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an MASW survey. The offset range free of these two effects is called an "optimum" offset.

(Below) Figure 2. A field record obtained by using a 12-lb sledge hammer source. Original

record is stretched (convolved) with a sweep function varying from 5 Hz to 50 Hz, and the low

(5-20 Hz) and high (35-50 Hz) frequency parts are displayed here to show the portions

contaminated by the near- and far-field effects, respectively.

The optimum offset for an MASW survey is the range of

distance from the source (i.e., offset) where surface waves are

best developed with minimal inclusion of adverse influences.

There are two factors that may impede the best development;

the near- and far-field effects.

**Near-Field Effects**

Surface waves are not developed at the vicinity of source point

because they are formed through interference of (P and SV)

body waves generated from reflections and refractions,

occurrence of which require a certain minimum distance from

the source. Therefore, the first receiver closest to the source

should be placed beyond this point, and this is called the near-

field effects. If a receiver is placed within this distance, it will

record either body or ambient noise wavefields.

This minimum distance changes with wavelength; a longer

wavelength needs a longer distance to be fully developed. A

rule of thumb is 25-50% of the wavelength. However, an MASW

survey deals with a wide range of wavelengths (e.g., 1-50 m),

and this range is determined by the receiver spacing (dx) (e.g.,

dx=1 m) and length of the receiver array (L) (e.g., L=50 m). In

practice, therefore, one distance compromising the entire range

of wavelengths is used to set the minimum distance between

the source and the closest receiver, and this is called the

source offset (X1) (Figure 1). It is usually between 25% and

50% of the receiver array length (L):

**Far-Field Effects**

Although surface waves are much stronger than body waves at

the time of generation near the source point, they attenuate

more rapidly as illustrated in Figure 1. In consequence, after

travelling a certain distance, their energy level may drop below

that of the body waves or ambient noise. It is, therefore,

important to place receivers closer than this maximum distance

(Xmax) to avoid the contamination by these noise wavefields.

This is called the far-field effects. Xmax depends on (1) source

power, (2) ambient noise level, and (3) attenuation property of

subsurface. The first, (1), acts positively, whereas the other

two, (2) and (3), act negatively in increasing Xmax. As a rule of

thumb for MASW surveys using a heavy sledge-hammer source

(e.g., 16-lb), Xmax is about 50 m in typical urban areas with

heavy traffic, while it can be extended to 100 m at relatively

quiet areas. A more powerful source (e.g., weight drop) can

extend Xmax by, for example, 30%.

Both near- and far-field effects are illustrated in a field record

(Figure 2) obtained by using a 12-lb sledge hammer source.

The original field record has been stretched (i.e., convolved)

with monotonically increasing frequency function (i.e., sweep) in

5-50 Hz. At the lowest frequency of 5 Hz that corresponds to

the longest wavelength, surface waves are not effectively

developed at near offsets (e.g., < 40 m) due to the near-field

effects (left panel in Figure 2), while body waves and ambient

noise dominate at the far offsets (e.g., > 30 m) due to the far-

field effects (right panel in Figure 2).

distance from the source (i.e., offset) where surface waves are

best developed with minimal inclusion of adverse influences.

There are two factors that may impede the best development;

the near- and far-field effects.

because they are formed through interference of (P and SV)

body waves generated from reflections and refractions,

occurrence of which require a certain minimum distance from

the source. Therefore, the first receiver closest to the source

should be placed beyond this point, and this is called the near-

field effects. If a receiver is placed within this distance, it will

record either body or ambient noise wavefields.

This minimum distance changes with wavelength; a longer

wavelength needs a longer distance to be fully developed. A

rule of thumb is 25-50% of the wavelength. However, an MASW

survey deals with a wide range of wavelengths (e.g., 1-50 m),

and this range is determined by the receiver spacing (dx) (e.g.,

dx=1 m) and length of the receiver array (L) (e.g., L=50 m). In

practice, therefore, one distance compromising the entire range

of wavelengths is used to set the minimum distance between

the source and the closest receiver, and this is called the

source offset (X1) (Figure 1). It is usually between 25% and

50% of the receiver array length (L):

- 0.25L <= X1 <= 0.5L (1)

the time of generation near the source point, they attenuate

more rapidly as illustrated in Figure 1. In consequence, after

travelling a certain distance, their energy level may drop below

that of the body waves or ambient noise. It is, therefore,

important to place receivers closer than this maximum distance

(Xmax) to avoid the contamination by these noise wavefields.

This is called the far-field effects. Xmax depends on (1) source

power, (2) ambient noise level, and (3) attenuation property of

subsurface. The first, (1), acts positively, whereas the other

two, (2) and (3), act negatively in increasing Xmax. As a rule of

thumb for MASW surveys using a heavy sledge-hammer source

(e.g., 16-lb), Xmax is about 50 m in typical urban areas with

heavy traffic, while it can be extended to 100 m at relatively

quiet areas. A more powerful source (e.g., weight drop) can

extend Xmax by, for example, 30%.

Both near- and far-field effects are illustrated in a field record

(Figure 2) obtained by using a 12-lb sledge hammer source.

The original field record has been stretched (i.e., convolved)

with monotonically increasing frequency function (i.e., sweep) in

5-50 Hz. At the lowest frequency of 5 Hz that corresponds to

the longest wavelength, surface waves are not effectively

developed at near offsets (e.g., < 40 m) due to the near-field

effects (left panel in Figure 2), while body waves and ambient

noise dominate at the far offsets (e.g., > 30 m) due to the far-

field effects (right panel in Figure 2).