Geophysical Methods

exploration, near surface and marine methods



Active and passive surface wave techniques are relatively new in-situ seismic methods for determining shear wave velocity (VS) profiles. Testing is performed on the ground surface, allowing for less costly measurements than with traditional borehole methods. The basis of surface wave techniques is the dispersive characteristic of Rayleigh waves when traveling through a layered medium. Rayleigh wave velocity is determined by the material properties (primarily shear wave velocity, but also to a lesser degree compression wave velocity and material density) of the subsurface to a depth of approximately 1 to 2 wavelengths. Longer wavelengths penetrate deeper and their velocity is affected by the material properties at greater depth. Surface wave testing consists of measuring the surface wave dispersion curve at a site and modelling it to obtain the corresponding shear wave velocity profile. Active Surface Wave Techniques Active surface wave techniques measure surface waves generated by dynamic sources such as hammers, weight drops, electromechanical shakers, vibroseis and bulldozers. These techniques include the spectral analysis of surface waves (SASW) and multi-channel array surface wave (MASW) methods. The SASW method is optimized for conducting VS depth soundings. A dynamic source is used to generate surface waves of different wavelengths (or frequencies) which are monitored by two or more receivers at known offsets. An expanding receiver spread is used to avoid near field effects associated with Rayleigh waves and the source-receiver geometry is optimized to minimize body wave signal. A dynamic signal analyser is typically used to calculate the phase and coherence of the cross spectrum of the time history data collected at a pair of receivers. During data analysis, an interactive masking process is used to discard low quality data and to unwrap the phase spectrum, as shown in the figure below. The dispersion curve (Rayleigh wave phase velocity versus frequency or alternatively wavelength) is calculated from the? unwrapped phase spectrum.

The MASW field layout is similar to that of the seismic refraction technique. Twenty-four, or more, geophones are laid out in a linear array with 1 to 2m spacing and connected to a multi-channel seismograph as shown below. This technique is ideally suited to 2D VS imaging, with data collected in a roll-along manner similar to that of the seismic reflection technique. The source is offset at a predetermined distance from the near geophone and the Rayleigh wave dispersion curve is obtained by a wavefield transformation of the seismic record via the frequency wavenumber (f-k) or slowness-frequency (p-f) transforms. These transforms are very effective at isolating surface wave energy from that of body waves. The dispersion curve is picked as the peak of the surface wave energy in slowness (or velocity) frequency space as shown. One advantage of the MASW technique is that the wavefield transformation may not only identify the fundamental mode but also higher modes of surface waves. At some sites, particularly those with large velocity inversions, higher surface wave modes may contain more energy than the fundamental mode. Passive Surface Wave Techniques Passive surface wave techniques measure noise; surface waves from ocean wave activity, traffic, factories, wind, etc. These techniques include the array microtremor and refraction microtremor (REMI) techniques. The array microtremor technique typically uses 7 or more 4.5- or 1-Hz geophones arranged in a two-dimensional array. The most common arrays are the triangle, circle, semi-circle and L arrays. The triangle array, which consists of several embedded equilateral triangles, is often used as it provides good results with a relatively small number of geophones. With this array the outer side of the triangle should be at least as long as the desired depth of investigation. Typically, fifteen to twenty 30-second noise records are acquired for analysis. A technique called spatial autocorrelation (SPAC) is used to obtain the Rayleigh wave dispersion curve. For a partic?ular frequency the phase velocity is equal to that which best fits a first order Bessel function to the SPAC function. The image shown is phase velocity versus frequency showing the degree of fitness of the Bessel function to the SPAC function for a wide velocity and frequency range. The dispersion curve, is the peak (best fit), as shown in the figure below.

Multichannel Analysis of Surface Waves (MASW) is a surface wave seismic technology which is better suited to a geological setting of interchanging stiff and less stiff geologic layers as this technology can deal with seismic velocity inversions (e.g. a higher velocity layer, stiffer geology, overlying a lower velocity layer, less stiff geology). MASW measures a 1D sounding at a station location over an array of geophones, averaging the shear-wave velocity, Vs, of the ground at the station location. The array can then be moved along a profile line to the next station location, station spacing can be up to 10 m. The 1D soundings along the profile line are then used to build-up a 2D Vs cross-section of the profile line. Why is MASW useful? The calculated Vs can be multiplied with known geological density (?) to provide an approximate bulk shear modulus (K) for that geology e.g. K = ?Vs2. . MASW is affected by external seismic noise and is recommended to be utilised in areas of low seismic noise. The MASW technique is noise resistant, but will be affected by severe background noise such as drill rigs, aircraft and earthmoving/excavation machinery. The resolution of the profiles degrades with depth. As a rule of thumb, the shear wave velocity is averaged over a vertical distance of about half the depth of burial. Surveys must be performed on a flat lying area free of obstructions, dirt piles or ditches; an area of constant gradient or an area where the elevation change between geophones is < geophone spacing.

Refraction MicroTremor (ReMi) is a surface wave seismic technology (providing Vs) which is also useful in the same geologic settings as MASW. The difference between the two methods is that ReMi records the ambient seismic noise of the geology (not using an active seismic source like MASW e.g. dropweight). Similarly, ReMi can also use an MASW set-up and provides an average of Vs over a deeper depth range. ReMi provides less Vs detail in the near surface to allow for a more general average of Vs at depth. Why is ReMi useful? The calculated Vs can be multiplied with known geological density to provide an approximate bulk shear modulus (K) for that geology.