I. INTRODUCTION

This fieldwork was done last November 11, 2012, Sunday at Greenwoods Executive Village,

Pinagbuhatan, Pasig City. Two (2) students under the tutelage of Dr. Arturo S. Daag joined a group

of scientist from PHIVOLCS in conducting Refraction microtremor survey at two (2) designated

areas inside the village.

Refraction microtremor (ReMi) is a surface-performed geophysical survey developed by

Dr. John Louie (and others) based on previously existing principles of evaluating surface waves and

in particular Rayleigh waves. The refraction microtremor technology was developed at the

University of Nevada and is owned by the State of Nevada. Optim of Reno, Nevada has the exclusive

license to develop the technology, and SeisOpt® ReMi™ has been available commercially from

Optim since 2004. Since Rayleigh waves are dispersive, the propagating waves are measured along

a linear seismic array and evaluated relative to wave frequency and slowness (or the inverse of the

velocity). Due to the dispersive characteristics of higher frequency waves travelling through the

more shallow conditions and lower frequency waves passing through deeper materials, a 1-D sub-

surface profile can be generated based on the velocity with depth.

ReMi is a new, proven seismic method for measuring in-situ shear-wave (S-wave) velocity

profiles. It is economic both in terms of cost and time. Testing is performed at the surface using the

same conventional seismograph and vertical P-wave geophones used for refraction studies. The

seismic source consists of ambient seismic "noise", or microtremors, which are constantly being

generated by cultural and natural noise. Because conventional seismic equipment is used to record

data, and ambient noise is used as a seismic source, the ReMi method is less costly, faster and more

convenient than borehole methods and other surface seismic methods, such as SASW and MASW

used to determine shear-wave profiles. Depending on the material properties of the subsurface,

ReMi can determine shear wave velocities down to a minimum of 40 meters (130 feet) and a

maximum of 100 meters (300 feet) depth.

ReMi can be used to obtain Vs profiles for: Earthquake site response, Liquefaction analysis,

Soil compaction control, Mapping the subsurface and estimating the strength of subsurface

materials, Finding buried cultural features, such as dumps and piers and Offshore surveys to

determine depth to bedrock for harbor and pier extensions.

II. OBJECTIVE

The main objective of this field work is to familiarize ourselves with the equipments and

proper technique in conducting a Refraction microtremor (ReMi) survey and correlate the resulting

data with another data using different geophysical survey.

III. LOCATION and ACCESSIBILITY

Greenwoods Executive Village, Pinagbuhatan, Pasig City is located 25 km east of Manila,

and can be easily access by public and private vehicles. A typical commute between Manila

and the village will take between 30 minutes to one hour depending upon traffic

conditions. It is bordered on the west by Quezon City and Mandaluyong City; to the north

by Marikina City; to the south by Makati City,Pateros, and Taguig City; and to the east by Antipolo

City, the municipality of Cainta and Taytay in the province of Rizal.

Two sites at Greenwood Executive Village were chosen for the survey. The first survey was

staged in an open field of concrete road with geographical coordinates of N14º54’93.9” and

E121º11’20.7”, and the second was inside the football field beside the village club house. The whole

area was a rice field before being converted to a first class residential village. Most of the areas that

are not yet develop are swampy with thriving water lilies and live fish particularly the local variant

of tilapia.

IV. EQUIPMENT AND MATERIALS

Geophones with 24 sensors

Seismograph (digitizer) Laptop (displays the data)

Battery (power source) Y-cables (connects the geophones to

the digitizer)

Refraction Cable 200 m Seismic source (hammer)

Metered tape (for measuring the required distance of each geophones)

Note: Note: Amplitude or frequency-response calibration of geophones is not needed - as

with refraction, ReMi uses only the phase information in the recorded wave field.

V. PROCEDURE AND METHODOLOGY

Setting up the array: Put the cable across your site in a line 200 m (600 ft) long, with about

8 m (25 ft) spacing between phones.

a.) You need to locate a reasonably straight stretch of flat ground at your site at least 200

meters long. Center it on your foundation or borehole location if possible.

b.) The absolute minimum array length needed for a 15%-accuracy ReMi is 100 meters.

c.) Avoid known underground cavities 3 meters or more in diameter - pass beside but not

over them.

d.) Avoid concrete slabs; thin pavements are OK as long as you are able to set the

geophones on them.

e.) For ease of deployment, avoid water more than 6 inches deep.

f.) An easy urban layout is to run the array along the sidewalk, with the geophones planted

into the parking strip or cracks in pavements. If the seismic cable must cross a street or

drive ways that cannot be blocked during the survey, put it between 2x4s nailed to the

pavement. Protect any geophones installed in a street with a traffic cone, or note any

channels that cannot be installed.

g.) A ReMi array can work extending into the ground floor of a building.

h.) For ReMi a deviation in the line of 5%, or 10 m from a 200-m-long line, will not affect the

stated 15% velocity accuracy of the method.

i.) This accuracy applies to elevations as well - in fact the line can have a constant

inclination that can safely be ignored, as long as geophone elevations do not deviate more

than 10 m from the incline.

j.) Simply chain out the geophone locations with a tape measure.

k.) Survey the geophone locations to 1-meter accuracy if the array is more than 10 meters

out of line, or 10 meters off constant incline. Include the survey data with the seismic

records if this is the case.

Installing the Array:

a.) Unreel your geophone cable along the array line, setting takeouts at the chained, marked

geophone locations.

b.) The geophones can be as much as 15 degrees off vertical without compromising ReMi

data quality. Visual inspection of verticality is thus adequate.

c.) Attach your multichannel seismograph and test for electrical continuity, and signal from

all geophones.

