PDS_VERSION_ID = PDS3 LABEL_REVISION_NOTE = "2016-07-21 R. Lorenz" RECORD_TYPE = STREAM OBJECT = INSTRUMENT INSTRUMENT_HOST_ID = "VL2" INSTRUMENT_ID = "VL2-SEIS" OBJECT = INSTRUMENT_INFORMATION INSTRUMENT_NAME = "VIKING LANDER 2 SEISMOLOGY EXPERIMENT" INSTRUMENT_TYPE = "SEISMOMETER" INSTRUMENT_DESC = " Instrument Overview =================== The Viking Seismic Experiment was conceived [ANDERSONETAL1972B] to observe ground motion on the surface of Mars and thereby determine the level of seismic activity on Mars, and deduce crustal properties and internal structure. It comprised a three-axis seismometer and data handling unit, fuller details of which are provided below and in [ANDERSONETAL1976]. The unit was mounted on the deck of each of the two Viking landers. Unfortunately, the protective caging mechanism (see later) on the Viking 1 seismometer failed to release and no useful data could be obtained. The strong scientific benefits of having two simultaneous observations at different locations were also lost. However, the Viking Lander 2 instrument uncaged as intended, and returned a rich dataset until Sol 560, when a lander memory unit failed [LORENZ&NAKAMURA2015]. Because the unit was mounted on the deck of each of the two Viking landers, it was susceptible to wind noise, and much of the signal content shows strong coherence with wind measurements [ANDERSONETAL1976; ANDERSONETAL1977; NAKAMURA&ANDERSO1979]. Relevant meteorology data for the interpretation of the seismic record record is derived from the Viking Lander Meteorology Experiment data (PDS data sets VL1/VL2-M-MET-4-DAILY-AVG-PRESSURE-V1.0, VL1-M-MET-4-BINNED-P-T-V-CORR-V1.0, VL1/VL2-M-LCS-5-ATMOS-OPTICAL-DEPTH-V1.0, VL1/VL2-M-MET-3-P-V1.0, and VL1/VL2-M-MET-4-BINNED-P-T-V-V1.0). Science Objectives ================== The originally-stated objectives [ANDERSONETAL1972B] of the Viking Seismic Experiment were: a. To determine the amplitude, spectrum, polarization, and source of continuous background microseism activity. b. To determine whether Mars is a tectonically active planet and to determine the level, nature, and location of tectonic activity. c. To determine the internal structure and composition of Mars; specifically, to determine if Mars has a crust and a core and to determine if the mantle of Mars is similar in composition to the mantle of the Earth. d. To determine the influx rate of meteorites. e. To determine the mechanical properties of the material near the lander. It later emerged that the instrument also contributed to meteorological observations [NAKAMURA&ANDERSO1979]. Instrument Design ================== The Viking seismometer package is located on top of the lander's equipment bay near the attachment of leg 1. It is 12 X 15 X 12 cm and has a mass of 2.2 kg and includes the sensors, amplifiers, filters and trigger electronics for automatic event detection, data compression and storage. The instrument consumes 3.5 W. The useful frequency range is 0.1-10 Hz with a minimum ground amplitude resolution of 2 nm at 3 Hz and 10 nm at 1 Hz. The Viking seismometer has a maximum sensitivity equivalent to that obtainable at a relatively quiet site on earth. The undamped natural frequency of each instrument is 4 Hz, the coefficient of damping is 0.6, and the generator constant is 177 V/(m/s). The natural undamped frequency of the sensors was chosen such that the instrument would meet the constraints of weight and volume and to insure that the sensors would operate over the largest expected tilt of the lander (15 degrees) without the use of any mechanical zeroing adjustments. The seismic sensors are three matched, orthogonally mounted (one vertical and two horizontal) velocity-sensing seismometers. A 16-g mass-coil assembly is supported on twin booms by two hinges such that the flat transducer coil is poised between the facing poles of two channel magnets. Motion of the frame causes the transducer coil to move in the field of the magnets and generates a signal which is proportional to the velocity of the coil relative to the magnet. The inertial mass of each sensor is individually caged to protect it from the vibration of launch, separation of the lander from the orbiter (peak shock of 1200 g), and landing. Spring-loaded plungers hold the mass firmly against a stop. The plungers are secured by a palladium-aluminum fusewire, which is electrically heated to break and release the plunger. All three axes failed to uncage on the Viking 1 lander. The Viking 2 lander seismometer uncaged as planned. Each sensor can be stimulated by a calibration coil, which deflects and releases the mass to produce a pair of pulse doublets. In addition to verifying instrument response over time, the asymmetry in the doublet constrains the tilt of the instrument, estimated to be ~9.5 deg down at an azimuth of 