Sea ice vibration observations on landfast ice

Brief description

Data was obtained from 31-Aug-2021 to 10-Oct-2021. The experiment consists of 3x Geophone loggers deployed on landfast ice just north of Casey station. Geophone loggers record vibrations using a tri-axis GS-ONE geophone. The tri-axis geophone was dug slightly in the top layer of the ice (few centimeters), and the hole was filled with a mixture of snow and fresh water to create a slush for rapid refreezing. The geophone converts motion of a mass suspended by a spring to a voltage. The voltage is proportional to the velocity of the mass' motion. The GS-One has a (open-circuit) sensitivity of 85.8 V/m/s (+- 3.5%) and a corner frequency of 10Hz. Spurious frequency is 240 Hz. The geophone will still respond to vibrations of frequencies <10 Hz, but is simply less sensitive (see Page 2 of the enclosed datasheet). The geophone output is 3x a voltage, corresponding to the x, y and z direction. The signals are first amplified (using an INA122). The z-signal is split and amplified in three different ways to increase the dynamic range of measurements of vertical vibrations. The following gains were used: x = gain of 5, y = gain of 5, z1 = gain of 5, z2 = gain of 50, z3 = gain of 1000. Data is recorded on a microcontroller at 1000Hz, at 12-bit resolution. GPS was used to record geographical coordinates and high accuracy time-fixes. Time-fixes are accurate at 1 millisecond to allow for cross-comparison of high frequency vibrations between instruments. Dataset includes timeseries of sea ice vibrations recorded by three vibration loggers deployed on fast ice between 31-Aug-2021 and 10-Oct-2021. These vibration loggers consist of a geophone to record vibrations, and are referred to as G1, G2 and G3. G2 failed to record when deployed on the ice so no useful data was obtained for this instrument. Each measurement record number has a unique number F0000XXXX, with XXXX increasing inclemently. Each measurement is 15 min long and consists of a 'Data' and 'GPS' file. The Data file has 5 columns, giving time, x, y, z1, z2 and z3, respectively. The analogue voltage signals were amplified with the following gains: x = gain of 5, y = gain of 5, z1 = gain of 5, z2 = gain of 50, z3 = gain of 1000. Note, z1, z2 and z3 are derived from the vertical axis of the geophone. We note that vibrations were in general very small and only z3 seems to have recorded noticeable vibrations. Output of geophones is in Volt, however, the units of the vibration signals are in bit-resolution. The microcontroller has a 12-bit ADC, meaning that the measurable range of 3.3V is converted to a range of 0 to 12^2, i.e., 0-4095. The geophones have a sensitivity of 85.8 V/m/s, but were not calibrated, thus cannot be converted to obtain surface elevation. A bias voltage of 3.3/2 V was applied, such that the signal in rest will show a value of about 2046, i.e., 1.65V. If a fix was obtained within the 15-minutes of recording, one GPRMC message is shown in the GPS file. It contains information about the location of the instrument to keep track of where it was deployed. Note, none of the instruments drifted out. As an example, the following typical GPRMC message may be shown: $GPRMC,231220.000,A,6613.1212,S,11037.8747,E,0.00,350.32,051021,,,D*7C where '6613.1212,S' means 66 deg. 13.1212 min South and '11037.8747,E' means 110 deg. 37.8747 min West. If no GPS fix was obtained, these coordinates will be empty in the GPS file. Also note, that if no GPS fix was obtained, the absolute time (date) cannot be provided, although the relative accuracy remains fine.

Lineage

Progress Code: completed

Statement: The magnitude of the vibrations at the site were initially unknown, and a wide dynamic range was considered for the z-component only. After data analysis, it looks like the records with the largest amplification (i.e., signal z3) provides useful data.

We note that the spurious frequency of the geophone is 240 Hz. Though signals were recorded at 1000Hz, we do not expect of the data above 240Hz to be reliable.

Instruments were sometimes moved during the field experiment and redeployed due to changing ice-conditions and empty batteries. Users are advised to monitor the GPS coordinates.