Earthquake extraction and correlation energy at Long Beach, California seismic survey

Introduction

The Long Beach oil field is a productive oil field located below the city of Long Beach. Due to the urban environment, traditional techniques for collecting data for seismic imaging and velocity analysis are disruptive and difficult to perform. One alternative is using passive seismic data for this type of subsurface analysis. The effectiveness of such data for these purposes at the reservoir scale and in the urban environment is relatively unknown, but the recently deployed Long Beach seismic array provides a great opportunity to investigate their potential. The array is unique given its station density, recording period, and location. These parameters make this survey ideal for testing passive seismic imaging and tomography techniques from earthquake and exploration seismology.

Earthquakes oftentimes have source location depths on the order of kilometers, thereby producing waves that travel deep through the Earth's interior prior to reaching the surface. Depending on the amount of energy that a given event releases, the signal can be recorded by seismometers around the world. These two characteristics make data recorded from earthquakes ideal for resolving structures and velocities at the crustal/mantle scale, whether using body waves (Kissling, 1988; Aki et al., 1977; Schmandt and Humphreys, 2010; Romanowicz, 2008) or very low-frequency (less than $0.025$  Hz) surface waves that can sample these depths (Yang and Ritzwoller, 2007; Tanimoto and Sheldrake, 2002). When dealing with these scales, what is considered dense station spacing can seem relatively sparse. For instance, USArray is a transportable array of $400$ broad-band seismometers spaced 'densely' at approximately $500$  km (Meltzer et al., 1999). With this denser Long Beach array, earthquake signal might be able to resolve structures and velocities at the reservoir scale.

A common drawback with using earthquake events for surface wave tomography is that surface wave dispersion is difficult to measure at frequencies higher than $0.05$  Hz (Yang et al., 2008). Seismic interferometry with ambient noise can handle dispersion at these frequencies. By cross-correlating recordings of ambient seismic noise at two receivers, the traveltime between them can be recovered. This traveltime information can then be used for purposes such as velocity modeling, particularly when using low-frequency signal. Shapiro et al. (2005) and Lin et al. (2008) have been able to create velocity maps at the crustal scale by performing ambient noise cross-correlations on data recorded by USArray. At the reservoir scale, de Ridder and Dellinger (2011) showed that virtual low-frequency ($0.35$ -$1.75$  Hz), omnidirectional Scholte waves along the ocean floor could be generated from ambient seismic noise. They were able to use the Scholte-wave traveltime information for tomographic imaging of structures in the near-surface (0 -$150$  m).

In this report, we will show data that have been extracted from the Long Beach dataset. First, we will present snapshots of interpolated waveforms generated by earthquakes near and far. Second, we will present cross-correlation results for the same station location at various times of the survey. Both these results are viewed with an eye toward determining the capability of such an array for continuous reservoir monitoring in an urban environment using passive seismic data.

Earthquake extraction and correlation energy at Long Beach, California seismic survey