Recording Data: Take 3 to 10 records of background noise, 20-60 seconds long each.

a.) Set the record to have 24 channels (if your recorder allows more).

b.) Set a time sample interval of 2 milliseconds (ms) or longer, up to10 ms.

c.) Set each record to have a time duration of 20-30 seconds. 4 seconds is the absolute

minimum. Records longer than 30 seconds can be difficult to translate between formats -

configure each recorded trace to have no more than 16,000 samples to avoid such problems

(8-sec record at 0.5 ms sampling; 32-sec record at 2 ms sampling; or 160-sec record at 10

ms sampling).

d.) Turn off any filtering before digitizing or plotting. If the recorder does not allow this, set

the lowest possible low-cut filter frequency (hopefully 4 Hz or less) and a high-cut

frequency equal to half the sampling frequency (e.g.: with 2 ms samples, sampling frequency

is 500 Hz, so a high-cut filter at 250 Hz is OK as a reasonable anti-aliasing filter).

e.) Record 3 to 10 records, triggered manually. You may want to wait for the passage of a

good noise source like a train, heavy trucks, or low-flying jet.

f.) Do not stack records in the seismograph's memory. Clear the stack memory before

triggering each record.

g.) Save each record separately to the seismograph's hard disk, or floppy

h.) Transfer records to a laptop on-site, for easy data transmission later.

i.) If the site is quiet, activate some sort of source during each record. Drive up and down

the geophone line in a truck. Run up and down the line. Pick up and drop 50-lb rocks. No

timing or locating of the source is needed

A. Site No 1. Open field along the concrete road

1. The geophones are placed in an array consisting of 24 geophones at an interval of 2 m

each. (Photo 1)

Photo 1

2. We device a stand to make sure that the geophones tip are directly pointed towards the

ground. (Photo 2)

Photo 2

3. Passive seismic source was used in this particular survey since the survey area is not in

a soft soil. (Photo 3)

Photo 3

B. Site No. 2. Football Field.

1. The geophones are placed in an array consisting of 24 geophones at an interval of 2.5 m

each. (Photo 4 & 5)

Photo 4

Photo 5

4. Since the survey is on a soft ground we utilize a seismic source, which is a hammer to

generate seismic waves. (Photo 6)

Photo 6

VI. RESULTS AND INTERPRETATION

A. Site No 1. Open field along the concrete road

Velocity model is on the right 1st column is the depth (in meters), 2nd column is the density,

and the 3rd column is the shear wave velocity (in meters/sec). At lower bottom right is the IBC site

class E, which is not surprising considering that the surveyed area is a former rice field. The lower

bottom figure is the graph of fit dots based on the velocity model on the right. The rms (~5) must be

less than 10, to consider the velocity model to be acceptable. The upper left figure is the surface

wave dispersions, the boxes are the picks which are reflected on the graph.

Note the decrease in velocity value below 4.15 m depth.

Here are the velocity values gathered from site 1:

Depth (m) Shear wave velocity, Vs (m/sec)

0.0 116.11

1.051 116.11

1.892 135.683

5.571 168.47

17.868 135.41

Velocity model from ReMi Comparison of velocity model with SPT-N of Spectrometer

B. Site No. 2. Football Field.

Here are the velocity values gathered from site 2.

Depth (m) Shear wave velocity, Vs (m/sec)

0.0 122.559

5.648 170.567

10.465 170.567

15.781 233.852

Velocity model from ReMi Comparison of velocity model with SPT-N of Spectrometer

In the comparison of velocity model with SPT-N of the spectrometer, the formula that I

used is Vs = 89.8*N^(0.341). (Ref: Imai, T. and Yoshimura, Y. (1975). The relation of

mechanical properties of soils to P and S-wave velocities for ground in Japan, Technical Note, OYO

Corporation.) This is also the same formula that Metro Manila Earthquake Impact Reduction Study

(MMEIRS) used in their reports.

In the MMEIRS report of Vs value of Taguig, which is close to Cainta, ranges from 120-250

m/s and is very consistent with our measurements. The results of the Spectrometer and ReMi

methods suggest that a soft soil layer underlies the village. The spectrometer method gives

a higher resolution of the shallower rock layer while the ReMi provide estimates of the Vs at deeper

depths. ReMi modelling suggested a site class type E (soft soil profile) following the 2003

International Building Code Site Class Definitions.

The difference in measured shear wave velocity results between these methods is in the

order of 10 to 20 per cent.

VII. CONCLUSION

The ReMi method offers significant advantages. In contrast to borehole measurements ReMi

tests a much larger volume of the subsurface. The results represent the average shear wave velocity

over distances as far as 200 meters (600 feet). Because ReMi is non-invasive and non-destructive,

and uses only ambient noise as a seismic source, no permits are required for its use. ReMi seismic

lines can be deployed within road medians, at active construction sites, or along highways, without

having to disturb work or traffic flow. Unlike other seismic methods for determining shear wave

velocity, ReMi will use these ongoing activities as seismic sources. There is no need to close a street

or shut down work for the purpose of data acquisition. And a ReMi survey usually takes less than

two hours, from setup through breakdown. Greater depth of investigation compared to borehole

and surface methods.

Surface waves are naturally considered to be the dominant component of the microtremors,

over body waves. The ReMi method is suitable for the estimation of subsurface structure, such as

required by the field of earthquake engineering.

IX. REFERENCES Comparative Study of the Refraction Microtremor (ReMi) Method: Using Seismic

noise and standard P-wave refraction equipment for deriving 1-D S-wave profiles.

Satish Pullammanappallil and Bill Honjas, Optim LLC, Reno, Nevada, USA. .

PHIVOLCS. Winchelle Ian Sevilla