THE FIRST DEEP SEISMIC INVESTIGATIONS IN NORTH VIỆT NAM

¹ĐINH VĂN TOÀN, ²STEVEN HARDER, ³PHẠM NĂNG VŨ, ¹TRỊNH VIỆT BẮC,
¹ĐOÀN VĂN TUYẾN, ¹LẠI HỢP PHÒNG, ¹TRẦN ANH VŨ, ¹NGUYỄN THỊ HỒNG QUANG

¹Intitute of Geological Sciences, VAST, 84, Chùa Láng Str., Hà Nội;
²Miller Geophysical Laboratory, University of Texas at El Paso, USA;
³Hà Nội University of Geology and Mining, Hà Nội.

Abstract: During the time period from December, 2007 to February, 2008 for the first time were carried out two profiles of deep seismic investigation in North Việt Nam. The designed profiles pass the most part of main structures in the territory of North Việt Nam. Two hundreds of Reftek-125 instruments were deployed along each profile for recording seismic signals produced from 3 explosion locations. In each explosion location were done 2 shots in boreholes to the depth of 26-32 m. The capacity of the explosive used for each explosion location varies from 500 to 1000 kg. The preliminary data processing was carried out by using the package of programs Seismic Unix and Ixseg2segy software. Six separated seismic wave sections were derived from the investigation, among them the seismic section produced by the explosion in Phổ Yên (E2 ) location showed clearly three main interfaces in the Earth crust along the whole profile. In the other seismic sections the crustal interfaces appear on limited sections surrounding the explosion locations. The summing of the wave fields produced by the explosions in two or three locations indicated better the reflection signal from the crustal interfaces. The crustal thickness obtained from the simple 1D velocity model for Phổ Yên location is 27 km. This value is smaller than the crustal thickness in previous studies more than 3 km, but this result may change later, because the first model applied for the estimation is a very simple one.

I. INTRODUCTION

An understanding of the crustal structure is a very important factor in the study on the geotectonic history of any region. In the region of more complicated geologo-geotectonic environment, the study of the Earth’s crust is more heavy and often requires a denser network of geophysical measurements with the application in certain areas of more reliable techniques, such as deep seismic investigations. North Việt Nam is located in the juction of two global tectonic belts: Mediterranean and Pacific ones. In the regional scale, the territory of North Việt Nam comprises two continental terranes: Southwest part belonging to the Indochina block and Northeast part being a portion of South China active platform. The boundary of these two structural blocks is the Red River fault zone. The Northeast Part extends from the Red River fault zone to the border with China. This region is characterized by relatively stable in the Earth’s crustal structures. The oscillation of the depth of the crystalline basement and the Moho surface is gradual from location to location. A large portion of the main tectonic faults here are arc-shape characteristics. On the contrary, the structure in the Southwest part of the Red River fault zone is much more complicated: a strong differenciation in depth of the crystalline basement is detected from location to location; the oscillation of the Moho surface depth is also bigger and most of the structures are elongated along the NW-SE direction (Fig. 1).

Figure 1. Distribution of seismic investigation profiles.

As a great regional geological discontinuity, the activity of the Red River fault has been playing a significant role in the formation of present-day geotectonic environment not only in the southwestern part of China, North Việt Nam, but also in the SE Asian countries. During the Cenozoic time the displacement between the sides of the Red River fault is estimated as some hundreds of kilometres. Rifting and folding were activated as the consequences of the above mentioned displacement, that are the reason of more complex geotectonic evolution of the region. As a production created by the regional geologo-geotectonic regime, the Earth’s crust in North Việt Nam is demonstrated complication in its structure. The strong separation in the structures of the crystalline basement and Moho surface cut by different systems of deep tectonic faults, as well as the sharp differenciation along the horizontal direction due to the change of rock composition was detected. The studies on the Earth’s crustal structures are carried out by different authors since the 1970 years and step by step allowed us to better understand the geotectonic environment. Most of the studies used the gravity data interpretation. In some limited locations the structures are also studied by magneto-telluric investigations, but with the sparse network of measurement points. The studies in the past time are indeed provided valuable results for elucidation of geotectonic history of the Việt Nam territory. However, the results of gravity interpretation obtained from the application of different techniques for data interpretation in different studies some time showed a big difference in determination of the depths to the crystalline basement, and particularly the Moho surface.

