G-force measurements at Mount Nyiragongo

To evaluate the mechanical stress on the volcanic edifice that results from lava lake level variations, we deployed a self-recording, differential capacitance (MEMS Inertial Sensor STMicroelectronics LIS3LV02DQ), 3-axis X6-1A accelerometer (Gulf Coast Data Concepts, LLC) at a distance of ~100m from the center of the Nyiragongo lava lake on freshly erupted lava flows. The device range was used in high (12-bit) resolution mode, which corresponds to a sensitivity of about 1 mg. The device was set to high-sensitivity mode with four additional bits to improve resolution, yet with a much lower signal-noise ratio. Once in position, the accelerometer continuously recorded data for three-day periods in June 2010. The system was oriented so that the X- and Y-axes form a plain parallel to the lava lake. During data collection, we did not attempt to calibrate the precision of the angle because relative G-force measurements were required instead of absolute G-force measurements. To distinguish the tiny accelerations caused by temperature differentials of the atmosphere, from the forces caused by magma movements, the temperature of the X6-1A device was continuously recorded. Temperature variations were corrected for by applying a de-correlation method to the recorded signal. Data was collected at 20 Hz, regrouped into batches that cover 1 hour per observation and associated with one averaged temperature measurement. This method was reproducible because diurnal temperature variations were the main cause for heating and cooling.