. In both
cases there were no significant teleseismic events during the
recording periods. 30 min recording intervals were used for
spectrum analysis and to obtain autocorrelation and
intercorrelation functions. We chose intervals during which
the ocean condition remained most calm: (1) sea surface
state at these study areas during the experiments was equal
to 3-5 on the Beaufort scale; and (2) near-bottom currents
were below 3 cm s-I.
20
Figure 3.
117
118
T. A . Dozorov and S. L. Soloviev
.. .
.. . .-
. . ..
.
. .
.
. ..
... .. - .
- .. - . . . .. .
..
.
.
..
t
L
0
T
A
2
I0
8
G
I2
I4
IG
-rt--ft
100 sec
Figure 4. A sample of a visualized record of a vertical seismic channel made by digital recorder in the ultra-low-frequency pass-band of
0.01-0.3 Hz,the Reykjanes ridge.
MAJOR CHARACTERISTICS OF BOTTOM
NOISE SPECTRA
Figure 4 shows a sample of a plot of signals from the vertical
channel recorded on the low-frequency digital channel of
the OBS.
Figure 5 shows noise spectra for vertical oscillations
recorded at the given study areas. The square root of the
spectral power of displacement is plotted on the vertical
axis. Instrumental responses are removed.
The considerable dynamic range covered by the spectrum
(about 160dB) was due, first, to non-uniform gain of the
seismic sensors which recorded long-period oscillations of
high intensity with low gain; second, automatic gain control
in the digital recorder circuits depending on microseism
level; third, non-uniformity of t h e response curve of the
analogue recorder; fourth, the combination of the results
obtained by the digital recorder and those obtained by the
analogue recorder (with dynamic ranges of the order of
70 dB each); and some other factors.
The data obtained show that the Reykjanes study area
differs from the Canary area, the former being characterized
by higher noise level in the 1-1OHz range and a lower
microsesmic noise value in the 0.01-0.1 Hz range.
Both spectral curves show two common regularities. The
first, which might have been expected, is as follows: in the
frequency range of approximately 0.1-1 Hz, high seismic
noise was recorded. In fact this is a well-known storm
microseism range which has been a subject of land and,
more recently, ocean-bottom seismic investigations since the
beginning of the 20th century. Different study areas are
characterized by different positions of maxima and minima:
10-5
10‘
40-~
10”
I
0.1
10
Figure 5. The bottom seismic noise spectra recorded under quiet condition of the ocean. C-€anary study area; R-Reykjanes
The line with dots shows the estimation of the instrumental equivalent noise. CPA-data from Sutton et al. (1988).
study area.
Spectra of ocean-bottom seismic noise
this may be due to regional peculiarities of the ocean surface
conditions and the depth of the ocean.
The second characteristic of the bottom noise spectra is of
fundamental significance for further development of marine
seismology. It is pronounced minimum in the 0.05-0.1 Hz
range.
The minimum in the seismic noise spectrum registered in
the area of the Reykjanes ridge occurs at a frequency of
0.07-0.08Hz and is 20dB below the level for 0.02 and
0.17Hz. The minimum in the noise spectrum curve in the
area of Canary upwelling occurs at a frequency of
0.09-0.10 Hz. It is 22 dB lower than the spectrum level for
0.03 and 0.25 Hz.
The considerably greater energy in the lower part of the
range under consideration, i.e. 0.01-0.1 Hz, recorded in the
area of the Canary upwelling is probably caused by direct or
indirect influence of the bottom current on the equipment
used. The results of hydrophysical studies reveal the
complicated and heterogeneous character of near-bottom
currents in the Canary upwelling area (Bulatov & Miroshkin
1986).
COMPARISON W I T H T H E P R E V I O U S L Y
REPORTED D A T A
To compare our results with those previously reported (see
the introduction), we used the most relevant data from Fix
119
(1972), Prothero & Schaecher (1984) and Sutton et al.
(1988). Spectral density curves were reduced to the same
scale and an averaged curve found for the two previously
described study areas in the Atlantic Ocean is shown in Fig.
