REFLECTION SEISMOLOGY USING MICO-SEISMISTY AS SOURCE SIGNALS

T. MATSUOKA, K. SHIRAISHI Kyoto University, Kyoto, Japan

T. AIZAWA Suncoh Consultants Co., Ltd., Tokyo, Japan

INTRODUCTION

Under the ground, there always exist various vibrations accompanying social life, such as the running of trains, subways, or cars and civil constructions and factories besides natural earthquakes or volcanic activities. These vibrations are called as the micro seismisty. In this paper we use this seismisty as the source signal for the reflection seismology. This underground servey technique is called “Seismic Interferometry" or “Daylight Imaging.” The method intends to image the subsurface structure using the seismic wave observed on the surface as a transmission wave field and to simulate the pseudo reflective wave field by cross correlation of transmission wave data.

COMPOSITION TRAMSMISSION WAVWS TO REFLECTION RECORS

The relationship between the transmission wave field and the reflective wave field in the one-dimensional multilayer structure was introduced by Claerbout (1968) and he called this as “Daylight Imaging.” Wapenaar (2003) was extended this idea to the case of the three dimensional. This relationship is obtained by using the reciprocity theorem and conservation of energy.

When the seismic wave fields from one buried impulsive source are observed at two places A and B on the surface of ground (Fig. 1), we can simulate the pseudo reflective wave field which corresponds the source at A and the receiver at B. The expression of this relationship of the transmission wave record T and the reflective wave record R is as following (Wapenaar 2003, the equation (14)).

Figure 1 The concept of seismic interferometry.

∑ −−

−=−+

iiBiA

BHAHBABA

txxTtxxT

txxtxxRtxxR

),,(*),,(

)()(),,(),,( ,, δδ (1)

Where T(XA, Xi, t) and T(XB, Xi, t) are the transmission wave fields at points A and B from the subsurface source point Xi respectively. R(XA, XB, t) is the reflective wave field corresponding to the receiver at XA and the source is placed at XB. Using this relationship, the reflective wave field can be simulated by the cross correlation with two transmission wave fields (the first term of left-hand side of Eq. (1) becomes causal part and the second term correspond to the anti-causal part). Moreover the second term of the right-hand side of Eq. (1) expresses the sum of the correlation results for many buried sources, when two or more buried sources are exist. In theory, the buried sources are required infinity and which are randomly distributed.

In this method, when the many geophones are arranged on the survey line, we can simulate pseudo reflection shot records as many shot records as the number of the receivers. This is a great advantage for improving the signal to noise ratio.

ABSTRACT: We applied acoustic daylight imaging technique to micro seismisity. Since this method can synthesize shot records of the same number as receivers, it is effective for weak signals. The reflection response can be obtained by the cross-correlation of the transmission responses. In this paper, we apply this to the field data observed by several different ground vibrations. Obtained subsurface images are clear enough to estimate underground structures.

A B

xi

T(xA,Xi,t) T(xB,Xi,t)

UNDER GROUND RADOM NOISE In order to investigate the applicability of the

seismic interferometry, we carried out a field test. The survey design is considered as close as possible to the theory, we adopted a field where there exist two tunnels under the ground. The idea is to generate the underground seismic noise using several different sources. Survey Layout

The geological structures of the test field and survey liens are shown in Fig. 2. In this area, it is known that the alternation of mudstone of a single inclination which is parallel to the east side slope of a mountain by a prior geological survey. Moreover, the driveway tunnel and the sidewalk tunnel pass in the ground, and these tunnels are used for a course of the moving vibration sources. A geophone group (Line-1) has 96 receiver points at intervals of 5m and it arranged on the mountain right above a sidewalk tunnel. We also acquired the standard reflection shot record for the comparisons with seismic interferometry sections. However, this line has 72 receiver positions (receiver #25 – 96 are used). Instrument

In order to apply the seismic interferometry, it is desirable that the long time measurement of the data can be acquired by many channels. On the purpose of this field work, the seismic instrument based on desktop PC was developed so that the continuation measurement of the receiver record of 96ch could be carried out simultaneously for a long time (Fig. 3). Geophone signals enter the A/D board of the PC and the data are recorded on a hard disk. The PC base data acquisition system is controlled by LabView.

