Randall, as a rule I very seldom respond to articles on PSN however, this i= s a rare exception. Your opinion and thoughts on seismology=A0are right on = and when you express your opinions one knows you are completely correct . W= hat a refreshing article. I commend you my friend. John Cole, Pearland,Tx. = ..( amateur seismologist)=A0=0A=0A=0A=0A=0A________________________________= =0AFrom: Randall Peters=0ATo: "psn-l@.............."= =0ASent: Mon, December 7, 2009 8:43:23 AM=0ASubject:= RE: Integrating in WinQuake=0A=0A=0AIt is high time that everybody in the = amateur seismology community (as well as the professional one) understand s= omething that is FUNDAMENTAL PHYSICS.. The ONLY thing that ANY seismometer'= s mechanical parts EVER respond to is ACCELERATION. Let the skeptic of this= claim go study not just Newton's laws as treated in elementary textbooks, = but also Einstein's principle of relativity. The direct response to acceler= ation that I'm talking about is the motion of the instrument's main mass (c= alled inertial) relative to the case of the instrument. The only thing that= can cause relative motion between these two components of the instrument i= s acceleration of the case. The case acceleration is the same as the accele= ration of that part of the (local) earth on which the instrument sits (at r= est relative to this piece of earth). The nature of the trace that is outpu= t from the instrument is determined not only by this relative motion of the= se two mechanical components but also by the nature of the electronics that monitors the motion. With a= coil/magnet (Faraday law) detector of classical type, the output voltage i= s proportional to the time rate of change (derivative) of the relative moti= on of mass and case. (assuming `flat electronics' for the amplifier circuit= s.) It is a velocity sensor only in terms of what it is measuring; i.e., th= e relative velocity of mass and case. This IS NOT, however, necessarily a m= easure of the velocity of the local-earth itself. The earth velocity is the= integral of the acceleration of the earth. The nature of the instrument's = response to the earth's quintessential acceleration depends on the frequenc= y of the harmonic acceleration that causes an output. There are two parts t= o this matter, both the frequency respnse of the mass/case and also the fre= quency response of the detector. The easiest mechanical type to understand = is a simple pendulum. If the frequency of the earth's acceleration is lower than the natural frequency of the pendulum, then the angular disp= lacement of the pendulum is proportional to the acceleration. For this freq= uency regime, the coil/magnet output is therefore proportional to the deriv= ative of local-earth acceleration. In engineering terminology, this is call= ed a `jerk' detector.=A0 On the other hand, for local-earth acceleration fr= equencies higher than the natural frequency of the pendulum, the output is = indeed proportional to the local-earth velocity. This is because the pendul= um response to acceleration for frequency higher than its natural frequency= is proportional to reciprocal frequency squared. Since Chris has mentioned= the highly-esteemed Erhard Wielandt, let me point out that Professor Wiela= ndt has agreed with me on this matter; i.e., the coil/magnet detector is a = measure of local-earth velocity for harmonic motions having a frequency hig= her than the natural frequency of the pendulum. Conversely, for frequencies lower than the natural frequency, the detector is a jerk detec= tor; i.e., it measures the time derivative of the acceleration of the earth= .. Now concerning a capacitive detector, it is important that one understand= s the architecture with which it is employed. With the VolksMeter, the most= important to me (non-integrated) output is one in which the symmetric-diff= erential-capacitive (SDC) detector measures the angular displacement of the= pendulum. For earth's harmonic accelerations that are at a frequency lower= than the natural frequency of the VM (0.92 Hz), the pendulum response (and= thus the SDC output) are proportionall to the earth's acceleration. One ob= tains then the earth's velocity (for frequency less than 0.92 Hz) by integr= ating this SDC output. In the case of a force-feedback instrument like thos= e of STS type (influenced by the expertise of Erhard Wielandt, built by Gun= ar Streckeisen), the output is=A0governed by=A0the poles/zeroes of the electronics designed to behave like a long-period-equivalent pendulum. The= total system (`pendulum plus all of the electronics) mimics that of a clas= sical coil/magnet=A0system