What is a MASW?

What is a MASW?

MASW is a shortcut of Multichannel Analysis of Surface Waves. The MASW method deals with surface waves in the lower frequencies(e.g.,1-30Hz) and uses a much shallower depth range of investigation ranges from a few meters to a few tens of meters. MASW deals with relatively lower frequencies and shallower investigation depth ranges than do conventional high resolution seismic methods. It provides shear-wave velocity information of near-surface materials in a highly cost-effectively manner. Because of the relatively significant value of this information in most geotechnical engineering projects and also because of the relatively simple in-field operation and data processing, it is gaining popularity among engineering communities.

The MASW method tries to utilize the dispersion property of surface waves for the purposes of profiling in 1D(depth) or 2D( depth and surface location) format. Basically it is an engineering seismic method dealing with frequencies in a few to a few tens of Hz recorded by using a multi-channel (24 or more channels) recording system and a receiver array deployed over a few to a few hundreds meters of distance(e.g., 2-200m). The entire procedures of MASW usually consists of three steps: 1) acquiring multi-channels fields records(or short gathers); 2) extracting dispersion curves(one from each record); and (3) inverting these dispersion curves to obtain 1D(depth) profiles( one profile from one curve).

Then, by placing each 1D profile at a surface location corresponding to the middle of the receiver line, a 2D( surface and depth) map can be constructed through a appropriate interpolation scheme.

Active MASW

The active MASW method was introduced in GEOPHYSICS in 1999. This is the most common type of MASW survey that can produce a 2D profile. It adopts the conventional mode of survey using an active seismic source(e.g., a sledgehammer) and a linear receiver array, collecting data in a roll-along mode. It utilizes surface waves propagating horizontally along the surface of measurement directly from impact point to receivers. It gives this information in either 1D(depth) or 2D(depth and surface location) format in a cost-effective and time-effective manner. The maximum depth of investigation is usually in the range of 10-30m, but this can vary with the site and the type of active source used.

Passive MASW

As the surface-wave method is gaining in popularity among engineers and geophysicists, demand for increased investigation depth is also growing. However, the amount of active-source energy needed to gain a few more Hz at the low-frequency end of a dispersion curve(e.g., 5-7Hz) and thereby to increase investigation depth by several tens of meters- often rises by several orders of magnitude, rendering efforts with an active source impractical and uneconomical. On the other hand, passive surface waves generated from natural(e.g., tidal motion) or cultural(e.g.,traffic) sources are usually of a low frequency(1-30Hz) nature with wavelengths ranging from a few km(natural sources) yo a few tens(or hundreds) of meters(cultural sources), providing a wide range of penetration depths and therefore a strong motivation to utilize them.

The passive MASW method, on the other hand, usually uses more(24 or more) channels to fully exploit the advantages of multi-channel recording and processing. It therefore has a greater flexibility in field logistics and an enhanced robustness in data processing with a increased resolution in the analysis of both the modal nature and azimuthal properties of surface waves.

Passive MASW is divided into two different types based on field logistics and types of profiles(1D or 2D) to be obtained,passive remote and passive roadside MASW surveys. The former seeks a 1D profiles of bulk materials ranging up to hundred meters along the surface and depth directions. On the other hand, the latter type can generate a 2D profile covering up to a hundred meters in depth and a surface distance determined by the survey length.These two passive MASW methods utilize those surface waves generated passively from ambient cultural activities such as traffic.

The passive remote method employs a 2D receiver array such as a cross or circular layout to record passive surface waves. This results in the most accurate evaluation of 1D shear-wave velocity at the expense of more intensive field operation and the burden of securing a wide-open space for the array. This can be a good choice if relatively regional 1D profiling is needed. Any type of 2D receiver array of fairly symmetric shape can be used. Any array of significant asymmetric shape, for example an elliptical or elongated rectangular shape, is not recommended due to bias toward a specific direction of incoming waves that do not necessarily coincide with the actual direction of major surface-wave energy.

The passive roadside MASW method adopts the conventional geophone array and tries mainly to utilize those surface waves generated from local traffic. It tries to overcome limitations with the passive remote method such as difficulty in securing a spacious area and inconvenience in field operations by sacrificing the accuracy. With this method, the geophone array can improve survey speed by as much as a few orders of magnitude. In addition, an active impact(e.g., by using a sledgehammer) can be applied at one end of the geophone array to trigger a long recording(e.g., 30s). This can result in a combined active-passive analysis of surface waves to obtain both shallow(e.g.,1-20m) and deep(e.g.,20-100m) information simultaneously. Although it an result in slightly overestimating(usually by less then 10%) values in comparison to the remote method using a 2D geophone array, this survey mode can be useful and convenient because of the significant advantage in field operations.

Combining active and passive surveys

It is often useful or necessary to combine dispersion images processed from active and passive data sets for two reasons: (1) to enlarge the analyzable frequency(therefore depth) range of dispersion and (2) to better identify the modal nature of dispersion trends.

The near future of MASW

As the land streamer can be effectively used for both active and passive MASW surveys, an acquisition system will be routinely used in the near future. Comparison of the data quality versus that of the conventional spike-coupled geophones showed insignificant difference because of the strong nature of surface waves. Relatively simply data processing procedures made the in-field data handling so effective that the vertical 1D profile of the surveyed point on the ground could be continuous added to the updated the 2D map almost in a real-time mode.

Due to the significantly increased mobility of the entire land streamer system, the surveys can be conducted by only a few field personal with one or two operating both source and geophones and another dedicated to in-field handling.

More to learn:

Seismic Acquisition System: https://www.geostuff.com/index.html

Land Streamer: https://www.geostuff.com/Land%20Streamer.html

Geophone: https://www.seis-tech.com/category/geophones/

Spread cable: https://www.seis-tech.com/6-12-24-48-channel-seismic-cable/