Passive seismic experiment design changes and tests for Viking Mars Progra

Anderson, D. L.

English

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3 
Cal i fo rn ia  I n s t i t u t e  of Technology 
Seismological Laboratory 
Pasadena, Ca l i fo rn ia  
Progress Report 
May 15, 1969 - August 31, 1969 
NASA GRANT NO. : NGR 05-002-129 
PROJECT TITLE : Development of Performance Criteria 
and Functional Spec i f ica t ions  f o r  a 
Passive S e i s m i c  Experiment on Mars 
PRINCIPAL INVESTIGATOR: Don L e  Anderson 
https://ntrs.nasa.gov/search.jsp?R=19700003091 2020-03-12T04:42:19+00:00Z
Seismometer-Development 
A new package of three sensors based on the  design of t h e  earlier 
experimental model i s  cur ren t ly  being constructed. Attention i s  being 
given t o  reducing t h e  weight and volume and t o  caging of the  mass f o r  
v i b r a t i o n  and shock surv iva l .  However, f ea tu res  are being re ta ined  
t o  f a c i l i t a t e  changes, replacements, and adjustments which would be 
replaced by "hard" assembly i n  f l i g h t  models e Although shape f a c t o r s  
f o r  weight reduction, r i g i d i t y ,  etc., are being sc ru t in i zed ,  conventional 
and r e a d i l y  obtainable materials are being u t i l i z e d  t o  avoid delay. 
The e f f e c t  of s u b s t i t u t i o n s  i n  fu tu re  generation models i s  predic tab le .  
Figure 1 shows a s i n g l e  seismometer un i t .  Its weight is 3 .8  02. 
The dimensions of t he  u n i t  are 1 1/8" x 1 1/8" x 2 3 / 4 " .  It i s  estimated 
t h a t  t h r e e  u n i t s  can be packaged i n  a container with dimensions of 
3" x 4" x 1 1/2". 
Seismic Data Prscessor 
The seismic d a t a  processor amplifies and conditions the  seismic 
s i g n a l  f o r  presenta t ion  t o  the  lander in t e r f ace .  
allow f o r  t h ree  modes of operation: 
It is  designed t o  
1. High da ta  rate, 20 samples/sec/channel, three channels. 
This mode is intended t o  be  i n i t i a t e d  by e a r t h  command at  
t i m e s  when the telemetry can accommodate the b i t  rate. 
2. Low d a t a  rate, 4 samples/min/channel, six channels. Thfs 
i s  the  normal Qperating mode where the  microseism background 
is  sampled wi th in  narrow frequency bands, 
2. 
3. Moderate d a t a  rate, 1 sample/sec/channel, s i x  channels. 
This mode is  i n i t i a t e d  e i t h e r  by Earth command o r  automatically 
upon the  de t ec t ion  of a seismic event by the  event t r i g g e r .  
I n  t h i s  mode some form of d a t a  compression must be used t o  
preserve t h e  wideband seismic information while consuming 
a minimum number of b i t s .  
Work has  continued on t h e  construction of t h e  Seismic Data Processor 
with the  goal of performing in t eg ra t ed  systems tests i n  conjunction 
with seismometers. Present ly  being assembled are the  pre-amplifiers, 
ampl i f i e r s ,  f i l t e r s ,  averaging circuits and t h e  event t r igge r .  These 
ind iv idua l  elements may b e  arranged such t h a t  the system takes on a 
number of configurations allowing f o r  t he  several modifications i n  
system concept which have evolved as more information on the  lander 
has become ava i lab le .  
t he  d i r e c t  wideband d a t a  and two channels of f i l t e r e d  and averaged 
d a t a  f o r  each of t he  three seismic axes. The bandwidths of the low-pass 
f i l t e r s  are 4 hz and 0.5 hz. 
due t o  t h e  seismometer frequency c h a r a c t e r i s t i c s ,  no add i t iona l  ampl i f ie rs  
are needed t o  normalize the  channels. 
s c a l i n g  of the signal is performed because f i rm  information 
po in t s  has  not  been received from the  Prime Contractor 
Pro jec t .  
The latest configuration presents  t o  the lander,  
These two bandwidths are unique i n  t h a t ,  
No conditioning of power o r  
these  
f o r  t h e  Viking 
The c i r c u i t r y  is being assembled on 4 by 5 inch p r in t ed  c i r c u i t  
boards f o r  ease of assembly and handling and t o  allow f l e x i b i l i t y  
i n  performing the systems tests 
t i m e  t o  package the  e l ec t ron ic s  t o  meet the  mechanical 
No attempt i s  being made a t  t h i s  
3. 
