Method and means for measuring geophone parameters

A geophone testing apparatus includes a plurality of force detectors upon each of which is mounted a corresponding geophone. An electrical step function is applied to the active elements of the geophones to generate mechanical output transients. The mechanical output transients are detected by the force detectors which generate corresponding electrical signals. The signals are analyzed to determine the sensitivity, damping and natural frequency of each of the geophones under test. A multiplexer is provided so that all of the geophones may be tested at the same time.

BACKGROUND OF THE INVENTION 
1. Technical Field of the Invention 
This invention is concerned with a method and apparatus for measuring 
desired geophone parameters such as sensitivity, damping and natural 
frequency. 
2. Discussion of Prior Art 
A seismometer is a device for detecting and measuring mechanical vibratory 
motion. Essentially, a seismometer consists of a mass, suspended from a 
spring secured to a support. A sensing element measures the relative 
motion between the support and the stationary mass. Seismometers are used 
in earthquake studies, geophysical exploration for oil, for foundation 
studies in engineering work, for intruder detection, for monitoring the 
movements of vibrating machinery and for many other applications. The 
design parameters of a seismometer are tailored to fit the technology 
involved. 
Various types of sensing elements are known. One common type consists of a 
spring from which is suspended a coil of wire, that constitutes the mass 
and moves relative to a magnetic field. In most instruments, the coil and 
the form upon which it is wound constitute the mass. Relative movement of 
the coil with respect to the magnetic field generates an electrical signal 
indicative of the amplitude and frequency of the mechanical vibration. The 
moving-coil element is widely used with seismometers, also known as 
geophones, that are used for geophysical exploration for oil. Other types 
of sensing elemets are based upon magnetostrictive or variable reluctance 
principles. The sensing elements have in common the ability to convert 
mechanical input motion into electrical output signals. Conversely, an 
electrical signal applied to the sensing element will induce a mechanical 
motion to that element. 
For purposes of this disclosure, by way of example but not by way of 
restriction, moving-coil elements, as used in geophones, will be the 
subject of the ensuing discussion. A typical moving-coil geophone is 
disclosed in U.S. Pat. No. 4,159,464, assigned to the assignee of this 
invention. That patent is incorporated herein by reference as a showing of 
a typical geophone. 
Geophones are manufactured to close tolerances. In the factory, they must 
be tested to assure uniformity of the output-signal characteristics. In 
the field, they must be tested to detect a change in parametric tolerances 
due to use and abuse. 
Parameters of interest are sensitivity, damping, natural or reasonant 
frequency, coil resistance and phase shift. 
In one form of test, a geophone to be tested is mounted on a shaking table 
next to a standard geophone. The two geophones are excited at various 
frequencies and their outputs are compared. If the output signals match 
within predetermined limits, the geophone under test is accepted. 
Another type of test may be conducted wherein a geophone is driven by a 
steady-state oscillatory electrical signal. The back EMF of the coil is 
compared with the driving signal and is expected to meet preset standards. 
Both of the above tests are qualitative but not necessarily diagnostic of 
specific parameters. 
A somewhat more analytical test involves applying a mechanical or 
electrical step function to the geophone being tested. A transient output 
signal, or signature, results from the step-function output. By use of 
equations well known in the art, the first four parameters listed above 
can be calculated. See for example, U.S. Pat. Nos. 4,043,175 and 
4,015,202. 
All of the above test procedures must be performed upon individual 
geophones, one at a time. Clearly such tests are slow and cumbersome for 
production testing in a factory as well as for field testing. 
As is well known in geophysical exploration, for purposes of signal 
enhancement, a plurality of geophones may be employed in an array. The 
geophones of the array are permanently electrically wired together as a 
single string. The individual output signals of the repsective geophones, 
being electrically combined, produce a single composite output signal. A 
diagnostic test of the individual geophones of such an array is simply not 
possible using prior-art methods, without violating the electrical 
integrity of the array as a whole. 
SUMMARY OF THE INVENTION 
It is an object of this invention to provide a method and apparatus for 
rapidly determining selected parameters of each one of a plurality of 
geophones, regardless of whether the geophones are independent of each 
other or are electrically connected together in an array. 
In a preferred embodiment of this invention, a geophone or seismometer is 
mounted on a force detector. An electrical step function, such as a square 
wave, is applied to the active sensing element such as the moving coil of 
the geophone. The step function momentarily disturbs the sensing element 
which generates a characteristic transient mechanical output or signature 
that is detected by the force detector. The output signal of the force 
detector is analyzed to determine desired parameters of the geophone. 
