Acquisition and analysis of microearthquake data is part of the range of services provided by Geosystem’s Seismic Imaging Group.

Microearthquakes are an essential tool for modern reservoir management. The applications span from geothermal to the oil & gas industries.

Geosystem has developed methods for the analysis of microearthquakes including 3D seismic tomography of P, S and Vp/Vs, high-resolution 3D location of seismic swarms based on waveform similarity and microearthquake moment tensor analysis including the isotropic (i.e. volumetric) components of the moment tensor.

ACQUISITION SERVICES

The detection of the seismic energy is performed automatically by using pattern-recognition algorithms based on the comparison of spectrograms or sonograms (i.e. frequency spectra versus time) of modeled microearthquakes with the actual recorded signals.

The database library of microearthquake spectrograms can be updated with the spectrograms of microearthquakes recorded locally and characteristic of the area. Previous experience indicated that the detection threshold obtained by means of pattern-recognition techniques is two/three times that of STA/LTA algorithms. In addition to this the analysis of spectrograms allows better discrimination of the signature of noise relative to that of seismic sources.

Sonogram detector picking up two closely spaced microearthquakes (magnitude = 0.2).

DATA ANALISYS SERVICES

Figure 1 displays the vertical and horizontal components, and power spectra of a microearthquake resulting from injection in a well. The triggering mechanism can be explained by a local increase of pore pressure following water injection, causing a reduction of the effective stress across the sides of the active fault.

By comparison, Figure 2 shows the three-component records of a volcanic quake (tremor). Notice the expanded time scale, the peak frequency on the power spectra (about 1.5Hz) and the harmonics of the fundamental frequencies.

Further analysis on the microearthquake data are performed to examine the characteristics of the recorded seismicity:

   o 3D tomography (Vp, Vs and Vp/Vs), including repetitions over time (time lapse) with well constraints
   o Swarm analysis based on spatial/temporal occurrence and waveform similarity of microearthquakes
   o High-precision relocation of seismic swarms with 3D tomographic velocity models and/or based on waveform similarity
   o Moment tensor analysis of microearthquakes including the isotropic (i.e. volumetric) components

3D Tomography of microearthquake data is performed to derive Vp, Vs and Vp/Vs ratio. The reliability of the tomographic images can be assessed by means of synthetic studies.

The time-lapse evaluation of the Vp/Vs ratio in geothermal fields is used to determine the depletion of the steam reservoir over time further to directly assessing the concentration of steam reserves and the boundaries of the reservoir. Tomographic inversion from earthquakes allows an improved calculation of hypocenter positions through the location with 3D rather than 1D velocity models.

Figure 1. Typical microearthquake induced by injection in an active fault.

Figure 2. Volcanic tremor.

High-resolution location of microearthquakes is used to describe the geometry of faults and cracks, through waveform analysis of the recorded microearthquakes. In the mapping of similar events each event is considered in turn as a possible master and its similarity with all the other (slave) events in the data set is calculated.

Similar-looking microearthquakes have close geographic locations and are generally the expression of strain release on the same portion of a fault. This characteristic is used to calculate time lags in the original picking between the master event and its correlated slaves to obtain corrections to P and S arrival times. The corrected dataset is then re-located obtaining a significant improvement in the geometrical description of the fault structure.

Moment tensor analysis including the isotropic components is used to discriminate the source processes that, for shallow microearthquakes related to man-related activity in the field (e.g. injection, production in the reservoir) may involve volumetric changes (i.e. non double-couple mechanisms) and not just shear deformations. The cross interpretation of source processes through moment tensor analysis and reservoir management activity provides significant control on the movement of fluids in the reservoir, leading to better recovery procedures.

Moment tensor analysis: A) Source-type plot depicting moment tensors of earthquakes in a manner independent of source orientation (events plotting at DC position are pure double-couple earthquakes, the others have significant volumetric components); B) Equal area upper-hemisphere plot showing pressure (P), tension (T) and intermediate (I) axes of the moment tensors for earthquakes having similar waveforms; C) microearthquake hypocentres superimposed onto the tomographic Vp/Vs ratio

top of page