We monitor daily thermo-elastic movements at select arches using a compact electrolytic tilt-meter. Our Jewell Instruments model 701-2(4x) biaxial tiltmeter has micro-radian resolution (1 micro-radian = 0.00006 degrees), well better than anticipated daily movement of the studied arches. The instrument is placed on the top surface of an arch limb, while the battery and data logger are stored nearby in a sealed plastic case. We measure relative tilt every second, and simultaneously monitor rock and air temperature plus relative humidity using additional data loggers. Each morning as sun strikes the arch and the rock warms, thermal expansion causes the arch to bow upwards and the limbs tilt outward. After sunset, the rock begins to cool and the arch sags, causing inward tilt. Tilt measurements help us calibrate numerical models and determine daily displacements at different locations along the arch. Preliminary model results shown at right predict vertical displacement at Delicate Arch in the range of mm as it warms and cools each day (displayed movements are greatly exaggerated, colors indicate temperature, red: warm, blue: cool).


Ambient vibration measurements are conducted in a site-to-reference configuration using two broadband seismometers: one placed on or very near the arch being assessed (termed the active sensor) and the other placed in a flat, stable position roughly 100 m away for reference. This allows us to distinguish regional ground motion and target only signals of interest related to resonance of the arch. We use ultra-sensitive seismometers for ambient vibration measurements: sensors are Nanometrics Trillium-Compact seismometers (flat frequency response between 0.05 and 100 Hz) with 24-bit Nanometrics Centaur data loggers. These are powered by 16 A-h rechargeable lithium-ion batteries (& optional solar panel), which together with a data logger and GPS clock sit inside a medium-size plastic case. Ambient vibration data are recorded at 100 Hz and processed for spectral content, polarization magnitude and orientation. Preliminary model results shown at left predict that the fundamental resonant frequency of Delicate Arch is approximately 2 Hz (two times per second), swaying in the out-of-plane direction. These movements are excedingly small and not visible to the naked eye. Higher-order modes have greater resonant frequencies.