In accordance with an aspect of this invention, a plurality of force 
detectors are provided for detecting the transient signatures of a 
plurality of geophones. An electrical step function is applied 
simultaneously to all of the geophones. The outputs of the force detectors 
are multiplexed by a multiplexer to a single-channel output. The 
multiplexer output is sampled, digitized and transmitted to a computer for 
determination of the desired parameters attributable to each one of the 
respective geophones. 
In another aspect of this invention, the known sensitivity of the force 
detectors is applied as a coefficient to the output signals of the force 
detectors to determine the absolute sensitivity of the geophones under 
test. 
In yet another aspect of this invention, a square-wave signal train is 
applied to a geophone to generate a plurality of transient signatures at 
the output of a force detector. The plurality of transient signatures are 
averaged before analysis to provide an average value for each determined 
parameter.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Let an electrical step function be applied as an input signal to the active 
sensing element of a damped seismometer such as a moving-coil geophone. 
The inertial mass represented by the coil is impulsively displaced to 
produce a damped mechanical transient force. If the geophone is mounted on 
a force detector or sensor, the applied mechanical transient force will 
cause the force detector to generate an electrical output signal such as 
the transient signature 10 of FIG. 1. From that signature, the natural 
frequency, sensitivity, and damping of the geophone may be calculated from 
suitable well-known equations. 
Referring now to FIG. 2, there is shown the preferred form of a force 
detector 14 that forms a part of the overall geophone tester system shown 
schematically in greater detail in FIG. 3. Force detector 14 includes an 
open-topped, hollow shell 16. A thin, flexible diaphragm 18, preferably 
metallic, is secured to a shoulder 20 at the bottom of shell 16. A 
piezoelectric crystal 22 is conductively cemented to the bottom of 
diaphragm 18. The bottom of crystal 22 is silvered so that a conductor 
lead 24 can be soldered thereto. A second conductor 26 is soldered to 
diaphragm 18 as the other signal output lead for crystal 22. Leads 24 and 
26 are brought outside shell 16 through suitable holes in the wall 
thereof. A geophone 28 may be inserted into shell 16 resting on diaphragm 
18. Geophone 28 of course, has leads 30 which are internally connected to 
the moving coil, not shown. An electrical transient applied to geophone 28 
generates a mechanical output force that is transmitted to crystal 22 
through diaphragm 18 to produce an output signal proportional to the 
applied force. 
It is important that the natural frequency of the force detector 14, when 
loaded with the mass of geophone 28 plus any internal supporting structure 
for the geophone, be at least an order of magnitude above the natural 
frequency of the geophone under test. That requirement is necessary to 
avoid distorting the output signal of the force detector relative to the 
force applied to it by the geophone. Thus, if the natural frequency of the 
geophone is on the order of 10 Hz, the natural frequency of the force 
detector must be at least 100 Hz or more. Force detector 14 must be 
accurately calibrated because the output signal of force detector 14 is a 
function of the product of the geophone sensitivity and the force-detector 
sensitivity. Accordingly, the force-detector sensitivity must be applied 
as a coefficient to the output signal to reduce the signal to that due to 
the geophone alone, as will be discussed below. 
Referring now to FIG. 3 there is shown schematically a plurality of force 
detectors 14, 14', 14" upon which are resting corresponding geophones 28, 
28', 28". Three such units are shown, but many more are used in practice. 
Geophones 28, 28', 28" are connected inseries with a constant current 
source 34. A normally-open switch 36 is connected in series with the 
geophones and voltage source. A mechanical switch is shown, but an 
electronic switch is preferred. In operation, to conduct a test, switch 36 
momentarily closed to apply a current i.sub.o to the moving coils of 
geophones 28, 28', 28". The applied current is sufficient to displace the 
moving-coil inertia elements a desired distance. When the current is 
released at time t.sub.o, the inertial elements seek to return to their 
rest position. In so doing, they generate a reactive mechanical transient 
force that is sufficient to cause the force detectors to develop a desired 
voltage E.sub.f.sbsb.1, at the force detector outputs. There will, of 
course, be a voltage E.sub.g developed across geophones 28-28" in 
proportion to the applied current i.sub.o and the total resistance across 
the circuit. The desired voltage E.sub.f.sbsb.1 is on the order of 200 
millivolts and the exciting current will be in the range of 0.2 to 5.0 
milliamperes. 