(weight, shock, and v ibra t ion)  cons t r a in t s  of f l i g h t  hardware. Although 
t h e  power consumption of the  d a t a  processor is s t i l l  approximately 
100 m i l l i w a t t s ,  t h e  weight, i n  i t s  present  form, of packaging is  
considerably over the  approximate one pound a l l o t t e d  f o r  the  e l ec t ron ic s .  
Seismic Event Trigger 
The Seismic Event Trigger i s  designed t o  d e t e c t  an event when 
i t s  amplitude exceeds a predetermined mul t ip le  of the average background 
level. 
t h e  t r i g g e r  po in t  so t h a t  i t  remains a f ixed  mul t ip le  of the  average 
background. 
It allows f o r  an unknown o r  varying background by ad jus t ing  
The mul t ip le  is s e l e c t a b l e  by Earth command. 
Testing of the breadboard t r i g g e r  has been performed using the  
vertical component of, t he  ee2sndc s i g n a l  which i o  tclemetered from 
Glamis, Cal i fo rn ia  t o  t h e  Seismological Laboratory i n  Pasadena. 
The t r i g g e r ' s  a b i l i t y  t o  de t ec t  seismic events and t o  accomadate 
v a r i a t i o n s  i n  t h e  microseism level were exce l len t .  
however, t h a t  on some small events,  t h e  t r i g g e r ,  a f t e r  de tec t ing  the  
P-wave, would reset before the  arrival of t he  S-wave, whereupon i t  
would t r i g g e r  again. 
delay following t h e  t r i gge r .  
of t i m e  t h a t  an event operates t he  t r i g g e r  and varies from zero f o r  
an impulse ( t y p i c a l  of seismic noise  which might cause f a l s e  t r igger ing)  
* 
The tests revealed, 
This w a s  corrected by the  addi t ion  of a va r i ab le  
The delay is  dependent on the  length 
* 
1 sec instrument with the  ve loc i ty  response peaking a t  4 hza 
4. 
t o  approximately one minute f o r  an event l a s t i n g  three minutes o r  more. 
The r e s u l t  is  t h a t  t he  t r i g g e r  has an on t i m e  which varies from 
approximately two seconds f o r  a f a l s e  t r i g g e r  t o  a maximum of four  
minutes f o r  the  longes t  event. 
Figure 2 shows a 2 4 h o u r  seismogram and the  t r i g g e r  output with 
nineteen earthquakes detected while accommodating a s i x  t i m e s  change 
i n  t h e  microseism background over t h i s  period. 
t y p i c a l l y  80% of the  t r i g g e r  t i m e  represented earthquakes. 
20% w a s  due t o  "gli tches" e i t h e r  of seismic o r  telemetered o r ig in ,  
o r  seismic events so bur ied  i n  t h e  microseisms t h a t  they were no t  usefu l .  
For the  G l a m i s  s i g n a l ,  
The remaining 
Data Compression 
5 As of t h i s  wr i t i ng  w e  are informed t h a t  1.7 x 10 b i t s  per  day are 
This would allow s i x  minutes of a l loca t ed  f o r  t h e  seismic experiment. 
operation i f  t h e  t h r e e  axis seismic da ta  were sampled a t  the 20 samples/ 
second rate necessary f o r  recovery of the  wideband data.  
i s  studying methods of da t a  compression which w i l l  allow reasonable 
recovery of the  wideband d a t a  while sampling a t  a moderate rate of 
one sampla/second during t i m e s  t h a t  t he  event de t ec to r  s i g n a l s  t he  
presence of a seismic event. 
M. Nafi Toksoz 
The most promising of t h e  da t a  compression schemes, which r e s u l t s  
i n  an approximate 10 t o  1 compression, is one t n  which only t h e  
amplitude and t i m e  of peak values of the  seismic wave are sampled. 
It is  f e a s i b l e  t o  implement t h i s  technique e l e c t r o n i c a l l y ,  b u t  it places 
a g rea t  burden upon t h e  lander i n  t h a t  i t  must service the  seismic 
experiment and s t o r e  d a t a  i n  a completely aper iodic  manner i n  order 
5 .  
t o  e f f e c t  t h e  savings i n  t o t a l  b i t s .  We have not  proceeded on t h e  c i r c u i t r y  
t o  implement t h i s  system because the  lander d a t a  acqu i s i t i on  u n i t  i s  not 
versatile enough t o  handle t h i s  function and weight cons t r a in t s  preclude 
implementation by t h e  experiment. The a l t e r n a t i v e  t o  the  lander 
providing aper iodic  se rv i c ing  is  f o r  t h e  Seismic Data Processor t o  perform 
sampling and temporary d a t a  s torage  of up t o  1000 b i t s  f o r  four minutes. 
