Seismic Data Acquisition

YOUNG GEOPHYSICIST

Seismic Data Acquisition

INTRODUCTION

            The seismic methods measure the travel-time of acoustic waves propagating through the subsurface. The shot point is the location of the source wave and the acoustic receivers are called geophones. Source waves are generally generated by percussion mechanisms located near the ground or water surface. Data are collected, stored, and processed in a seismograph.

The seismic method is by far the most important geophysical technique and its predominance is due to high accuracy, high resolution and great penetration.

            The objective of seismic exploration is to deduce information about the rocks, the structure of subsurface formation especially attitudes of the beds, from the observed arrival times and to lesser extent from vibration in amplitude, frequency, phase and wave shape.

            Seismic waves are messengers that convey information about the earth’s interior. The capacity of a material to be temporarily deformed by passing seismic waves can be described by its properties of elasticity. These physical properties can be used to distinguish different materials. Thus a medium having anomalous structural feature affects both velocity and direction of a propagating seismic waves. Observation of such effects is the essence of seismic prospecting.

            The basic technique of seismic exploration consists of generating seismic waves and measuring the time required for the wave to travel from the source to a series of geophones usually disposed along a straight line directed towards the source. From knowledge of travel times and the velocity of the waves the path of the seismic waves are reconstructed.

            Sedimentary basins are composed of layers of different kinds of rocks mostly deposited smoothly on one another. There is a ‘velocity interface’ a change from one velocity of propagation to another, at each change from one kind of rock to another. Density of rock enters into, but velocity contrasts are usually much greater, so people tend to speak of velocity interface without monitoring the density difference. Moreover, the rigidity(shear) modulus(defined as the stress-strain proportionality constant in case of simple shearing-tangential stress) being the physical property possess by the solids only and cannot be applied to ideal liquids, this property is used to distinguish different interface. The higher the rigidity the greater is the energy transmission and vice-versa.

            In seismic prospecting we require both types of approach, travel time and waveform studies. The first approach is for the purpose of defining the subsurface anomalous structure and the second mainly for enhancing the signal to noise ratio, and for investigation subsurface stratigrahic anomalies.

            The single most important purpose of seismic data acquisition is to meet the exploration objectives of the prospect area. With the focus of geological objectives shifting to more complex and subtle features, the need to sharpen the survey parameter design process and meticulous monitoring becomes extremely demanding.

Geophysical Party No-16 has been assigned to carry out 3D seismic data acquisition in Dabka-Sarban area under SIG. No.G-436. The area is located on the north-eastern rising flank of the Broach Depression of Cambay Basin .

PROPAGATION OF SEISMIC WAVES

Being a wave motion, seismic waves spread out from the source following the propagation principles. Wave types speed and propagation directly vary in accordance with the physical properties and dimensions of the medium. The simplest medium is the one which is homogeneous, isotropic and perfectly elastic. Is such and idealized modes the waves travels along straight ray paths, and with constant velocity.

In nature, media are bounded and commonly stratified into layers of different physical properties. In such circumstances a seismic wave suffers from a number of changes every time it hits an interface wave speed, propagation direction spectral structure and energy content all changes as the wave passes from layer to layer. In addition, new wave phases may be generated at interface.

SEISMIC REFLECTION METHOD

The seismic reflection method by far the most widely used geophysical technique. With this method the structure of subsurface formation is mapped by measuring the times required for a seismic wave generated in the earth by a near-surface explosion, mechanical impact or vibration to return to the surface after suffering reflection from interfaces between formations having different physical properties. The reflections are recorded by detecting instruments responsive to ground motion. They are laid along the ground at distances from the point of generation, where the far offset distance from the source are generally nearly equal to the depth of investigation. Variations in the reflecting times from place to place on the surface usually indicate structural features in the strata below.

PRINCIPLE OF SEISMIC REFLECTION METHOD

The principle of seismic reflection method is based on Snell’s law.

The principle holds for normal incidence or for a small dip (up to 20^o). For greater dipping interfaces the principle does not offers the desired result.

“Acoustic Impedance” is the principle parameter which defines the existence of an interface and thus the structure of subsurface formation can be mapped.

For reflection, an interface exists if both velocity and density of the media in the adjacent layers are different. “The parameter which expresses the combined effect of velocity (v) and bulk density (ρ) is called the acoustic impedance (Z), where,

Z = v * ρ

Thus the greater the contract in the value of the acoustic impedance the stronger the reflection becomes.

The reflection coefficient depends upon the acoustic impendence as given by the following relation,

R = (Z₂ – Z₁)/ (Z₂ + Z₁)

= (ρ₂v₂ - ρ₁v₁)/ (ρ₂v₂ + ρ₁v₁)

Where Z₂ and Z₁ are the acoustic impedance of second and the first layer.

OBJECTIVE OF 3D SEISMIC DATA ACQUISITION

The geological features in the subsurface that are of interest in hydrocarbon exploration are 3D in nature for example salt diapers, over thrust and folded bed, mature unconformities, and deltaic sands. The 2D seismic section is, in reality, a cross-section of 3D seismic wave field. A typical land 3D survey carried out by laying out a number of receiver lines parallel to one another and placing the shot points along the line perpendicular to the receiver lines(swath shooting).

The 3D seismic data acquisition is similar to 2D seismic data acquisition except it gives the closer grid information. The main objective of the 3D seismic data acquisition is the further development of an oil field, sand geometry and delineation of minor faults of the subsurface.

The 3D seismic data acquisition gives the detailed image of the subsurface which makes the interpretation more reliable. In geologically complex areas, the 3D seismic is essential to effective exploration for hydrocarbon. 3D survey is performed after the discovery of reservoir by 2D and other reconnaissance methods. 3D seismic survey is confined to a desired depth of investigation.

OBJECTIVE OF THE SURVEY

The objective of the survey in the area assigned to the party is as under:

1. To map prospects in the Dhadar formation

2. To delineate the pinchout prospects of Hazad sands.

3. To delineate structural prospects of Olpad and Deccan Trap.

ZONE OF INTEREST : 800 to 2000m

TWO WAY TIME : 800 to 1800m

RESERVOIR THICKNESS : less than 10m

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