Aiming to receive more reliable information about the crustal structure of North Việt Nam during the period of 2007-2008 time were carried out the first deep seismic investigations along two profiles in North Việt Nam.

II. DEPLOYMENT OF INSTRUMENTS AND DATA COLLECTION

The instruments used in this study are produced by the USA Refraction Technology Company (Reftek-125) and supplied by the Miller Geophysical Laboratory, University of Texas at El Paso in the framework of the cooperation with the Institute of Geological Sciences, Việt Nam Academy of Sciences and Technology. These instruments are the lightweight, low power consumption with 24-bit dynamic range and no communication cable instruments.

The target of our investigation is a better understanding the position of the main tectonic fault systems, the distribution of the main interface in the Earth’s crust along the observational profiles, such as crystalline basement, Moho surface and maybe the middle crust, where this boundary exists.

As mentioned above, the main structures in North Việt Nam are predominantly elongated along the NW-SE direction, so investigated profiles are better designed in the NE-SW direction. To avoid the strong separation of high mountainous regions and dense population localities the position of the two profiles is selected in the midland areas. The first profile starts mostly from the centre of Northeast structural zone, at the position: 105.95° E; 21.74° N and finishes at the point: 105.35° E; 20.76° N with more than 130 km long. This profile cuts a number of arc-shape faults belonging to the Northeast structural zone, then passing all tectonic faults of the Red River zone and finishes at the vicinity of the Đà River fault zone (Fig. 1). The depth to the crystalline basement along this profile is estimated by gravity from 1 to 3 km, depending on what location. The thickness of the Moho in some studies was estimated as about 32-34 km at the beginning and ending of sections and reduced to 31 km at the central part of the profile. In other studies on the depth to the Moho surface revealed as 30-32 km at the beginning and ending sections and reduced to 25 km at the central part of the profile.

The second profile starts from the southern side of the Red River fault (Fig. 1) at the point 105.70° E; 20.52° N extending southward in the submeridian direction and finishes at the point 105.39° E; 19.49° N. This profile passes most the main systems of tectonic faults of NW-SE trending in the Southwest region of Red River zone. The crystalline basement under this profile changes more in depth in comparing with the first profile and varies from 1 to 3 km. The depth to the Moho surface in some studies is estimated as from 31 to 34 km. In some other studies a stronger change of the Earth’s crust thickness from location to location is accepted. Thus, in the northern section of the profile the thickness of the crust is estimated as 26 km, meanwhile in its southern part it is in average of about 33 km from previous studies.

For producing seismic waves in this study were conducted the explosions at three positions along each profile. In every shot point the explosions were carried out in two boreholes at the depths of from 26 to 32 m with Emulsion-NT explosive mass of from 500 to more than 1000 kg. For the data collection 200 instruments were deployed along each profile. The GPS time was synchronized for every instrument just before installation. The spacing between the seismometers on the profile varies from less than 500 to 700 m. The collected data showed that, 2 to 4 instruments deployed along the first and second profiles did not record signal.