6 together with some other curves. The comparison shows
that for 0.2-0.8 Hz, the noise levels recorded in the Atlantic
Ocean (curve 1) and in the Santa Barbara Channel (curve 2)
fully coincide. A considerable divergency for frequencies
above 1Hz probably has a regional character because
spectral curves, described in Prothero & Schaecher (1984)
which were found for the deeper waters (depth 3660 m) give
noise density data in the range of 1-10 Hz, similar to ours.
The reliability of the results by Prothero & Schaecher in
the frequency range of 0.01-0.15Hz causes some doubt.
Their instrument used short-period geophones of magnetoelectric type operating at natural frequency of 1Hz and
correcting amplifiers with a gain of more than 3 X lo3 at
0.033 Hz, and for this reason the instrumental input noise of
the seismic channel in the low frequency range was probably
considerable. Curve 3 shows the equivalent noise level in
the seismic channels. This curve is taken from Prothero &
Schaecher (1984). The comparative analysis of curves 2 and
3 shows that the working parameters of the equipment used
have given no reliable data about constituent values in
spectra below 0.15 Hz. It is known that spectral density of
two non-correlating intensity equivalent processes occurring
greater than each of the given
randomly gives a level
~
io-3
10=
\
E
r
: IO-~
v)
z
w
D
cl
4
a
lo4
a
m
4n4
1U
0,Ol
1
0.l
F I? E 12 11 E N C Y,
I0
HZ
Figure 6. The seismic noise spectra obtained as a result of different investigations. 1-averaged
bottom noise spectrum at the Atlantic Ocean
(on the Canary and Reykjanes study areas); 2-bottom noise spectrum obtained in the Santa Barbara Channel near California (Prothero &
Schaecher 1984); %the instrumental equivalent noise level registered during the experiment in the Santa Barbara Channel; &the spectrum
of the noise obtained in the result of the land-based observations [in the western part of North America, in a mine near Queen Creek,
Arizona, USA (Fix 1972)l.
120
T. A . Dozorov and S. L. Soloviev
processes (Bendat & Piersol 1980). It is to be noted that at
given receiver noise values (curve 3) and resulting natural
data (curve 2) Prothero & Schaecher should have drawn an
actual seismic noise spectrum curve in the frequency range
of 0.01-0.15 Hz which is a factor of 1.4 below curve 3.
The equivalent spectral noise level of the transducers
which we used was up to
m HZ-’’’ at the frequency
value of 0.01 Hz, which allows hope that our results are
more reliable than those described in Prothero & Schaecher
(1984).
Additional comparison of our spectra with spectra found
from observations carried out with the help of the
Columbia-Point Arena installation (Sutton et al. 1988) for
the 0.01-6 Hz pass-band shows (Fig. 5) that frequencies of
spectral minima and maxima respectively below and above
0.1 Hz in ocean-bottom seismic noise to the WSW of San
Francisco and to the south of Iceland coincide more or less.
But a small secondary maximum at the frequency of about
0.06 Hz was registered at the Pacific site. The level of noise
on the Reykjanes site in the 0.2-5Hz range is higher
perhaps because of shallower water (980111 instead of
3900m) and heavy sea (4-5 Beaufort numbers) during the
experiment.
To compare ocean-bottom and land seismic noise levels,
curve 4 (Fig. 6) is also used. This curve was described in Fix
(1972). It reflects the spectrum of noise as recorded in a
‘quiet’ zone on land. It is clear that in the frequency range
of 0.04-0.3 Hz microseisms prevail.
The comparison of curves 1 and 4 shows that the
ocean-bottom spectrum appears to be shifted towards the
higher frequencies and raised relative to the land spectrum.
If on land the main maximum of the spectra is seen at
0.11 Hz, with the secondary maximum at 0.055 Hz and the
minimum at O.O35Hz, on the sea-floor these extrema are,
respectively 0.30, 0.15 and 0.08 Hz, i.e. at frequencies which
are higher approximately by a factor 2.3-2.7. If the extrema
of the spectra of ocean-bottom and land noise are reduced
to one and the same frequency, then the noise level on the
sea-floor will be by 20-30dB higher than on land, which
agrees with the other results obtained in comparing noise
data on the sea-floor, in bottom boreholes and on land
(Adair et al. 1986; Duennebier et al. 1987). The bottom
seismic noise minimum occurring at 0.05-0.1 Hz is very
important regardless of any other data that can be obtained
in ultra-low-frequency domain. The occurrence of this
minimum makes it possible to propose some substantially
new methods of marine seismological investigations, i.e.
methods of routine bottom recording of both short-period
body seismic waves in the generally occurring (Ostrovskiy
1982) minimum at the frequency of 8-10 Hz [which actually
makes it possible to investigate hypocentral fields of local
microearthquakes (Soloviev et al. 1990)], and 10-20 s period
surface waves from strong earthquakes. This means that the
newest perspectives of the adequate bottom remote
earthquakes recording are opened; new methods of the
velocity section reconstructions for the ocean crust and
upper mantle using surface wave dispersion can be
determined.