monitoring a pendulum whose period is about 30 s= .. In other words, for earth accelerations in which the frequency is=A0highe= r than the lower corner frequency of the STS (approximately 0.03 Hz), the o= utput is proportional to earth velocity. For the spectral region of interes= t to many of us (teleseismic observations), one thus sees that the integrat= ed output of the VM can be compared directly and meaningfully with the heli= cord display of conventional force balance sensors whose pendulum-equivalen= t natural frequency is no=A0greater than about 0.03 Hz. The same can be sai= d of the garden-gate instruments that many of you use, IF your period has b= een adjusted to=A0be at least as=A0great as 20 s. So can one talk about ear= th displacement or earth velocity or earth acceleration on the basis of what one's instrument is measuring? The answer is obviously a qualified ye= s, but it is imperative that the transfer properties of both the mechanical= part (such as a pendulum) and also that of the electronics be properly fac= tored into what is concluded. In summary, different systems give different = results concerning what is direct output, as opposed to indirect, depending= on the detector type. A capacitive sensor as used in the VM measures earth= -acceleration directly for frequencies lower than 0.92 Hz. STS instruments,= though they use a capacitive sensor, but with sophisticated electronics in= volving an actuator, measure earth-velocity directly for frequencies=A0high= er than about 0.03 Hz. Consequently, the integrated output from the VM can = be compared directly with the output from an STS. Keep in mind that integra= ting a second time may or may not yield earth displacement, depending on th= e natural frequency of the instrument. One must `back out' the system response (total system transfer function) if the result is to have any mea= ning. For those of you using geophones, where the natural frequency is high= er than 1 Hz; your output is proportional to the derivative of earth accele= ration.=A0 Relative to earth motion (not the geophone mass) your detector i= s for frequencies of interest to most of us (not the local-environment high= frequency parts) -- a jerk detector=A0and not=A0a velocity detector, even = Randall, as a rule I very seldom respond to articles o= n PSN however, this is a rare exception. Your opinion and thoughts on seism= ology are right on and when you express your opinions one knows you ar= e completely correct . What a refreshing article. I commend you my friend. = John Cole, Pearland,Tx. .( amateur seismologist)=0A=0A
=0AFrom: Randall Peters <PETERS_RD@= mercer.edu>
To: "psn-= l@.............." <psn-l@..............>
Sent: Mon, December 7, 2009 8:43:23 AM
Subject: RE: Integrating in WinQuake<= BR>
=0A= =0A=0A=0A=0A=0AIt is high time that everybody in the amateur seismology community (as wel= l as the professional one) understand something that is FUNDAMENTAL PHYSICS= ... The ONLY thing that ANY seismometer's mechanical parts EVER respond to i= s ACCELERATION. Let the skeptic of this claim go study not just Newton's la= ws as treated in elementary textbooks, but also Einstein's principle of rel= ativity. The direct response to acceleration that I'm talking about is the = motion of the instrument's main mass (called inertial) relative to the case= of the instrument. The only thing that can cause relative motion between t= hese two components of the instrument is acceleration of the case. The case= acceleration is the same as the acceleration of that part of the (local) e= arth on which the instrument sits (at rest relative to this piece of earth)= .. The nature of the trace that is output from the instrument is determined = not only by this relative motion of these two mechanical components but also by the nature of the electronics that monitors the mot= ion. With a coil/magnet (Faraday law) detector of classical type, the outpu= t voltage is proportional to the time rate of change (derivative) of the re= lative motion of mass and case. (assuming `flat electronics' for the amplif= ier circuits.) It is a velocity sensor only in terms of what it is measurin= g; i.e., the relative velocity of mass and case. This IS NOT, however, nece= ssarily a measure of the velocity of the local-earth itself. The earth velo= city is the integral of the acceleration of the earth. The nature of the in= strument's response to the earth's quintessential acceleration depends on t= he frequency of the harmonic acceleration that causes an output. There are = two parts to this matter, both the frequency respnse of the mass/case and a= lso the