A MASW Seismic Survey
assists with determining shear wave velocities at depth.
As discussed in other pages, MASW depends on the dispersion properties of surface waves, primarily Rayleigh waves. Many of the equations derived for general physics to describe or define wave propagation can be solved or applied to the study of seismic waves. Observed changes in seismic waves are associated with the geology the waves pass through. This fusion of physics and geology is called geophysics. Thus, multi-channel analysis of surface waves or MASW is a geophysical application derived from the physics used to study wave propagation for the purpose of mapping shear wave velocities at depth. As a geophysical application there is a process for extracting the shear wave velocities from the MASW seismic record. The process calculates shear wave models using an inversion algorithm. A computer program provides solutions for the algorithm that are expected to correlate reasonable well with the true geologic conditions. While there are not any unique solutions, a well designed MASW seiscmic survey or MASW test is expected to yield desirable results. Budgetary costs and time constraints imposed by a client who hires a geophysicist to conduct a MASW geophysical survey can lead to less desirable results.
The level of confidence one has of the inversion process
to produce reasonable results is often a direct result of the level of effort placed on the field effort.
A greater level of effort in the field often leads to geophysical results that more closely represent the geologic conditions at hand. As an example, it is common for an active MASW survey using a hammer source and a linear 24 geophone array with geophones spaced 5 feet apart to be “rolled along” to acquire enough data to create a velocity panel. While this level of effort is greater than acquiring a single record from a single shot point, the 5 ft spacing limits the depth of exploration to approximate 50 or 60 feet.
To achieve greater depths of exploration with a higher level of confidence
one either needs to increase the length of the array or supplement the active MASW results with passive data. Now, not all passive data are equal. Often, because is takes less of a field effort, passive data may be record using the linear array placed down for the active results. With a little more effort, the linear 1-D array would be reconfigured into a larger 2-d array. The larger 2-d array improves the inversion process with larger wave length data and makes fewer assumptions.
A second option is to increase the length of the active geophone array.
A 24 channel system could expand the 5 ft geophone spacing to 10 ft or better yet use a 48 channel system. There is some discussion as to how many channels are needed; however, it is difficult to argue that the longer array length doesn’t increase the depth of penetration and improve the analysis by adding potential redundancy. In a perfect world, one good reading may be all you need; but geophysicists often find great comfort in knowing the results are repeatable coherent events.
As a final option, increasing the amount of energy placed into the ground can improve the signal to noise.
This not an issue for shallow surveys but for greater depths it can be a real issue. Thus, for a higher level of confidence one should consider using a weight drop. One that clearly generates large amounts of energy. As an example, try an elastic wave generator that sling shots a 700 lb weight in to the ground with an approximately 5 inch wide 20 ft long rubber band . When it hits the ground, you know it.
an active 48 channel linear array with a large weight drop and a 2-d geophone passive array is likely to yield a higher level of confidence for greater depths than active and passive results collected with a closely spaced 24 channel 1-D linear array. Geometrics is one of several good sources of information about MASW.