The comparison of the vertical and horizontal components
of seismic oscillations has shown that in the 1-10Hz range
they are of much the same intensity, but the horizontal
components grow stronger as the frequency falls below
1Hz, whereas in the 0.01-0.1 Hz range, on the average, the
horizontal components are 10 times higher than vertical
ones. The spectra of fluctuations in the near-bottom
pressure resemble the spectra of vertical oscillations of the
bottom, but in the 0.01-0.1 Hz band pressure fluctuations
are relatively stronger than bottom oscillations and in the
1-10 Hz range they are relatively weaker. The autocorrelation functions and the functions of mutual correlation
plotted for all recording channels and for both study areas
showed a well-pronounced oscillatory character with periods
of 600, 540, 470 and 290 s. However, all the results listed in
this section need verification and further analysis, and are
therefore not discussed in this paper.
CONCLUSIONS
(1) Bottom seismic noise has been recorded in the wide
frequency range from 0.01 to 10 Hz in two study areas in the
Atlantic Ocean. The results for the ultra-low-frequency
band of 0.01-0.1 Hz, which has been studied little, are of
special interest.
(2) Although scanty, the observational data show a
minimum in the microseismic noise level in the range of
0.05-0.1 Hz. This minimum would be useful for bottom
recording of strong teleseismic events (provided that there
are no strong near-bottom currents).
ACKNOWLEDGMENTS
The authors wish to thank Dr 0. V. Kopelevich for his
general management of the 49th cruise of the R/V
‘Akademik Kurchatov’, Dr E. A. Kontar for his general
management of the experiments on the Canary upwelling
and Reykjanes ridge study areas, Mrs S. A. Mkhitaryan, 0.
R. Mkrtchan and V. P. Pleshka for their assistance in
processing the experimental data, Dr M. A. Zhdanov for
valuable comments, and two anonymous reviewers whose
constructive criticism has helped us to improve the form and
content of the paper.
REFERENCES
Abramov, 0. K., Grafov, B. M., Ermolin, V. I., Iskhakov, I. Kh.,
Negmatullin, R. Sh., Novitskiy, M. A., Sirotinskiy, Yu. V. &
Fedorov, S. A., 1978. Electrochemical sensors of mechanical
oscillations and possibilities to applicate them in seismometry,
Seismicheskie Pribory (Seismic Instruments), 11, 203-208 (in
Russian).
Adair, R. G., Orcutt, J. A. & Jordan, Th.H., 1986. Low-frequency
noise observations in the deep ocean, J. acowt. SOC. A m . , 80,
633-645.
Adair, R. G., Orcutt, J. A. & Jordan, Th.H., 1987. Preliminary
analysis of ocean-bottom and sub-bottom microseismic noise
during the Ngendei experiment, Init. Repts of DSDP, 88/91,
357-375.
Akhsakhalyan, G. A., Beiburdyan, G. E., Karapetyan, S. V.,
Mkrtchan, 0. R., Mkhitaryan, S. A,, Mkhitaryan, S. Kh. &
Sargsyan K. R., 1989. Analogue-digital bottom seismic station
with microprocessor control, Morskaya Seismologiya i Sebmometriya (Marine Seismology and Seismometry ), pp. 49-54,
Institute of Oceanology, Moscow (in Russian).
Auld, B., Latham, G., Nowroozi, A. & Seeber, L., 1969.
Seismicity off the coast of Northern California determined from
ocean bottom seismic measurements, Bull. seism. SOC. A m . ,
69, 2001-2015.
Spectra of ocean-bottom seismic noise
Bendat, J. S. & Piersol, A. G., 1980. Engineering Applications of
Correlation and Spectral Analysis, Wiley, New York.