Upon command at time t.sub.o (FIG. 1) the switch is opened, releasing the 
respective moving coils. The mechanical force generated by movement of the 
coils generates a signal in force detectors 14, 14', 14". The signals from 
the detectors are amplified by amplifiers 38, 38', 38" and are then 
transmitted to multiplexer 40. At multiplexer 40, the signals are 
multiplexed into a common output and are sampled in sample-and-hold 
circuit 42. The sampled signals are digitized in analog-to-digital 
converter 44. From experience, it has been found that about 50 samples per 
transient cycle are adequate. Accordingly, for a 10 Hz geophone, the 
sample rate is about two milliseconds. The digitized signals are then 
processed in a suitable data processor such as a digital computer 45. 
Operation of switch 36, multiplexer 40, sample-and-hold circuit 42 and A/D 
converter 44 are synchronized by a controller 47 of conventional design. 
Controller 47 may be sequenced by the computer 45 or computer 45 itself 
may be programmed to perform the control functions without a separate 
controller. 
The details of a typical buffer amplifier 38 are shown in FIG. 4. Output 
from a force detector such as 14 is fed to the non-inverting input of an 
operational amplifier 46. An RC filter 48 controls the gain of the 
amplifier and also acts as a low pass filter to minimize high frequency 
noise. The gain of the amplifier may be unity; preferably it may be 20 dB. 
The amplifier circuit is conventional and per se forms no part of this 
invention. 
Returning now to FIG. 1, geophone sensitivity, G, damping, b.sub.o and 
natural frequency, fn, are calculated by computer 45 from the following 
well known equations: 
EQU G=E.sub.f.sbsb.1 /(S.times.i.sub.o .times.10.sup.7) V/cm/sec, 
where E.sub.f.sbsb.1 is the maximum amplitude of transient 10, S is the 
known sensitivity of force detector 14 and i.sub.o is the driving current. 
The coefficient 10.sup.7 is a unit-conversion factor. If the gain of 
amplifier 38 is other than unity, then a coefficient k.sup.-1 must be 
applied where k is the amplifier gain. 
Damping b.sub.o is found from 
EQU b.sub.o =((.pi./1n(E.sub.f.sbsb.2 E.sub.f.sbsb.3)).sup.2 +1).sup.-1/2, 
where E.sub.f.sbsb.2 and E.sub.f.sbsb.3 are the amplitudes of the second 
and third maxima of FIG. 1. The amplitude ratio between the first and 
second maxima could be used in the above equation, but for practical 
reasons, the second and third maxima are preferred. 
Natural frequency f is calculated from 
EQU f.sub.n =(T.sub.x (1-b.sub.o.sup.2).sup.1/2).sup.-1 Hz, 
where T.sub.x is the time difference between the first and third axis 
crossings, the reciprocal of which is the damped frequency. 
The phase shift of the geophone is a function of natural frequency and 
damping. So long as those two values lie within a selected tolerance, then 
so also will the phase shift be within tolerance. 
If a single geophone is under test, the effective coil resistance can be 
computed from 
EQU R=E.sub.g /i.sub.o. 
If a plurality of series-connected geophones are being tested then of 
course, R is the total resistance of the string. Dividing R by the number 
of geophones in the string, yields the average resistance of each coil. 
Statistically, the average individual resistance will closely match the 
true resistance of the respective coils. A gross difference between the 
specified resistance and the observed average resistance is indicative of 
a faulty geophone in the string. The faulty geophone can usually be 
identified because one or more of the computed parameters will be out of 
tolerance. 
In FIG. 3, a switch was used to apply a step function to the geophones. In 
an alternate embodiment, a square wave train is applied. Thus, in place of 
constant current source 34, a square wave oscillator 50 may be used as 
shown by the dashed lines. The frequency of the square waves is not 
critical but the period of one half-cycle must be long enough to allow the 
amplitude of a preceeding transient to settle to less than 1% of the 
maximum amplitude, E.sub.f.sbsb.1 before a new impulse is applied. See 
FIG. 5 where the square wave train is represented by 52 and the series of 
transients are plotted along axis 54. In operation, several square waves 
are applied to the same string of geophones. Each transient is separately 
analyzed by computer 45 and the results are averaged. 
I have disclosed a geophone test apparatus suitable for the simultaneous 
testing of a plurality of geophones, well adapted to mass-production 
testing either in a factory or in the field maintenance shop. The 
apparatus is suitable for use with any type of transducer that produces an 
electrical output in response to a mechanical input and that produces a 
mechanical output in response to an electrical input. My invention has 
been disclosed in terms of a moving-coil geophone but it is not restricted 
thereto. 
The invention has been described with particular reference to a 
step-function input. Other periodic functions such as a sine wave may be 
equally well employed without departing from the scope and spirit of this 
invention which is limited only by the appended claims.