Sampling ratio is considered as 1000Hz (variable) of sampling frequencies, and measurement of arbitrary time period is available. After starting measurement, the vibration source in the tunnel begins to move and stop at the pre-specified period of time (or the specified number of the shot times). Although the maximum measurement up to 96ch is possible now, we have a plan to develop 360ch system. Data Acquisition

Table 1 shows the list of 4 kinds of vibration sources used on a driveway tunnel and a sidewalk tunnel. The length of the each data acquisition is also shown. Fig. 4 shows the pictures of the air knocker, the Digipulse, and a track which we adopted as the seismic sources. A hammer, an air knocker, and Digipulse hit the tunnel road

Figure 2. Geology, topography, and the survey line.

Figure 3. The picture of data acquisition instrument.

Table 1 :The vibration sources and data aquisiotion length. Source Set place Data Lngth (sec)

Hammer sidewalk tunnel 1,600 Air knocker sidewalk tunnel 2,140 Digipulse driveway tunnel 1,840 2t Track driveway tunnel 1,600

Figure 4. The Vibration sources. Digipulse (upper left), air knocker (below), 4t track

at equal intervals (every 2.5m - 5m), moving from the entrance to the exit of the tunnel after the data acquisition started. Moreover a 4 ton track continues running the inside of a driveway tunnel intently, and the track was drove 10 times round trips. Since a trigger was not applied during the data acquisition, the initial shock appearance time of each record is not common. However, the necessity of matching these times does not exist since taking cross correlation based on Eq. (1). Data Processing

The main processing flow is shown in Fig. 5. In order to remove the measurement and PC instrument noise, a median filter and a band path filter are applied first. Then, pseudo shot records are generated by cross correlation with observed records, and the header information is added and finally the SEG-Y Format data are obtained. This is synthesized pseudo shot records. Fig. 6 shows this pseudo records obtained from the observation data to each seismic source which are shown in Fig. 4. Form Fig. 6, we can recognize the reflection events in all the cases which may corresponds the geological boundaries. From Fig. 1, an inclined structure which is parallel to the east side slope of a mountain can be recognized. This structure crosses the west side slope of the mountain, and is considered as the mud rock formation.

In order to do imaging of the subsurface structures, the conventional data processing was performed to pseudo shot records. The processed sections are shown in Fig. 7 which corresponds to after NMO stack using the pseudo 72 shot records. Being able to recognize the reflective event which has an east inclination, this result strongly supports the validity of the analysis result of the observation record which used the micro seismisty.

We obtained subsurface image of the mountain by using the hit type sources and also by track running vibrations. The long data acquisition may enable to improve the signal and noise ratio. This result shows the applicability of seismic interferometry to field data not using the artificial sources such as the dynamite explosive. We also can presume to estimate underground structure from the vibrations produced in a social activity, and this has suggested that this technique can play an active part in a many field. DISCUSSIONS

In this paper, a field examination towards utilization

of "Daylight Imaging" or “Seismic Interferometry” was done. Since this method requires a long record length, we developed PC base seismic instrument with 96 channels in this purpose.

Figure 5. Data processing flow for seismic interferometry

Figure 6.1. The simulated shot record by hummer.

Figure 6.2. The simulated shot record by air knocker.

The method discussed here is a newly proposed reflection seismology technique by using positively the micro seismisty and do not require artificial sources but has so far been considered to be a noise in reflection seismology. Also the application to three-dimensional survey is theoretically easy.

REFERENCES Claerbout J. F. (1968): Synthesis of a layered medium

from its acoustic transmission response, Geophysics, 33, 264-269.

Wapenaar, C. P. A. (2003): Synthesis of an inhomo- geneous medium from its acoustic transmission response, Geophysics, 68, 1756-1759.

Figure 6.3. The simulated shot record by digipulse. Figure 6.4. The simulated shot record by 4t truck. Figure 7. Stacked sections corresponding, hummer, air knocker and truck.