Bradner, H. & Dodds, J., 1964. Comparative seismic noise on the
ocean bottom and on land, 1.geophys. Res., 69, 4339-4348.
Brune, J. N. & Oliver, J., 1959. The seismic noise of the Earth’s
surface, Bull seism. SOC.A m . , 49, 349-353.
Bulatov, R. P. & Miroshkin, K. V., 1986. The dynamic
characteristicsof the Canary upwelling, Fisicheskie i Okeanograficheskie Issledovaniya Tropicheskoy Atlantiki (Physical and
Oceanographic lnvestigations in the Tropical Atlantic), pp.
254-261, Nauka, Moscow (in Russian).
Byrne, D. A., Harris, D., Duennebier, F. D. & Cessaro, R., 1987.
Technical review of the ocean subbottom seismometer system
installed in deep sea drilling project site 581 C, leg 88. Init.
Repts of DSDP, 88, 65-88.
Carmichael, D., Carpenter, G., Hubbard, A. & McDonald, W.,
1973. A recording ocean bottom seismograph, J . geophys.
Res., 78, 8748-8750.
Duennebier, F. K., McCreery, C. S., Harris, D., Cessaro, R. K.,
Fisher, G. & Anderson, P. N., 1987. OSS IV: noise levels,
signal to noise ratios, and noise sources, Znit. Repts of DSDP,
88,89-103.
Fix, E., 1972. Ambient Earth motion in the period range from 1 to
2560sec, Bull. seism. SOC.A m . , 62, 1753-1760.
Francis, T. J. G. & Porter, I. T., 1977. Experience gained with the
Blacknest ocean bottom seismograph, Mar. geophys. Res., 3,
143-150.
Goodman, D., Yamamoto, T., Trevorrow, M., Abbot, Ch.,
Turgut, A,, Badiey, M. & Ando, K., 1989. Directional spectra
observations of seafloor microseisms from an ocean-bottom
seismometer array, J . acoust. Soc. A m . , 86, 2309-2317.
Harris, D., Cessaro, R. K., Duennebier, F. K. & Byrne, D. A.,
1988. A permanent seismic station beneath the ocean bottom,
Mar. geophys. Res., 9, 67-94.
Haubrich, R. A. & MacKenzie, G. S., 1965. Earth noise, 5 to 500
millicycles per second. 2, Reaction of the earth to oceans and
atmosphere, 1. geophys. Res., 70, 1429-1440.
Latham, G. V. & Nowroozi, A. A., 1968. Waves, weather and
ocean bottom microseisms, J. geophys. Res., 73, 3945-3956.
Mark, N. & Sutton, G. H., 1969. Particle motion studies from an
ocean bottom seismograph, EOS, Trans. A m . geophys. Un.,
50,644.
Murphy, A. J. & Savino, J . R., 1975. A comprehensive study of
long-period (20-200 sec) earth noise at the high-gain world
wide seismograph stations, Bull. seism. SOC. A m . , 65,
1827-1862.
Nowroozi, A. A., 1973. Seismicity of the Mendocino escarpment
and the aftershock sequence of June 26, 1968, ocean bottom
seismic measurements, Bull. seism. SOC.A m . , 63, 441-456.
Nowroozi, A. A., Sutton, G. & Auld, B., 1966. Ocean tides
recorded on the sea floor, Ann. de Geophysique, 22, 512-517.
Nowroozi, A. A., Kuo, J. & Ewing, M., 1969. Solid Earth and
oceanic tides recorded on the ocean floor off the coast of
Northern California, J. geophys. Res., 74, 605-614.
Nowroozi, A. A., Ewing, M., Nafe, J. & Fliegel, M., 1968. Deep
ocean current and correlation with the ocean tide off the coasts
of northern California, J. geophys. Res., 73, 1921-1932.
Odegard, M. E. & Sutton, G. H., 1969. Earthquakes detection
capabilities of an ocean-bottom seismometer, EOS, Trans.
Am. geophys. U n . , 50, 644.
Odegard, M. E., Mark, N., Letourneau, N. J. & Kwon, T. H.,
1969. Ocean-bottom seismographic station (OBS) catalogue of
events for the period from 1 February, 1967 to 31 January,
1968, Haw. Inst. Geophys. Data Rep. N 13, HIG-69-18.
