Comparative analysis of various seismic refraction interpretation techniques over granitic bedrock

Mundhra A.¹, Pathak O.P.², Pamnani S. ¹, Seshunarayana T. ²

¹Indian School of Mines, Dhanbad, India, ²National Geophysical Research Institute, India

[e-mail]

In recent years acquisition of seismic refraction data has increased manifolds thereby making interpretation an imperative section for research work and development. In the present paper, onus is on applying different interpretation techniques over the data set obtained above granitic bedrock of Dharwar craton. Different conventional interpretation techniques viz. intercept time method, delay time method, reciprocal methods and tomography have been worked over and results have been compared.

Geographical location of study area is in Uppal, Hyderabad, India. Area is comprised of granites and rock formations which date back to Precambrian period and has the host rock which is medium to coarse grained granite rocks of predominantly grey or pink colour. The rocks have undergone weathering process completely at few places. The fresh granites consists fractures and joints. Area is largely composed of 2.7 – 2.5 Ga old granites and gneisses with minor narrow tracts of meta-volcanic sedimentary schist belts. Morpho-structurally, this region appears to be an area exposed near circular granite massifs, which are approximately bounded by the Krishna and Manjeera rivers in the north and south, respectively. It largely exposes Achaean medium to coarse grained granites, porphyritic granite with small inclusion of amphibolite and migmatitic gneisses at places.

Data was acquired using hammer as an energy source and 48 geophones were used to receive signals. Geophones were placed at 2.5 meters spacing making a total spread of 117.5 meters. For better quality signals vertical stacks of hammer strikes were used. Total of 19 shot points were selected, both forward and backward. Depth of penetration can be safely estimated as about 25-30 metres.

Interpretation techniques are simple to apply barring tomography as they can be done over spread sheets with easy going calculations and uses first arrival time to compute velocity and depth of refractors. All the techniques worked out had their share of advantages and disadvantages.

Intercept and delay time methods provided preliminary information such as depth and velocity of reflectors and were found to deviate nominally from computations using other interpretation techniques. This deviation may be due to personal error in manual calculation and plot of travel time curve on graph. For delay time computation first arrival were picked and extrapolation of travel time curve was done to obtain second and third layer velocity and consequently depth of respective interfaces.

Figure 1: Velocity Model obtained from ITM Figure 5: Velocity model after Tomography

Figure 4: Depth section computed by DTM  

The delay time method tends to find out depth to the bedrock below each receiver. However, it fails in the determination of lateral velocity variation and identification of thin layer.

Conventional Reciprocal Method (CRM) carries the advantage of resolving simple departures from the plane interfaces and homogeneous velocities of the intercept time method. Reciprocal methods are computationally effective for the region having gentle dip i.e. less than 15 degrees. In case of pronounced changes in depth, assumption of plane refractor does not hold and artifacts occur.

Reciprocal methods were able to accurately predict depth of second layer. But, they cannot calculate depth of third layer as velocity analysis function for the same could not be figured out. Depth section and velocity model for both conventional method and reciprocal method was found to be at par, with generalized reciprocal method (GRM) having more gentle lateral variation.

Advantage of reciprocal method over delay time method is that lateral velocity variations can be found along with the capability of depicting the fractures and joints, causing the velocity of granitic layer somewhat less than the original value.

Among all the methods illustrated, a) tomography appears to provide optimum results. Lesser undulations, accurate depth section and precise velocities for different layers marked the operation of tomography. b) Generalized reciprocal method is proved to be better interpretation technique than conventional reciprocal method. c) Results obtained were verified with borehole data. While comparisons deduced by us are not definitive however they do offer some guidance over performance of refraction interpretation techniques over granitic bedrock.

References:

1. Cerveny, V. and Ravindra, R.. (1971) Composition, Theory of Seismic Head Waves, (Editors) Toronto Press, 296 p.

2. Hawkins, L.V. (1961) Composition, The Reciprocal method of routine shallow seismic refraction investigations, (Editors) Geophysics, 26,

p. 806–819.

3. Hagedoorn, J.G. (1959) Composition, The plus-minus method of interpreting seismic refraction sections, (Editors) Geophysical Prospecting, 7,

p. 158–182.

4. Leung, T.M. (1995) Composition, Examination of the optimum XY value by ray tracing, (Editors) Geophysics, 40, p. 1151–1156.

5. Palmer, D. (1980) Composition, The generalized reciprocal method of seismic refraction interpretation, (Editors) Society of Exploration Geophysics, 104 p.

6. Redpath, B. (1973) Composition, Seismic refraction for engineering site investigation, (Editors) Explosives Excavation Research Lab., TR E-73-4,

51 p.

7. Zhang, J., and Toksöz, M.N. (1998) Composition, Nonlinear refraction traveltime tomography, (Editors) Geophysics, 63, p. 1726–1737.

Дата добавления: 2015-10-29