This is not  f e a s i b l e  wi th in  the  weight and volume a l loca ted  t o  the  seismic 
experiment. 
Because of the foregoing, another approach t o  d a t a  compression has 
been developed which requi res  only t h a t  t h e  lander be capable, upon the  
command of t h e  event de t ec to r ,  of changing the  sampling rate on the  seismic 
channels from t h e  low t o  the  moderate rate. I n  an informal discussion with 
t h e  r ep resen ta t ive  of t h e  Pr ime  Contractor i t  w a s  ind ica ted  t h a t  t h i s  is  
poss ib le  wi th in  t h e i r  present framework. 
This da t a  compression scheme cons i s t s  of a seismic event 
envelope de tec to r  and an ax i s  crossing counter f o r  each seismic 
axis. 
one sample/second gives a 10 t o  1 d a t a  compression. 
of t he  o r i g i n a l  seismogram is performed by passing the  seismic envelope 
s i g n a l  through an inverse  f i l t e r  and convolving with t h e  instrument 
response according t o  t h e  axis crossing count wi th in  each sampling i n t e r n a l .  
Two da ta  words pe r  sample are required which when sampled a t  
Reconstruction 
Implementing t h i s  system a l s o  m a k e s  mul t ip le .use  of e x i s t i n g  
hardware i n  t h a t  t he  seismic envelope de tec to r  can be  constructed 
out of t h e  low-pass f i l t e r  and averagin c i r c u i t s  already required,  
6. 
Only t h e  addi t ion  of t he  ax i s  c ross ing  counter (approximately 10 m i l l i w a t t s  
of power) and some e l e c t r o n i c  switches are required t o  implement and 
commutate the  d a t a  compression system. A block diagram f o r  one a x i s  
of t he  complete system is  shown i n  f igu re  3. 
cont ro ls  t h e  pos i t i on  of t he  switches which i n  t u r n  changes the  system 
from two channels of f i l t e r e d  and averaged da ta  t o  one channel f o r  
the  seismic envelope and one f o r  the  axis-crossing count. For both 
Lhe low and t h e  moderate da ta  rates, s i x  channels of analog signals 
are presented t o  the  lander i n t e r f a c e  f o r  d i g i t i z a t i o n  and s to rage ,  
Figures 4 and 5 show seismograms and t h e  output from the  envelope 
de tec to r  using t h e  technique of f igu re  3. 
The seismic event t r i g g e r  
Problem Areas 
1, 
2. 
3. 
W e  need f i rm assurance from the  Pr ime  Contractor t h a t  t he  t h r e e  
modes of operation of t he  da t a  processor are f u l l y  compatible 
with t h e  lander ' s  c a p a b i l i t i e s  f o r  commutation, sampling and 
s to rage  of data.  
W e  need firm idiferrmation pe r t a in ing  t o  the electrical i n t e r f a c e  
iwith the  lander as t o  s i g n a l  levels, impedances and power 
conditioning. 
The seismic d a t a  processor is  designed around what w e  deem are 
f lightxmrthy -components. I f ,  however, a mandatory p a r t s  list i s  i n  
ex is tence  o r  i s  going t o  be generated, w e  should know about i t  as 
soon as poss ib l e  so t h a t  w e  may incorpora te  these  p a r t s  i n t o  our 
design, o r  have some influence on the  p a r t s  which are approved 
f o r  f l i g h t .  
7. 
4 .  As mentioned earlier, the present  e l e c t r o n i c  packaging of t h e  
d a t a  processor,  although no t  designed as f l i g h t  hardware, is 
considerably overweight. 
weight t o  an acceptable level. No problem is an t i c ipa t ed  i n  the  
sensor weight. 
Further work is  needed t o  reduce the  
E T E R  
SHOWN ACTUAL SIZE 
F I G U R E  1 
h TYPICAL TRIGGER OUTPUT 
1 M I N U T L  t+ EVENT DETECTOR RECORD 
v TYPICAL EARTHQUAKES S E I S M O G R q M  
S E I S M I C  E V E N T  D E T E C T O R  TEST 
FI-GURE 2 
SEISMOMETER PRE-AMP AMP 
HIGH DATA RATE 
e--+ 
lsal 0 LOW/MODERATE DATA RATE 
n LOW/MODERATE SAMPLING COMMAND 
TRIGGER I/ 
I OVERRIDE, EARTH COMMAND 
SEISMIC DATA PROCESSER 
SINGLE CHANNEL 
FIGURE 3 
SEISMIC ENVELOPE DETECTOR T E S T  
F I G U R E  4 
SEISMIC ENVELOPE DETECTOR 
FIGURE 5 


Development of performance criteria and functional specifications for a passive seismic experiment on Mars Progress report, 15 ~MAY - 31 ~AUG. 1969

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