Ostrovskiy, A. A., 1982. The generalized spectra of bottom seismic
121
noise of the world ocean, Okeanologiya (Oceanology), XXII,
980-983 (in Russian).
Prentiss, D. D. & Ewing, J. I., 1963. The seismic motion of the
deep ocean floor, Bull. seism. Soc. A m . , 53,765-781.
Prothero, W. A. Jr, 1979. An operationally optimized oceanbottom seismometer capsule, Phys. Earth planet. Inter., 18,
71-77.
Prothero, W. & Munk, W. H., 1974. Benthic Array, Advance
Ocean Engineering Laboratory Report No. 65, SIO Reference
Number 74-24.
Prothero, W. A. Jr & Schaecher, W., 1984. First noise and
teleseismic recordings on a new ocean bottom seismometer
capsule, Bull. seism. SOC. A m . , 74, 1043-1058.
Savino, J. M., McCamy, K. & Hade, G., 1972. Structures in Earth
noise beyond twenty seconds, a window for earthquakes, Bull.
seism. SOC. A m . , 62, 141-176.
Schneider, W. A., Farrell, P. J . & Brannian, R. E., 1964.
Collection and analysis of Pacific Ocean bottom seismic data,
Geophysics, 29, 745-771.
Soloviev, S. L., 1985. History and Perspectives of the Marine
Seismology, Nauka, Moscow (in Russian).
Soloviev, S. L., 1986. Seismological Ocean Bottom Observations in
the USSR and Abroad, Nauka, Moscow (in Russian).
Soloviev, S. L., 1990. Unquiet life of the seafloor, Priroda
(Nature), 4, 27-31 (in Russian).
Soloviev, S. L., Kontar, E. A., Dozorov, T. A. & Kovachev, S. A.,
1988. A deep-water ocean bottom pop-up seismological station
ADS-8, Fizika Zemli (Physics of the Earth), 9, 75-85 (in
Russian).
Soloviev, S. L., Kuzin, I. P., Kovachev, S. A., Ferri, M., Guerra,
I. & Luongo, G., 1990. Tectonic and volcanic
microearthquakes in Tyrrhenian sea as revealed by joint land
and sea-bottom experiment, Mar. Geol., 94, 131-146.
Sutton, G. H., Odegard, M. E. & Hussong, D. M., 1973.
Telemetering ocean-bottom seismograph, Earthquake Notes,
44,12.
Sutton, G. H., Barstow, N. & Carter, J . A., 1988. Long-period
seismic measurements on the ocean floor, Proceedings of a
Workshop on Broad-Band Downhole Seismometers in the Deep
Ocean, April 26-28, 1988, pp. 126-142, Woods Hole
Oceanographic Institution, MA.
Sutton, G. H., Macdonald, W. G., Frantiss, D . D. & Thanos, S.
N., 1965. Ocean bottom seismic observations, Proc. IEEE, 53,
1909-1921.
Sutton, G. H., Odegard, M. E., Mark, M. & Le Tourneau, N. J.,
1970. Research in seismology related to the Colombia
ocean-bottom seismograph, Techn. R e p . Univ. Haw, HIG-7012.
Trevorrow, M. V., Yamamoto, T., Turgut, A. & Goodman, D.,
1989a. Measurements of ambient seabed seismic levels below
1.0Hz on the shallow eastern U.S. continental shelf, J . accoust.
SOC.A m . , 86, 2131-2327.
Trevorrow, M. V., Yamamoto, T., Turgut, A., Goodman, D. &
Badiey, M., 1989b. Very low frequency ocean bottom ambient
seismic noise and coupling on the shallow continental shelf,
Mar. geophys. Res., 11, 129-152.
Webb, S. & Cox, Ch., 1984. Pressure and electric fluctuations on
the deep seafloor: background noise for seismic detection,
Geophys. Res. Lett. 11,967-970.
Webb, S. & Cox, Ch., 1986. Observations and modelling of seafloor
microseisms, J . geophys. Res., 91, 7343-7358.
Whorf, T., 1972. Teleseismic and noise monitoring with the
Block-Moors quartz accelerometer, Geophys. J . R. ustr. Soc.,
31, 205-238.