260-300 deg from north. Each axis has an amplifier with command-selectable (six steps) amplification, although as noted in [ANDERSONETAL1976] the instrument was almost always (99%) operated in the maximum-gain state (116 dB). The original signals are converted to a 7-bit plus sign digital word at the rate of 121.21 samples per second and then averaged and compressed as described later. The instrument contains dual 2048-bit data buffer memories. Two are required for continuous operation so sampling into one continues while the other full buffer is transferred to the lander's memory or magnetic tape recorder for buffering before transmission to Earth. The instrument operates in three modes: High, Event and Normal. High Rate Mode Each channel is digitally filtered and the 7-bit plus sign word is sampled at 20.2 samples/s/channel. The low-pass filter is a sixth-order Butterworth low-pass filter with command-selectable cutoff frequencies of 0.5, 1.0, 2.0, and 4 Hz. Since this mode directly records the velocity signal, it is in principle the richest dataset, although lander data volume limitations restricted operation in this mode to short periods. Event Mode (also 'triggered mode') This mode compresses the signal, by reporting at a lower sample rate (1.01 sample per second per axis) the number of zero crossings to indicate a dominant frequency, and the envelope (amplitude) of the signal by smoothing through a 0.5-Hz cutoff frequency digital low pass filter. This combination of sampling the envelope (7-bit word) and axis crossing (5-bit word) results in a 12.3 to 1 reduction in the data required to encode the original signal over the high data rate. As reported in LORENZ&NAKAMURA2014, Event mode data is the major data type returned (273,000 records vs 49,000 High rate). Event mode can be commanded directly, or is triggered by monitoring in Normal mode and detecting when the signal level exceeds a multiple of the long-term (1081 s) average microseismic level. The multiple is selected by ground command from values of 4, 8, 12, 16, or 20. When the level drops below the average background, the instrument reverts to the normal mode. To reduce the number of data taken from false triggering from lander thermal 'pops,' the event detector on time beyond the normal off time is controlled to be proportional to the time that the level of the event is above the trigger level. This time increase varies from a minimum of 2 s to a maximum of 1 min for the longest event. Normal mode The normal mode is the lowest data rate mode, reporting at only 4.04 samples/min per channel. It indicates only the microseismical background and is not discussed further here, although see NAKAMURA&ANDERSO1979 for a review of the large-scale correlation of these data with wind. Operation on Mars ================== Fuller details on Mars operation are given in [ANDERSONETAL1976 and ANDERSONETAL1977]. After landing the azimuths of the horizontal (Y and Z) seismometer axes were determined in the lander inertial reference system. The positive Y component corresponds to lander motion toward the northwest (N 31 deg W), and the positive Z is towards southwest (S 59 deg W). The polarity of the vertical (X) component is positive upward. The initial calibration pulse amplitudes were in good agreement with predicted values, but showed a steady increase in amplitudes (18%) over 140 sols due to changing temperatures. Signal amplitudes were consistently smallest on the Y component (10-20% less than those on X) and largest on the Z component (20-50% greater than X). Further, the coherence between channels in high rate observations suggest the lander rocked about a line joining footpads 1 and 2. This also seems consistent with wind-induced vibrations are highest for a given velocity when winds come from the East. Wind noise dominates the signal [ANDERSONETAL1976; ANDERSONETAL1977; NAKAMURA&ANDERSO1979], although a single event on Sol 80 was tentatively identified [ANDERSONETAL1979; LORENZETAL2016) as a real seismic event. Data were returned from the seismic instrument until Sol 560. " END_OBJECT = INSTRUMENT_INFORMATION OBJECT = INSTRUMENT_REFERENCE_INFO REFERENCE_KEY_ID = "ANDERSONETAL1972B" END_OBJECT = INSTRUMENT_REFERENCE_INFO OBJECT = INSTRUMENT_REFERENCE_INFO REFERENCE_KEY_ID = "ANDERSONETAL1976" END_OBJECT = INSTRUMENT_REFERENCE_INFO OBJECT = INSTRUMENT_REFERENCE_INFO REFERENCE_KEY_ID = "ANDERSONETAL1977" END_OBJECT = INSTRUMENT_REFERENCE_INFO OBJECT = INSTRUMENT_REFERENCE_INFO REFERENCE_KEY_ID = "NAKAMURA&ANDERSO1979" END_OBJECT = INSTRUMENT_REFERENCE_INFO OBJECT = INSTRUMENT_REFERENCE_INFO REFERENCE_KEY_ID = "LORENZ&NAKAMURA2014" END_OBJECT = INSTRUMENT_REFERENCE_INFO OBJECT = INSTRUMENT_REFERENCE_INFO REFERENCE_KEY_ID = "LORENZETAL2016" END_OBJECT = INSTRUMENT_REFERENCE_INFO END_OBJECT = INSTRUMENT END