Maximum investigation depth (Zmax) will be determined by the longest wavelength (Lmax) of surface waves
used for the analysis as Zmax ≈ 0.5Lmax. Lmax is then governed by the impact power of the seismic
source, which can be a controlled type like a sledge hammer in an active survey (or a car moving over a
road bump in the case of roadside passive survey). In general, a longer Lmax (therefore, a deeper Zmax) is
achieved with a greater impact power.
A fairly heavy sledge hammer (e.g., 20 lb) will be a good choice, although other more-sophisticated
sources that can deliver more impact power into ground (e.g., a weight drop) can be an advantage over a
sledge hammer because of its potential to generate lower (longer) frequencies (wavelengths) of surface
waves. The gain from using these other sources is often not enough to warrant cost of the equipment and
inconvenience in field operation unless they are carefully designed and built. For example, a mere
increase of impact power not accompanied by a careful consideration of energy coupling mechanism
many not achieve the goal. Using an impact plate (also called base plate) will help the source impact point
intrude less into soil. A detailed study on the role of the base plate in surface wave generation has not yet
been undertaken and needs to be done in the near future. See tables for optimum source for different
Recently, it has been reported that a non-metallic plate (e.g., a firm rubber plate) can generate noticeably stronger energy at the lower frequency part of surface waves (e.g., <
10 Hz) than a conventional metallic plate. This seems related to the speculation that car tire may act as an effective shock-absorber that releases impact power gradually,
resulting in a larger-scale deformation of surface around the source point by avoiding permanent (plastic) deformation caused by an abrupt release of impact power. For
unusually shallow investigation, a relatively light source has to be used so that the dominant frequency can be shifted towards higher frequencies.
Vertical stacking of multiple impacts can suppress ambient noise significantly and is therefore always recommended, especially if the survey takes place in an urban area.
The optimum number of stacking impacts can be determined when there is little change in signal-to-noise ratio (S/N) in the displayed seismic record during the stacking. 3-5
vertical stacks are often used. This number, however, should increase as the ambient noise level increases and/or total receiver array length (D) increases.
Fig. 1. Schematic of the active MASW field survey.
|(Right ) Fig. 2. Typical terrain conditions favorable and unfavorable for the MASW survey.
Vertical (instead of horizontal) phones must be used. Low-frequency geophones (e.g., 4.5 Hz) are always recommended. The high end of geophone frequency is not as
critical as in the reflection survey where any minor drop in sensitivity may become critical. For instance, recording and analysis of surface waves up to 450 Hz have been
reported by using 4.5-Hz geophones (Miller et al., 2000). Effectiveness of somewhat higher-frequency phones (e.g., 10-20 Hz), however, is often comparable to that of much
lower-frequency phones. Although spike-coupled geophones always give the highest sensitivity, the coupling provided by a land streamer can be equally efficient and is a
significant convenience in field operation (Fig. 3). In fact, using a land streamer can speed data acquisition by orders of magnitude; nowadays, it is becoming one of the
routine field apparatus, often operated with a small field vehicle (Fig. 3). See tables for optimum type of receiver for different investigation depth.
Length of the receiver spread (D) (Figure 1) is directly related to the longest wavelength (Lmax) that can be
analyzed, which in turn determines the maximum depth of investigation (Zmax). D usually has to be equal to or
greater than Zmax:
D = mZmax (1 ≤ m ≤ 3) (1)
On the other hand, receiver spacing (dx) is related to the shortest wavelength (Lmin) and therefore determines the
shallowest resolvable depth of investigation (Zmin).
Zmin=kdx (0.3 ≤ k ≤ 1.0) (2)
In practice, however, Lmax (therefore D) in an active survey is usually limited by the seismic source as it is the
most significant governing factor (Fig. 4), usually in the range of 50-100 m. If D becomes excessively long,
surface waves generated by most active sources become attenuated below noise level at the far end of the
receiver spread. However, weak low-frequency surface waves often exist even though no visually obvious on raw
record because of the much stronger ambient noise. This can help image the lower-frequency portion of
dispersion, leading to an increased Zmax. In this sense, the maximum D to provide the maximum Zmax is
usually slightly beyond the distance data start to appear noisy.
The source offset (x1) controls degree of contamination by the near-field effects that indicate a congregate of all
adverse influences (for example, not-fully developed surface waves) on data acquisition because of the source
being too close to the receivers. Its optimum value has been a subject of debate. A value of about 20% of D (e.g.,
x1=5 m when D=25 m) is suggested as a minimum and 100% as a maximum. A large x1 value (e.g., > 100 %)
and a large D (e.g., > 100 m) will increase the risk of higher-mode domination and reduce S/N for the
fundamental mode. See tables for optimum field parameters for different investigation depth.
Interval (dSRC) of Source-Receivers Configuration (SRC) Move
An interval between 1dx-12dx is recommended for 24-channel acquisition. 1dx is most commonly used in the case of 24-channel acquisition. This interval is one of the
variables directly related to the horizontal resolution. See tables for optimum interval under different situations.
A one millisecond of sampling interval (dt=1 ms) is most commonly used with a 1-sec total recording time (T=1 sec). Use of a smaller dt (e.g., 0.5 ms) is recommended if any
body-wave processing (e.g., refraction and reflection) is planned as by-product(s). In the case of extremely low velocities (e.g., Vs < 100 m/sec), a longer T (e.g., 2 sec) will be
a better choice. A longer T (e.g., 2 sec) is also recommended if a long receiver spread (D) (e.g., > 100 m) is used. In any case, an excessively long T (e.g., T >= 5 sec) is
discouraged in an active survey because it can increase the chance of recording ambient noise (e.g., traffic). Usually, 24-channel acquisition will be optimal. If 48-channel
acquisition is available, shortening dx is recommended rather than increasing D. Or, combining the two (shorter dx and longer D) is also recommended. The effect of
shortening dx when more channels are available will be an increased signal-to-noise ratio (S/N) during data analysis because of the redundancy as well as the possibility of
increasing resolution at shallow depths, whereas the effect of increasing D will be an increased Zmax. See tables for optimum recording parameters under different situations.
Fig. 3. Schematic illustration of
receivers on a land streamer.
Fig. 4. A field record illustrating usable offset range.
Field procedure for active MASW survey is explained here. The field procedure for passive MASW is different and explained under Passive MASW section. The active survey is
the most common type of MASW survey that can produce a 2-D Vs profile. The overall setup is illustrated in Fig. 1. The maximum depth of investigation (Zmax) that can be
achieved from the survey is usually in the 10-30 m range, but this can vary with site and type of active sources used. Field procedures and data processing steps are briefly
explained in Park et al. (1999). Surface waves are best generated over a ‘flat’ ground within at least one receiver-spread length (D) (Fig. 2). If this is the case, then overall
topographic variation within an entire survey line should not be critical. However, any surface relief whose dimension is greater than, say, 10% of D will cause a significant
hindrance to surface wave generation.
The following describes most of parameters related to data acquisition. A slight variation in any parameter can always be expected. A summary of optimum acquisition
parameters is displayed in separate tables. Optimum parameters for active MASW are also described in Park et al. (2002). They have been, however, continuously updated by
investigators and practitioners, and those most-recently used are listed in tables.