III. PRELIMINARY RESULTS OF DATA PROCESSING

The first step in the seismic investigation is the conversion of the collected data into SEGY format, that can be accessed by different seismic data processing package of programs. In our study the package of programs SeisUnix developed by the Center for wave phenomena, Colorado School of Mines and IxSeg2Segy developed by Interpex Limited was used. During this step, the positions and elevations of the surveyed points are integrated with seismic data. The desire time window of wave field is cut from the larger record. Due to the thickness of the Earth’s crust under the investigated profiles estimated as less than 35 km, the time windows are choosen as 17 and 27 seconds of duration. To reduce the influence of noise the banpass filtering technique in the frequency range of from 1 to 20 Hz was applied. The summing of the wave fields produced by two separated explosions at the same shot location was conducted after the filtering. For more easily watching the wave form in seismic sections a 6.0 km/sec reducing velocity was used. That is the source-receiver distance divided by 6.0 km/sec, which yields the time in seconds. Each trace was then moved earlier in time by this amount. This operation allowed us to obtain the seismic section more rectangular with arrivals coming in near horizontal rather than diagonal. Next step in our data processing is the application of automatic gain control with a 5-second window. It is expected to increase the amplitude of later wave arrivals, such as PmP (reflection from the Moho). The wave sections were plotted by using GMT package of programs (Generic mapping tools) with time-distance dependence on the graph.

Six seismic sections were derived from six explosion locations along both profiles; among them the wave field generated by the explosions in the second explosion location E2 of the first profile (Phổ Yên location) is much better than the others (Fig. 2). On this seismic wave section we can see clearly the main reclection interfaces with two way travel time approximating 0.2, 5.0 and 9.0 sec. at the place surrounding the shot point. In accordance to the Earth’s structure obtained from previous studies these reflection boundaries demonstrate the seismic signal from the crystalline basement, the middle crustal interface and the Moho under the northern surveyed profile T1. The capacity of this explosion is not the largest among the explosions done in this experiment, maybe because the geological environment surrounding the explosion boreholes is more capable to produce seismic waves of higher amplitude. Probably the rock in the thick weathered layer as well as in the underlying bedrock in this location is softer in comparing with other shot points. The evidence is indicated by smaller seismic wave velocities detemined by the shallow seismic exploration conducted for all the explosion places before drilling. On other hand, the rich groundwater environment here may be also the reason causing larger amplitude of seismic wave from this location. Obviously the reflection signals from the crustal interfaces in the area of the Red River fault zone are expressed weaker than other along the profile (from point 30 to 78 km on the section – Fig. 2). This characteristic of the wave field may be caused by the strong destruction inside the Red River fault zone. The remaining 5 seismic sections derived from the other 5 locations of explosion did not allow us to get clear seismic boundaries in the whole profile, even among them two sections were produced by much larger quantity of explosions in An Nghĩa (E5) and Bãi Tranh (E6) locations. Both of them belong to the second profile T2. In these sections the reflection from the interfaces of the Earth’s crust appears mostly at the limited sections surrounding the shot points (Fig. 4). The reason of weak signal of seismic wave is

Figure 2. Seismic wave section produced by shots at Phổ Yên (E2) (Explosion capacity: 760 kg).

Figure 3. Seismic wave section of profile T1 after summing two seismic wave sections produced by shots at Phổ Yên (E2) and Hòa Bình (E3).

probably caused by harder rocks detected by higher seimic wave velocity in the subsurface soil and bedrock layers and then the drilling in those places. The poor water content in the geological environment of these locations is also prevented generation of seismic wave of larger amplitude. Obviously, the noise created by human activities is also a factor reducing the signal/noise ratio, because all explosions were done in day time and, of course, it is affected by the quality of the seismic wave sections. However, the summing of the wave sections produced by explosions in two or three locations along the same profile indicates a better correlation between the wave fields and structural elements of the Earth’s crust (Fig. 3, 5). The seismic section demonstrated on the Fig. 4 is derived from the summing of wave fields produced by the explosions in E2 (Phổ Yên) and E3 (Hòa Bình) locations. The seismic wave interfaces appear in the section similarly to the wave field in Fig. 2. In fact, the summing wave section is included the wave field from the Fig. 2. Related to the seismic wave sections obtained from the southern profile T2 the situation changes in the summing of all the three wave sections produced by explosion from the three locations along the profile. It is difficult to watch for the seismic interface on separated wave fields (Fig. 4), but in the summing wave section the seismic boundaries seem to appear on the result feature (Fig .5).

