ALBERT

Description: A low-frequency passive seismic experiment using an array of 49 3C broadband seismometers was conducted over an onshore oil field in the emirate of Abu Dhabi in the United Arab Emirates. The aim of the experiment was to understand the characteristics and origins of the microseism (0.15–0.4 Hz) and microtremor (about 1–6 Hz) signals recorded, the latter having been reported as being a hydrocarbon indicator above several reservoirs in the region. The recorded array data were analyzed for their polarization, apparent velocities, and wavefront azimuths using various techniques, including spectral and time-frequency analyses, particle motion, H/V spectral ratios, and high-resolution frequency-wavenumber ( f-k ) analyses. In the frequency range of 0.15–0.4 Hz, the dominant feature observed consisted of double-frequency microseisms peaks generated by the nonlinear interactions of ocean waves with the shoreline along the coasts of the Arabian Sea and the Arabian Gulf. The f-k analyses confirmed that microtremor events in the frequency range of 2–3 Hz have an azimuth pointing toward a major oil pipeline and oil plant facilities to the west–southwest of the study area, as well as a motorway to the southeast. This would indicate that the microtremor events are probably caused by local sources, including the continuous movement of oil through the pipeline, the noise from oil plant facilities, as well as nearby traffic noise. This interpretation was confirmed by the polarization analysis performed on the data. The data also indicated that no clear correlation exists between the microtremor signal and local meteorological conditions. Although some body waves with an infinite apparent velocity generated by earthquakes were recorded, no other body waves that could have possibly been generated by hydrocarbon reservoirs were observed using the analyses techniques used in this study. Therefore, our results indicated that for the site under investigation, the microseism and the microtremor signals detected could not be related to the presence of hydrocarbon accumulations in the subsurface, but instead they may be attributed to local anthropogenic sources.

Description: The characteristics of ambient noise over an onshore oil field in Abu Dhabi, United Arab Emirates, have been investigated using arrays of three-component broadband seismometers by means of spectral amplitude and array wavenumber analysis within a frequency range of 0.1-10 Hz. The experiment was conducted to better understand the characteristics and origins of microseism (0.15-0.4 Hz) and microtremor (about 2.0-3.0 Hz) signals that have been reported as being a hydrocarbon indicator above several reservoirs in the region. The results of this study indicate that the long-period double-frequency peaks of microseism signals are generated by oceanic storms in the Arabian Sea as confirmed by data acquired throughout the impact of Cyclone Gonu on the coast of Oman. The study demonstrates that a narrowband of microtremor signals has no clear correlation with the recorded microseism signals. Cyclical daily and weekly variations in the spectral amplitudes of the signals clearly correlate with human activity. The results of this study, therefore, indicate that in this location the microseism and microtremor signals are not related to the presence of hydrocarbons in the subsurface but may be attributed to meteorological and anthropogenic effects, respectively.