IN THE NAME OF ALLAH THE MOST BENIFICENT AND THE

MERCIFUL.

SEISMIC EXPLORATION BY REFRACTED WAVES

IN PETROLEUM PROSPECTING

Contents

Introduction

Seismic exploration in petroleum basins

Head Waves

Methods

Conclusion & Recommendation

Overview

The method of refracted (head) waves rarely used in petroleum prospecting has a high

Exploratory potential and can be applied to regional- and local-scale problems.

Special experimental work in petroleum basins will be useful to investigate the influence of oil and gas as fields on the behavior of refracted waves.

Seismic Refraction This is basically the same as seismic

reflection but this time the waves are refracted through the layers before returning to the surface.

These waves hit the boundary between 2 rocks and then travel along the boundary before returning to the surface.

Reflection and Refraction

sin f1 = sin f3 = Vp1 Vs1

sin f2 = sin f4 = Vs2 Vp2

p (ray parameter)

When a seismic ray strikes on geological boundary at

certain angles, it is refracted, or bent. The amount of bending depends on the difference between the velocities of the two

geological layers and the angle of incidence.

Critical Refraction

Seismic Refraction

Waves passing from slow to fast medium

Head Waves

The shortest path, but as source– Geophone distances become greater, seismic waves travelling by longer paths through rocks of higher seismic velocity may arrive earlier. Such waves are called head waves, and the refraction method involves their interpretation

Head Waves

Seismic exploration in petroleum basins

Seismic exploration of oil and gas fields is most commonly run with reflected wave methods.

Reflection profiling allows high resolution and quite prominent vertical source orientation, and is good to image layered

structures using multicomponent techniques at variable offsets and radiation patterns.

Limitation: The use of refracted waves is very limited, even combined with reflections, and restricted to regional-scale crust and upper mantle soundings.

Reason.....

The reason is that the refracted wave method requires stronger technologies providing very long offset and

Powerful sources and is troubled with difficulties in high-frequency recording as in head waves.

Refracted reflections R/S with offset Record

Records of refracted reflections vary with offset, which is technologically correct for the standard seismic exploration. However, both refracted and reflected waves from different depth layers are recorded simultaneously at any finite offset.

Short-offset refracted wave records:

Longer-offset records:

Short-offset refracted wave records: Short-offset refracted wave records image the uppermost section and are almost never used in studies of specific fields.

Longer-offset records: contain refracted waves and occasionally also reflections with subcritical or critical incident angles.

These waves have been especially well studied in DSS experiments when wide-angle reflections from the crustal base (Moho) appear systematically, with prominent amplitudes.

Example:

DSS in East Siberia detected wide-angle reflections from the crustal base at <20 km. The network was too scarce to obtain travel time curves, but the amplitude pattern left no doubt that the waves were wide-angle reflections.

Example:

Wide-angle reflections were recorded at shallow depths in a petroleum province in Belarus from a two-layer syncline rather than from a flat horizontal reflector. This case, however, cannot be considered typical before the results are supported by additional experiments.

Although yet few, experiments with wide-angle reflections demonstrate that recording is possible in different petroleum provinces at different depths in sediments or in consolidated crust, provided that special experiments are run in favorable conditions.

wide-angle reflections

Multiple wide-angle reflections are possible though have never been recorded so far in field experiments.

Both P and S refracted waves can be assigned to interfaces at different depths. In some downhole experiments,

converted waves are recorded along with monotypical P or S waves, especially in the presence of reflectors with

high velocity contrasts. The most stable are PPS waves or PS in simpler models.

Note that converted waves, though less prominent and stable, were recorded in terrigenous sections in regional-scale experiments. Therefore,

identification and processing of the more numerous refracted waves can be difficult

Refracted waves Typeswithout diving phases, are of three main types:

(1) monotypical head waves in layered media;

(2) PPS head waves especially prominent in sedimentary sections; and

(3) Transmitted PP or PS waves from three or four relatively sharp interfaces located at certain depths (occasionally, relatively deep).

Data from all wave types (wide-angle reflections, head refracted waves, including multiple refractions) were

formerly processed on a nondigital basis. Broader use of advanced digital recording and processing techniques will significantly improve data quality.

method of refracted waves

First major geophysical method applied to subsurface investigation of relatively deep oil-bearing geologic structures

No longer the primary method in oil exploration, but has found use for near-surface, high-resolution subsurface investigation

Common applications for civil engineering and environmental studies include depth-to-bedrock and groundwater investigations; also used for shallow fault and stratigraphic studies

Main objective is to measure the time of the “first break”, that is, the time when a given geophone first moves in response to a seismic energy source. Simply stated, since time and relative distances of sources and geophones are known, the velocity of the subsurface can be calculated

Conclusions

The method of refracted (head) waves rarely used in petroleum prospecting has a high

exploratory potential and can be applied to regional- and local-scale problems.

Special experimental work in petroleum basins will be useful to investigate the influence of oil and gas fields on the behavior of refracted waves.

The experiments are expected to be run in petroleum provinces, and detection and investigation of hydrocarbon reservoirs by various methods is a top-priority target.

Parallel refraction and reflection surveys are useful in addition to refracted wave experiments postdating

reflection profiling, as refracted waves appear at lower frequencies than reflected waves. Yet, separate recording of refracted and reflected waves requires more logistic facilities.

Therefore, it is reasonable to try simultaneous recording at about 10 Hz and progressively increase the frequency of P waves in data processing.

The optimum solution, however, can be achieved in the course of experiments.

Recommendations

Proper site selection of experiments is especially important

The site should have satisfactory surface conditions and include a number of deep exploratory wells to provide a consistent idea of the deep structure.

Good reflection coverage of the prospect is desirable.

First experiments should be undertaken in a summer season to use inhole shots at depths of 20 or 30 m.

Information on noncompressional waves is very important, but at the initial stage the main focus should be on compressional and converted waves, including wide-angle reflections from sediments.

Special people training for the refracted-wave methods is essential, as most engineers and technicians have been trained lately for reflection profiling exploration

THANK YOU .......