Figure 4. Seismic wave section of profile T2 produced by shots at the middle point An Nghĩa (E4) (Explosion capacity: 1005 kg).

Figure 5. Seismic wave section of profile T2 after summing the seismic wave fields produced by shots at 3 locations (E4, E5 and E6).

The first estimation of the depths to the main boundaries of the Earth’s crust is applied for the seismic section in Fig. 1 by using 1D velocity model of three layers. The calculation was carried out by Dr. Steven Harder. The depths to the crystalline basement, the middle crust and Moho surface received from the calculation are approximately 0.9 km, 15.0 km and 27.0 km with P-wave velocities 5.0, 6.0 and 6.6 km/sec respectively in the three layers.

IV. DISCUSSION AND CONCLUSIONS

1. The preliminary results of data processing have been indicating a possibility of using first deep seismic investigations in Việt Nam for improving one step of the study on the Earth's crust of the North Việt Nam.

2. The crust thickness obtained from the first estimations from seismic data is thinner more than 3 km in comparison with results from previous studies by using the gravity data. The results may change later, because the first model applied for calculation is very simple, meanwhile the Earth’s crust in North Việt Nam is complicated.

3. The seismic wave is much better generated in the location of softer rock layers and rich water content. It is not easy to find this kind of location for doing explosion in the dense population areas like Việt Nam.

Acknowledgments: We would like to acknowledge the field work help provided by scientists of the Institute of Geological Sciences, the drilling teams of Hải Quang, Đông Ích companies, the Bắc Trung Bộ Geological Division, the Faculty of Geology, Hà Nội University of Natural Sciences as well as the North Trung Bộ and Việt Bắc Companies of Mining Chemical Industry. We also would like to express the deep thanks to J.W. Miller Geophysical Laboratory, University of Texas at El Paso, USA for providing of the instruments, technology and experience for the experiments.

The investigations were covered by Việt Nam National Project No. KC.08.06.

REFERENCES

1. Brink Uri S. Ten, Abdallash S. Al-Zoubi, Steven Harder et al., 2006. Seismic imaging of low velocity zone beneath the Dead Sea Basin and transform fault: Implications for strain localization and crustal rigidity. Geophys. Res. Lett., 33 : 1-6.

2. Cao Đình Triều, S. Tatiana, 2008. Mô hình cấu trúc vận tốc sóng P khu vực Đông Nam Á. TC CKHvTĐ, 30/2 : 176-184. Hà Nội.

3. Desert Group, 2004. The crustal structure of the Dead Sea transform. Geophys. J. Int., 156 : 655-681.

4. Đặng Thanh Hải, 2003. Nghiên cứu một số đặc điểm cấu trúc vỏ Trái đất và phân vùng địa chấn kiến tạo miền Bắc Việt Nam. Luận án TS. Vật lý, Viện Vật lý Địa cầu. Hà Nội.

5. Liu Fu-Tian, Wu Hua, Liu Jian-Hua, et al., 1990. 3D velocity images beneath the Chinese continent and adjancent regions. Geophys. J. Int., 101 : 379-394.

6. Mooney W.D., 2001. The crustal structure of China from deep seismic sounding profiles. On the Website: http:quake.usgs.gov/ research/ structure/ CrustalStructure/ China/ index.html.

7. Nguyễn Ngọc Thuỷ, 1999. Seismic velocity structure in North Việt Nam using observa-tional data and Ray method. Proc. of the NCST of Việt Nam, 11/2 : 103-113.

8. Wu Hsin-Hung, Y-Ben Tsai, Tung-Yi Lee, Ching-Hua Lo, Dinh Van Toan, 2005. 3-D shear wave velocity structure of the crust and upper mantle in South China Sea and its surrounding regions by suface wave dispersion analysis. Marine Geophys. Res., 25 : 5-27.