Utilization of seismic refraction data for the study of structure of bang hot water source, le thuy, quang binh VJES 38

This paper presents the preliminary results of the structural study obtained by the seismic refraction data in the area of hot water spring Bang, Le Thuy, Quang Binh.. Based on the above

Vietnam Journal of Earth Sciences Vol.38 (4) 394-409

(VAST)

Vietnam Academy of Science and Technology

Vietnam Journal of Earth Sciences

http://www.vjs.ac.vn/index.php/jse

Utilization of seismic refraction data for the study of structure of Bang hot-water source, Le Thuy, Quang Binh

Tran Anh Vu*P

1,2

P

, Dinh Van ToanP

1

P

, Doan Van TuyenP

1

P

, Lai H op PhongP

1

P

, Duong Thi NinhP

1

P

, Nguyen Thi H ong QuangP

1

P

, Pham Ngoc DatP

1

P

1

P

Institute of Geological Sciences, Vietnam Academy of Science and Technology

P

2

P

Graduate University of Science and Technology, Vietnam Academy of Science and Technology

Received 01 June 2016 Accepted 10 December 2016

ABSTRACT

Study of the geological structural elements in the area of geothermal sources is important for identifying the geothermal reservoir, the object is capable of energy production This paper presents the preliminary results of the structural study obtained by the seismic refraction data in the area of hot water spring Bang, Le Thuy, Quang Binh The exploration was carried out in 2014 by using 150 wireless Texan instruments produced by Refraction Technology Company - USA and provided by the Institute of Earth Sciences, Academia Sinica, Taiwan The data were collected from 4 profiles, cutting several tectonic faults around the exposed hot water source The seismic signals were strong on the records of each instrument, especially the signals of refraction wave The 2D seismic tomographic technique is applied for data interpretation to create the velocity structural models from 4 observation profiles Based on the velocity structures, the area can be separated into three main structural layers, characterized generally by three velocity ranges: 3,0-4,1 km/s; 4,2-5,1 km/s and 5,2-6,1 km/s, respectively

The block separation by the faults of different size with the subsidence tendency from southwest to the northeast parts of the region is apparently reflected in the seismic data obtained in this study The narrow lower velocity vertical structure detected inside the southern well-consolidated rock uplifted block away from the exposed hot water source more than 2 km, under the sub meridian extension Quaternary structure probably related to the breaking up of the bedrocks caused by the tectonic activity in the region Perhaps, the object played a role as the thermal fluid channel in the geological history time and is closely related to the geothermal reservoir predicted recently by magnetotelluric investigations in this location

Keywords: seismic refraction, 2D modeling, structure, geothermal Source Bang

©2016 Vietnam Academy of Science and Technology

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Corresponding author, Email: anhvu3110@yahoo.com

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is the basic way to get a more reliable velocity structural model under observation profiles If the velocity of each layer in the horizontal layered media is assumed to be a constant, the travel time is described by a straight segment with the slope to horizon decreasing by increasing the velocity on the time - distance graph In practice the time - distance graph for each layer is not completely obey the linear law, since the velocity is increased with depth

in the same layer and strongly changed at the boundary of two layers (White, 1989; Berryman, 1991; Zelt, 1999) Based on these properties the separation of environment into different layers followed the time - distance curve can be realized not so difficult The same properties can be applied to separate the velocity structural model into different layers

In this case instead of the slope change along the time-distance curve the difference in velocity gradient of different layer represented

by the density of velocity isolines of the model is used The infringement of linear law

of the time - distance graph can happen when the interface between two layers is inclined or

Vietnam Journal of Earth Sciences Vol.38 (4) 394-409 represented by the strong uplifted and

subsidence, etc… The above-mentioned

indications need to be taken into account

during the process of data processing and

interpretation According to practical

experience, a sudden velocity change along

horizontal direction, the strong offset along

vertical axis as well as the existence of narrow

vertical block penetrated deeply into

environment are the indications for

identifying the tectonic faults and tectonic

fracture zones These objects also can be

defined on seismogram by the strong offset of

the same phase of waves along the time axis,

or the change to hyperbolic shape of the time -

distance graph caused by wave diffraction

Related to the petrology, the stratigraphic

of the study area is characterized successively

from the surface to the depth by Quaternary or

weathered soil, the Paleozoic formations:

Long Dai, Dai Giang and Tan Lam with

composition of mainly claystone, sandstone,

siltstone, limestone and dolomite Though

there aren’t physical properties of the rock

samples obtained from laboratory analysis, the

consolidation degree is increasing with

age was revealed by the investigations at a

number of outcrops Therefore, the

environment in the study area is expected to

generate refraction waves

Based on the above - mentioned analysis

and the purpose of this study, in the

framework of the National Scientific Project

(Code KC.08.16/11-15), 4 seismic profiles

were conducted in the area of hot water spring

Bang - Le Thuy - Quang Binh (Figure 1) This

paper provides the information about

structures in the area Bang based on the

preliminary results of the refraction data

analysis

3

am

2 Tectonic setting

The hot water spring Bang is located in the southern margin of Quang Binh geotectonic unit On the regional scale, the study area belongs to the eastern part of a large Truong Son uplifted structure, its geotectonic evolution is dominantly driven by the activity

of the Khe Giua - Vinh Linh fault According

to the geological map of scale 1:1.000.000 (Tran Van Tri et al., 2004), this northwest - southeast trending fault is stretching from Nakay plateau (Lao territory), entering into Vietnam at the south of the mountain Co Ta Run; the fault section in the territory of Vietnam is estimated 120 km long with the first segment paralleled with the upper stream

of Long Dai River; the next segments are passed successively Khe Giua, Khe Bang (Le Thuy), North of Ben Quang, South of Ho Xa (Vinh Linh) and reaches the coastal line at Cua Tung Cutting the study area and

East Vietnam Sea

Tran Anh Vu, et al./Vietnam Journal of Earth Sciences 38 (2016) experienced a long evolution history, the

activity of this regional fault is profoundly

affected to the neotectonic structural feature

of the region In the study area, about 15 km

surrounding the hot water spring Bang, the

fault segment named F1 is stretching nearly along the sub parallel direction and separates the study area into two main structural blocks:

Le Thuy in the northern and Vinh Linh in the southern parts, respectively (Figure 2)

3

The Le Thuy block occupies the area

belonging to three villages: Son Thuy, Truong

Thuy and Van Thuy The structure is

developed on the basement of Truong Son

folded uplifted belt, which was consolidated

in the Paleozoic time and consists of

continental, continental carbonate materials

belonging to the Long Dai, Dai Giang

and Tan Lam formations The strong

differentiation of movement during Cenozoic

time had created a number of higher order

structures, their boundaries are mainly the

northwest - southeast trending faults The subsidence rate is increasing from southwest

to northeast, meanwhile the age of basement rocks is decreased from Early-Midle Paleozoic at the vicinity of Khe Giua - Vinh Linh fault to Middle Paleozoic age in the northeastern part of the block The Quaternary sediment is also spread more popularly and thicker in this part It is noted that, the basalt extrusion outcrop of Late Pliocene - Early Pleistocene age is distributed along the northwest - southeast direction fault and

Vietnam Journal of Earth Sciences Vol.38 (4) 394-409 located away from the hot water spring Bang

more than 5 km to the Northeast

The Vinh Linh block occupies the southern

part of the study area in the territory of Vinh

O, Vinh Ha, Ngan Thuy and Vinh Khe

villages This structure is developed on the

fold basement consolidated in Paleozoi and

consists of continental, continental carbonate

materials of the Long Dai and Tan Lam

formations In relation with the northern

structural unit, the southern structure can be

fairly accepted in term of the uplifted block

The strong differentiation of movement

during Cenozoic time had separate the block

into the high order structures, the boundaries

of which are mainly the faults of northwest -

southeast direction Though the age of the

basement rocks is the same Early - Middle

Paleozoic, younger tendency is demonstrated

from southwest to northeast In addition, in

this block the density of sub meridian faults is

higher than in the northern one, especially in

the south of the hot water spring Bang

Despite of the complexity of structural

feature in the study area, the step subsidence

tendency is apparently reflected in general

from southwest to northeast and the most

strong subsidence is revealed along Khe Giua

- Vinh Linh fault (F1), which is the boundary

between two main blocks In relation to the

faults, the northwest - southeast trending

system is the most popular system spread in

the study area The younger sub meridian

faults possibly related to the present day

geothermal activity, including the hot water

spring Bang, which was formed as the

consequence of the Quaternary extension

movement in the region

3 Field measurements and data interpretation

techniques

3.1 Field measurements

The seismic investigations aimed to

provide the information about deep structure

in the area of hot water spring Bang and

surroundings for improving confidence of the study of geothermal system structures and the prediction of geothermal reservoir existence based on the magneto-telluric data As mentioned above, the seismic refraction investigations were selected According to the previous studies (Flynn, Hoang Huu Quy, 1997; Hoang Huu Quy, 1998; Doan Van Tuyen, 2016), the hot water spring Bang is generated by the activity of the northwest - southeast trending fault system, including the F1, F6 and the smaller size sub meridian faults It will be better if the measurement profiles are designed to cut as much faults as possible and their prolongation needs to reach the maximum value to increase investigated depth Based on geological survey data (Tran Van Tri et al., 2004), the maximum thickness

of all Paleozoic sediments may reach 3500 m

in the region If it will be the desire depth to investigate, the measurement profiles must be prolonged from about 4 times greater than that (Reynolds,