frequency response of the detector. The easiest mechanical type to = understand is a simple pendulum. If the frequency of the earth's acceleration is lower than the natural frequency of the pendulum, then the= angular displacement of the pendulum is proportional to the acceleration. = For this frequency regime, the coil/magnet output is therefore proportional= to the derivative of local-earth acceleration. In engineering terminology,= this is called a `jerk' detector. On the other hand, for local-earth= acceleration frequencies higher than the natural frequency of the pendulum= , the output is indeed proportional to the local-earth velocity. This is be= cause the pendulum response to acceleration for frequency higher than its n= atural frequency is proportional to reciprocal frequency squared. Since Chr= is has mentioned the highly-esteemed Erhard Wielandt, let me point out that= Professor Wielandt has agreed with me on this matter; i.e., the coil/magne= t detector is a measure of local-earth velocity for harmonic motions having= a frequency higher than the natural frequency of the pendulum. Conversely, for frequencies lower than the natural frequency, the detector= is a jerk detector; i.e., it measures the time derivative of the accelerat= ion of the earth. Now concerning a capacitive detector, it is important tha= t one understands the architecture with which it is employed. With the Volk= sMeter, the most important to me (non-integrated) output is one in which th= e symmetric-differential-capacitive (SDC) detector measures the angular dis= placement of the pendulum. For earth's harmonic accelerations that are at a= frequency lower than the natural frequency of the VM (0.92 Hz), the pendul= um response (and thus the SDC output) are proportionall to the earth's acce= leration. One obtains then the earth's velocity (for frequency less than 0.= 92 Hz) by integrating this SDC output. In the case of a force-feedback inst= rument like those of STS type (influenced by the expertise of Erhard Wielan= dt, built by Gunar Streckeisen), the output is governed by the poles/zeroes of the electronics designed to behave like a long= -period-equivalent pendulum. The total system (`pendulum plus all of the el= ectronics) mimics that of a classical coil/magnet system monitoring a = pendulum whose period is about 30 s. In other words, for earth acceleration= s in which the frequency is higher than the lower corner frequency of = the STS (approximately 0.03 Hz), the output is proportional to earth veloci= ty. For the spectral region of interest to many of us (teleseismic observat= ions), one thus sees that the integrated output of the VM can be compared d= irectly and meaningfully with the helicord display of conventional force ba= lance sensors whose pendulum-equivalent natural frequency is no greate= r than about 0.03 Hz. The same can be said of the garden-gate instruments t= hat many of you use, IF your period has been adjusted to be at least a= s great as 20 s. So can one talk about earth displacement or earth velocity or earth acceleration on the basis of what one's instrument= is measuring? The answer is obviously a qualified yes, but it is imperativ= e that the transfer properties of both the mechanical part (such as a pendu= lum) and also that of the electronics be properly factored into what is con= cluded. In summary, different systems give different results concerning wha= t is direct output, as opposed to indirect, depending on the detector type.= A capacitive sensor as used in the VM measures earth-acceleration directly= for frequencies lower than 0.92 Hz. STS instruments, though they use a cap= acitive sensor, but with sophisticated electronics involving an actuator, m= easure earth-velocity directly for frequencies higher than about 0.03 = Hz. Consequently, the integrated output from the VM can be compared directl= y with the output from an STS. Keep in mind that integrating a second time = may or may not yield earth displacement, depending on the natural frequency of the instrument. One must `back out' the system response (tota= l system transfer function) if the result is to have any meaning. For those= of you using geophones, where the natural frequency is higher than 1 Hz; y= our output is proportional to the derivative of earth acceleration. R= elative to earth motion (not the geophone mass) your detector is for freque= ncies of interest to most of us (not the local-environment high frequency p= arts) -- a jerk detector and not a velocity detector, even though= it uses a Farady law detector.
=0A&n= bsp; Randall Peters
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