Low Frequency Passive Seismic Experiments in Abu Dhabi, United Arab Emirates:
Implications for Hydrocarbon Detection
 M.Y. Ali, K.A. Berteussen, J. Small and B. Barkat

Summary

 
So far we conducted two low frequency passive seismic experiments utilising arrays of 3-component broadband seismometers in Abu Dhabi. The experiments were conducted in the vicinity of a producing oilfield, and around a dry exploration well to better understand the characteristics and origins of microtremor signals (1–6 Hz) which had been reported as occurring exclusively above several hydrocarbon reservoirs in the region.
The results of the experiments revealed that a strong correlation exists between the recorded ambient noise and observed meteorological and anthropogenic noises. In the frequency range of 0.15–0.4 Hz, the dominant feature is a double-frequency microseism peak generated by the non-linear interactions of storm induced surface waves in the Arabian Sea. We observed that the double-frequency microseism displays a high variability in spectral amplitude, with the strongest amplitude occurring when Cyclone Gonu was battering the eastern coast of Oman; this noise was present at both sites, and so is not a hydrocarbon indicator. Moreover, this study found that very strong microtremor signals in the frequency range of 2-3 Hz were present in all of the locations surveyed, both within and outside of the reservoir boundary and surrounding the dry exploration well. This microtremor signal has no clear correlation with the microseism signals, but significant variations in the characteristics of the signals were observed between day time and night time recording periods which clearly correlate with human activity.
High resolution frequency-wavenumber (f-k) spectral analyses were performed on the recorded data to determine apparent velocities and azimuths of the wavefronts for the microseism and microtremor events. The f-k analyses confirmed that the double-frequency microseism originates from wave activity in the Arabian Sea, while the microtremor events have an azimuth pointing towards the nearest motorways, indicating that they are probably being excited by traffic noise. Results drawn from particle motion studies confirm these observations. The vertical-to-horizontal (V/H) spectral ratios of the data acquired in both experiments show peaks around 2.5–3 Hz with no dependence on the presence or absence of subsurface hydrocarbons. Therefore, this method should not be used as a direct hydrocarbon indicator in these environments. Furthermore, the analyses provide no direct evidence to indicate that earthquakes are capable of stimulating the hydrocarbon reservoir in a way that could modify the spectral amplitude of the microtremor signal. Although our results clearly indicate that microtremor analysis cannot be used as a hydrocarbon indicator for this area, the data does however have the potential to significantly contribute towards geotechnical characterization of shallow sediments for seismic hazard assessment.
The results of the project have considerably increased our understanding on the causes of microtremor signals. The results have also highly impacted both academia and oil industry. 		 
     

 
Fig. 1. (a) Time frequency display for data recorded from the vertical component of the seismometer at location A of the oilfield on 26th May 2007. (b) Time frequency display for data recorded from the vertical component of the seismometer close to the dry exploration well on 20th January 2009. On both locations the microtremor signal exhibit strong diurnal variations with a strong signal during the day and a weaker signal at night.

 
Fig. 2. (a) Peak spectral amplitude of horizontal and vertical components of double-frequency microseism (about 0.25 Hz) obtained at location B of the oilfield. (b) Peak spectral amplitude of microtremor signal (2.5 Hz) horizontal and vertical components recorded at location B of the oilfield. The spectral amplitudes of microseisms increased dramatically when Cyclone Gonu approached the coast of Oman, whereas the microtremor signal remained unchanged. Spectral amplitudes of microtremor signals exhibit strong daily and weekly cyclical variations. Signals were significantly stronger during normal working hours on weekdays compared with night times and across weekends (Fridays in the UAE).
 
     

Fig. 3. Normalized Energy response in slowness space for arrays of varying aperture sizes at (a) Location A, oilfield: Centre frequency = 0.2 Hz, array aperture = 3600 m, 4th June 2007 at 0:00-01:00. (b) Location B, oilfield: Centre frequency = 0.2 Hz, array aperture = 3600 m, 16th June 2007 at 0:00-01:00. (c) Location A, oilfield: Centre frequency = 2.5 Hz with array radius of 225 m, 26th May 2007 at 0:00-01:00. (d) Dry well: Centre frequency = 2.5 Hz with array radius of 100 m, 20th January 2009 at 0:00-01:00. For locations of the sensors see Figure 1. In each figure the symbol “X” indicates the peak values. The distance between the centers and “X” gives the slowness of the waves at the frequency and the line at the centre to “X” gives the direction of the wave propagation. The phase velocity and propagation azimuth (from the source) determined from the maximum peak are written in the lower left-hand corner in each plot.