Determination of Shear-Wave Velocity to 30 m Depth from Refraction Microtremor Arrays (Remi-Test). Applied in Some Greek-Roman Archaeological Sites in Alexandria, Egypt

Abstract

Preservation of monuments all around the world and increasing their stability against earthquakes is a matter of great importance. Main purpose of the present study is to investigate the dynamic characteristics of the underground monuments at the ancient sites in Alexandria, Egypt (Catacomb of Kom El-Shoqafa, El-Shatbi Necropolis, and Necropolis of Mustafa Kamil, Amd El-Sawari site (Serapium and ancient library)) and identify the main damage mechanism, in order to evaluate the risk of structure damage or collapse in case of future events using microtremors recordings. Array measurements at three sites in the city of Alexandria were performed to estimate the Vs velocity of soil/rock formations for site effect analysis. Our study includes a detailed geological and geotechnical survey of the areas, measurement, analysis and interpretation of ambient noise data using the refraction microtremor (ReMi) method.

A thorough assessment of shallow shear velocity is important to both earthquake-hazard assessment and efficient foundation design. The only standard procedure for determining shear velocity, crosshole seismic (ASTM D4428), requires at least two boreholes with high-precision positional logs. The refraction microtremors method is based on recording ambient ground noise on simple seismic refraction equipment (as in ASTM D5777). Wave field analysis of the noise allows picking of Rayleigh-wave phase velocities. It works well in dense urban areas and transportation corridors.

The shear velocities estimated from ReMi method is a fast commercial effective method as borehole velocities for estimating 30- m depth-averaged shear velocity for foundation design and other purposes.

The importance of soil shear wave velocity (Vs) can not be over emphasized in engineering work. Because of its vital importance, many field and lab techniques have been devised to obtain soil Vs including, SPT, CPT, SCPT, P-S logging, suspension logging, cross-hole, seismic refraction and reflection (e.g. Imai 1981; Japan Road Association 1990; AIJ 1993; Kramer 1996). All of these techniques are inaccurate, costly, intrusive (require boring), laborious, time-consuming, or not urban-friendly.

Current commonly used techniques of estimating shallow shear velocities for assessment of earthquake site response are too costly for use in most urban areas. They require large sources to be effective in noisy urban settings, or specialized independent recorders laid out in an extensive array. The refraction microtremor (ReMi) method overcomes these problems by using standard P-wave recording equipment and ambient noise to produce average one-dimensional shear-wave profiles down to 100 m depths. The combination of commonly available equipment, simple recording with no source, a wave field transformation data processing technique, and an interactive Rayleigh-wave dispersion modeling tool exploits the most effective aspects of the microtremor, spectral analysis of surface wave (SASW), and multichannel analysis of surface wave (MASW) techniques. The slowness-frequency wave field transformation is particularly effective in allowing accurate picking of Rayleigh-wave phase-velocity dispersion curves despite the presence of waves propagating across the linear array at high apparent velocities, higher-mode Rayleigh waves, body waves, air waves, and incoherent noise. It has been very effective for quickly and cheaply determining 30-m average shear wave-velocity (V30).

Use of “active source” methods such as seismic reflection and refraction. In geotechnical applications in particular, seismic refraction with surface seismic sources has gained widespread acceptance as a viable investigation tool (Whiteley 1994). The effectiveness of this approach, especially in urban situations, is limited by the presence of seismic noise and in the choice of a source with sufficient energy to achieve the required depth penetration. Additionally, the seismic refraction method is inherently “blind” to the presence of a velocity inversion (Whiteley and Greenhalgh 1979).

An alternative approach is to use “natural” microtremors (the “noise” in traditional seismic surveying), as a source of wave energy. The measurement of high-frequency seismic noise, or microtremors, is a well-established method of estimating the seismic resonance characteristics of relatively thick (tens of metres and above) unconsolidated sediments. This approach is described by Nakamura (1989), where the fundamental resonance period (TS) of a site can be obtained from surface waves and used in the assessment of potential seismic hazard to structures founded in soft soils.

The refraction microtremor technique is based on two fundamental ideas. The first is that common seismic refraction recording equipment, set out in a way almost identical to shallow P-wave refraction surveys, can effectively record surface waves at frequencies as low as 2 Hz. The second idea is that a simple, two-dimensional slowness-frequency (p-f) transform of a microtremor record can separate Rayleigh waves from other seismic arrivals, and allow recognition of true phase velocity against apparent velocities.

Two essential factors that allow exploration equipment to record surface-wave velocity dispersion, with a minimum of field effort, are the use of a single geophone sensor at each channel, rather than a geophone “group array,” and the use of a linear spread of 12 or more geophone sensor channels. Single geophones are the most commonly available type, and are typically used for refraction rather than reflection surveying. The advantages of ReMi from a seismic surveying point of view are several, including the following: It requires only standard refraction equipment already owned by most consultants and universities; it requires no triggered source of wave energy; and it will work best in a seismically noisy urban setting. Traffic and other vehicles, and possibly the wind responses of trees, buildings, and utility standards provide the surface waves this method analyzes.