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Seismology is a field uniquely suited to addressing both exploration and science goals. It can be used not only to ascertain the physical attributes of a planetary body’s interior (which are relevant to its formation and evolution), but also to gauge the amount, magnitude, and distribution of seismicity that may present significant risk to future surface activities.
In this proposal, we present our plans to leverage existing MSFC capabilities in the development of a seismometer instrument concept for planetary exploration. The primary goal of this work is a proof-of-concept demonstration that piezoelectric strainmeters are adaptable to seismic frequencies and robust to the expected planetary environmental conditions.
The planetary seismology community has spent the last 20 years heavily advancing the development of very-broadband (VBB) seismic instruments. VBBs maximize scientific return in instances where only small numbers of instruments are able to be deployed, such as for the upcoming InSight mission to Mars. However, they are much heavier and more technologically complex than the instrumentation that is typically used in terrestrial field surveys. Consequently, new development efforts are exploring alternative technologies, including micro- electromechanical systems (JPL), fluid seismometers (GSFC/Arizona State), opto-mechanical geophones (Univ. of Arizona), and strainmeters (U.C. Berkeley).
MSFC is currently pursuing the development of piezoelectric crystal oscillator strain and vibration sensors, for use in structural integrity testing and monitoring. ES21 and ES63 build the sensors and accompanying wireless technology in-house, and we herein propose to adapt these technologies to seismic frequencies of interest (~0.1 – 50 Hz). Piezoelectric sensors are not currently considered mainstream in terrestrial seismology. However, they may port well to planetary implementations where small size, low power, robustness, and high sensitivity are desirable. The current sensor prototypes use an innovative application of a crystal oscillator as the sensing element, which efficiently represents changes in mechanical strain as variations in crystal impedance. In the case of the strain sensor, this approach has resulted in a ten-fold increase in sensitivity over the current state-of-the-art. ES21 will use additive manufacturing to develop a compliant structure to convert crystal impedance into an applied load on the crystal oscillator material, based on its mechanical properties.
In this proposal, we will 1) determine the bounding mechanical properties of the existing piezoelectric sensor using tensile and compression testing; 2) determine the frequency sensitivity and dynamic range of the sensor, and 3) benchmark the sensor against a commercial-off-the-shelf geophone. Geophones are the most commonly deployed terrestrial seismic instrument, and are both inexpensive and robust.
A second and equally important goal of this effort is to develop the capabilities of MSFC science personnel such that their effectiveness to both current and planned future projects and missions is maximized. MSFC’s planetary seismology efforts to date have focused on data reduction and analysis, the end products of fully developed and successfully deployed instruments. However, with many new planetary seismometer efforts on the rise, instrumentation development skills are becoming more and more in demand. While leveraging existing MSFC engineering capability in testing and benchmarking the piezoelectric sensor, we expect to gain the necessary skills required to understand seismometers quite literally from the ground up – from ground motion to counts. This knowledge will give MSFC involvement to planned mission efforts a competitive edge.
A seismometer is a mechanical device that measures and amplifies ground motion at a point on a planet’s surface. A strainmeter similarly measures small changes in the distance between two points that occur during a seismic wave’s passage. Ground motions provide critical information on both seismicity and planetary structure, however no seismometer provides a perfect representation of ground motion: each one has an imperfect response.
In this work, we will develop a test procedure for determining a sensor’s response: its free period, damping factor, and relative frequency sensitivities. We will then apply that procedure to the piezoelectric strainmeter being developed by ES21. Our proposal focuses on further development of a 2016 TIP-funded piezoelectric strain sensor developed by P. Hull/ES21. This type of sensing element was designed for measuring low strains characteristic of metal and composite-based structures. It consists of a piezoelectric crystal oscillator similar to those employed in AM radios. The crystals are cut precisely to vibrate at a resonant frequency. When mechanical strain is applied, the resonant frequency of the crystal shifts. This electro-mechanical behavior can be monitored electronically in response to simulated or real seismic shaking Seismometers are designed to measure seismic energy within a particular frequency range, centered around the instrument’s natural period: the resonant period of un-damped oscillations. The displacements of long-period seismometers (0.01 – 0.1 Hz) are directly proportional to ground motion, while short-period instruments are more sensitive to the acceleration of ground motion, responding to frequencies of 1 up to tens of Hz.
ES21 will acquire commercial electronic crystal oscillators (< $1 each), disassemble them, mount them, apply mechanical strain, and monitor their electrical response. The development of the sensor system will combine both the piezoelectric crystal and advanced manufacturing to create a sensor capable of operating in cold (planetary) conditions and responsive to a wide range of loads. The bounding mechanical properties of the sensors will be characterized using traditional tensile and compression testing. Response under appropriate environmental conditions will be tested in a cold chamber. A shake table will be used to test functionality in a random vibration environment.
ES63 will develop a test procedure to establish the seismic response of the piezoelectric sensing element. To determine the free period of the seismometer, a DC forcing function is applied and the resulting seismometer oscillation (un-damped) is recorded. The damping of the seismometer is related to the rate at which motion of the sensor decays to zero. It can be controlled using resistance inserted into the readout electronics, and optimized to minimize offshoot (ratio of pre/post event rest signal). Acceleration and displacement responses are determined by measuring output in response to a range of sinusoidal forcing functions. We will test response across a range of seismic frequencies – 0.1, 0.5, 0.8, 1.0, 2, 4, 8, 10, 25 and 50 Hz – using a shake table and accompanying readout electronics.
ST13 will field test the piezoelectric sensor in both passive and active configurations, and benchmark its performance against a commercial geophone currently being developed for planetary purposes. Geophones are the most commonly deployed terrestrial seismic instrument, and are both inexpensive and robust. In the active experiment, both the piezoelectric sensor and the COTS geophone will be deployed in the field equidistant from the seismic source, which consists of a sledge hammer that is hit against a plate on the ground, at a known distance and time. The signal recorded by the piezoelectric sensor will be compared to the geophone output to verify that seismic frequencies of interest are being recorded. In the passive experiment, the instruments will be installed co-located within a vault, and left for several weeks to listen for naturally occurring (teleseismic) earthquakes. GPS timing provided by the datalogger will permit comparison with known events in the U.S. Geological Survey earthquake catalog.
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