2011) 3Since3 3the3 7total7 3thickness3 3of3 7Paleozoic 4s ediments4 3was3 3roughly3 3estimated3 3by3 3the3 3geolog ical3 3survey3 3data,3 3the3 3results3 3are3 3bearing3 3mainl

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Tran Anh Vu, et al./Vietnam Journal of Earth Sciences 38 (2016)

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Vietnam Journal of Earth Sciences Vol.38 (4) 394-409

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nt3 3slope3 3to3 3the3 3abscissa3 3from3 3segment3 3to3 3segm ent.3 By intuition it is not so clear to see the separation into different straight segment due

to a small scale of the seismic section, however 3 distinguished near straight segments of the different slope to the horizon corresponded to different refraction interface with different velocity in the environment were identified In this study the seismograms recorded by individual instrument is used for picking the first arrival time, since the signal

on it is much more clear than on the seismogram created by combination of the data records of all instruments

3

Tran Anh Vu, et al./Vietnam Journal of Earth Sciences 38 (2016)

3

3

receiver 3 3 in 3 3 the 3 3 investigated 3 3 environment

3.2 Method of data analysis

Construction of seismic velocity model

under observation profile is the purpose of

data analysis The 2D seismic inversion will

be applied and tomographic technique is used

to construct the velocity model under the

investigation profiles

3.2.1 The algorithm and software used for

data analysis

Our problem is seeking the velocity

structural model under the observation

profiles, so that the difference of theoretical travel time in comparison with the observation

is need to be small enough This paper just deals with the first arrival refraction wave appeared on the seismograms and generated

by underground layered environment The first step of inversion is to solve the forward problem, i.e the initial structural model must

be constructed by an interpreter and than the wave travel times from sources to receivers will be calculated The comparison between the theoretical calculated travel time curve and the observation data is the next step 3If3 3the3 3difference3 3between3 3them3 3is3 3not3 7sm all7 7enough,7 3the3 7parameters7 3of3 3the3 3model,3 3inclu ded3 3the3 3depths3 3to3 7different7 3interfaces3 3as3 3well3 3a

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Vietnam Journal of Earth Sciences Vol.38 (4) 394-409 omography3 3is3 3realized3 3by3 3using3 3the3 3software3 3n

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TR j R= AΣ

i = N

i = 1

E

SR i RLR ji R j = 1,2, , M (1)

Here M - the number of instruments; N -

the number of segments along the wave ray

passed the environment and separated by the

grid network

To calculate the theoretical travel time

curve both the initial layered structural model

as well as minimum and maximum velocity

values corresponded to the subsurface and

deepest layers must be given by an interpreter;

the number of layers can be also changed

during the iterative calculation process In this

study the increasing velocity with depth

obeyed the exponential law will be calculated

and it’s values at each node of the grid

is automatically accepted during the

calculations As mentioned above, if the

difference between the theoretical and

observation travel time curves TR ilt R and TR iqs R is

still not small enough, the iterative calculation

is continued on the basic of least square

techniques to change the model parameters:

E = 1

M AΣ

i = M

i = 1

E

[ TR ilt R - TR iqs R]P

2

P

= Min (2) Since the function E can reach a minimum

when ∂E/∂mR j R =0, therefore the parameters

need to be changed in the next iteration are

calculated from formula:

TR j RP

k+1

P

= TR j RP

k

P

+ ∂E/∂mR j R (3) Here ∂E/∂mR j R- partial derivative of the

parameter mR j R, possibly the velocity or the

depths in the nodes of grid; k - the numerical

order of iteration

Thus, to prepare the input data for modeling, the definition of the source - receiver observation travel times for all the instruments along the profiles is needed to be performed firstly Since the instruments used

in this study are wireless, so the travel times are determined by subtraction of the travel time at the instrument located in 2 to 4 m around a shot point from the travel times at the instruments distributed along the profiles The refraction signals are clearly reflected from the collected data, especially when the seismograms were read by using the program Seismogram2K developed by the Antony - Lomax company, USA The frequency, amplitude filtering and zoom functions can be operated by this program Though the first arrival wave is indicated stronger than the noise on all the recorded seismograms, the band pass filtering operation was applied to increase the resolution in time for the signals

In consequences the pick of first arrival times

is became more easy and more reliable (fig 7) 3All3 3the3 3travel3 3times3 3related3 3to3 3each3 3explosi

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point2 3N4;3 3the3 3value3 320.553 3ms3 7is7 3the3 7largest7 3err

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and3 329.243 3ms3 3are3 3the3 3largest3 3interpolation3 3err ors3 3related3 3to3 3the3 3common3 3explosion3 3at3 7the2

Tran Anh Vu, et al./Vietnam Journal of Earth Sciences 38 (2016) point2 3N93 3for3 3the3 3profiles3 3T33 3and3 3T4,3 3respectiv ely

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