30 | Halliburton Landmark
By: Owen Sutcliffe
Seismic analysis is the main tool for defining prospects for drilling, and for assessing the
presence of hydrocarbons. Asset models are commonly carried through to production and
are delivered as geological frameworks that illuminate the value of the asset when populated
with rock property information (Figure 1). Understanding the uncertainties that surround these
models is an important part of any asset evaluation. Gathering, processing, and interpreting of
seismic data is computationally and operationally complex, and only a broad overview can be
provided here. This article will also only deal with seismic reflection methods.
APPLICATIONS
The analysis of seismic data remains the main tool for subsurface evaluation. It provides:
» Models of the tectonostratigraphic evolution of a basin
b)
c)
» Input into models revealing the depth variation of petroleum system elements
» Models of structural and stratigraphic geometries capable of revealing prospects for
drilling
» An assessment of trap-density
» Rock properties models that reveal the value of an asset, when integrated with well data
THE SEISMIC METHOD
The propagation of sound (seismic) waves in the Earth provides a way to explore the
subsurface. The seismic reflection method works by generating sound waves artificially,
bouncing them off lithological interfaces, and recording the time taken to reach an array of
receivers (e.g. McQuillan et al., 1984; Ashcroft, 2011) (Figure 2A).
d)
The reflection and refraction of seismic waves reflects lithological properties (Figure 2B), since
the speed of propagation is governed by acoustic impedance (a product of density). Once a
seismic wave strikes an interface, only a limited amount of energy is reflected back to a receiver.
The rest is refracted to deeper levels that may also bounce off a deeper interface (Figure 2B).
Therefore, seismic waves are recorded at receivers as a series of overlapping pulses (Figure
2C). As waves pass deeper, they lose energy, causing the resolution of seismic data to decay
(McQuillan et al., 1984).
SEISMIC SURVEYS
Seismic surveys extend over a number of kilometers, with receivers typically spaced every 20
m. A source of sound is activated and the arrival times and amplitudes of the derived waves
are recorded at the receivers (Figure 2C). This process is repeated along the array (Figure 2D).
During acquisition, every effort is made to maximize the signal to noise ratio of the data. On
land, the seismic source includes dynamite or vibroseis (a truck-mounted vibrator), while air
guns are used in water (e.g. McQuillan et al., 1984). Land-based receivers are referred to as
geophones and in the water hydrophones. Seismic surveys are normally oriented in dip and
strike directions relative to structural grain.
2D surveys provide tied grids of data. These are most commonly shot in the earliest stages of
exploration. 3D surveys are representations of densely spaced 2D surveys, providing cross-
Figure 1> Example of a framework model using publically available data from the Volve Field, Norway. A) A sealed framework model;
B) Structural grid of the top of the Hugin Formation; C) Isopach of the Hugin Formation; and D) 2D cross section showing structural
horizons, faults, and seismic data.
sectional views along any azimuth, as well as horizontal or
strata-parallel sections. 3D seismic is normally shot once
prospects are selected and a more refined model of the asset
needed. 4D seismic data (collected over a period of time) also
exists, but is not used in exploration.
a)
Seismic Data: Interpretation and
Analysis in Exploration
Exploration Handbook | 31