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2.1 Introduction

This chapter presents the seismic reflection data used in this study, including the principles behind the acquisition, processing and interpretation of seismic-reflection data, and is followed by a brief introduction of the seismic dataset used in this thesis.

2.2 Seismic-data acquisition

Marine seismic data are acquired by creating an acoustic pulse using airgun arrays on ships (Fig. 2.1). The sound wave travels down through the sea water into subsurface rocks. Some of the sound waves are reflected back to the sea surface whenever interfaces with changes in physical properties are found (e.g. density and velocity changes). The reflected waves are then received and recorded by a series of hydrophones on the ships (Fig. 2.1). The time seismic wave travel from the source (i.e. airguns) to the receivers (i.e. geophones) is measured in two-way travel time (TWTT).

2.3 Seismic waves

Despite many types of waves that travel through the Earth, most seismic datasets use compressional waves or P-waves. The key variables defining the shape of a wave are amplitude, period, frequency and wavelength (Fig. 2.2). The wave display changes in amplitude, which can be represented by a maximum (i.e. peak) and a minimum value (i.e. trough) (Fig. 2.2). The separation (in time) between two successive peaks or troughs is the wave period (𝑇) (Fig. 2.2). The frequency of a wave is the number of cycles, i.e. peaks or troughs that pass through a specific point in a specific period. For example, 40 Hz indicates that 40 peaks or troughs pass through a specific point in one second. The wave frequency can also be shown as the inverse of the period:

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Figure 2.1. Schematic diagram showing the common geometry of a marine seismic survey. An acoustic source mounted on a boat emits a sound wave into the water. When the acoustic properties of the rock change, the P-waves are reflected back to the surface and detected by hydrophones in a long cable towed behind the boat (streamer) (modified from Bacon et al. 2003).

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Figure 2.2. A simple cosine geometry illustrating traveling waves in distance A) and time B) (from Hart, 2011).

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The wavelength is the distance between successive repetitions of the waveform (e.g., troughs or peaks) (Fig. 2.2). It is a function of the frequency (𝑓) and velocity (𝑣) in which waves travel through rocks:

𝜆= 𝑣/𝑓 ---(Eq. 2.2)

However, as seismic waves commonly contain a range of frequencies, the dominant frequency is generally used to calculate wavelength.

The energy of the P-waves is reflected at interfaces where changes in acoustic impedance are recorded (𝑍). Acoustic impedance is a function of the rocks’ P-wave velocity (𝑣) and density (𝜌):

𝑍 = 𝑣 x 𝜌 ---(Eq. 2.3)

In this equation, velocity and density are associated with rock physical properties such as texture, porosity and fluid content.

The relative amplitude of the reflected wave can be predicted by the zero-offset reflection coefficient (𝑅0):

𝑅0= 𝑍2−𝑍1

𝑍2+𝑍1 ---(Eq. 2.4)

where 𝑍1 and 𝑍2 are the acoustic impedances of the layer above and below the interface,

respectively.

2.4 Data resolution

The vertical resolution of seismic data is defined as the vertical distance between two seismic reflections that can be resolved separately, and this separation relates to the distance between two interfaces (Hart, 2011). A value of 𝜆/4 is generally considered to define the vertical resolution of seismic data (Fig. 2.3). Therefore, the higher the frequency and lower the velocity of the wave, the

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Figure 2.3. The relationship between bed thickness and tuning thickness. The tuning thickness is the bed thickness that two reflections become indistinguishable in time. The vertical resolution is related to the interaction between two closely spaced wavelets (from Brown, 2004).

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better will be the vertical resolution.

However, as velocity increases and frequency decreases with depth, vertical resolution decreases. In addition, constructive interference or tuning occurs when the interfaces of layers spaced more closely than 𝜆/4, leading to the overlap of reflections and boosted seismic signals (Fig. 2.3).

Lateral resolution of seismic data is associated with the Fresnel zone (Fig. 2.4). Because the seismic energy travels as wave fronts, the region where the seismic energy is reflected constructively is known as the Fresnel zone. The width of an object equal or greater than the diameter of Fresnel Zone can be resolved in seismic data. The diameter of Fresnel zone depends on the average velocity down to a specific horizon (𝑣), two-way travel time (𝑡) and frequency (𝑓) (Hart, 2011):

𝐷𝐹= 𝑣√ 𝑡

𝑓 ---(Eq. 2.5)

2.5 Seismic-data visualisation

Three-dimensional (3D) seismic volumes can be considered as a series of data cubes, or voxels (Hart, 2000). Each voxel consists of four pieces of information: the x, y, and z showing the location and an amplitude value. The x-y dimension is associated with bin space during data acquisition and is represented by inlines and crosslines during interpretation, whereas the z dimension reflects the location in the depth direction (Hart, 2000). Arbitrary lines comprise a single transect or multiple transects extracted from the data set, in order to image structural or stratigraphic features oriented obliquely to the inline or crossline. Apart from seismic cross sections, 3D seismic data can also provide plan views of features by using time slices, and flattened time slices, allowing the detailed imaging of geological features.

For example, seismic attributes are extensively used in the seismic interpretation of stratigraphy, structural geology, and reservoir properties. Submarine channels can be clearly imaged on attribute maps because of their linear and sinuous aspects. In this thesis, dip, RMS amplitude and variance maps are used to document the variations in pathways of submarine channels.

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Dip maps show gradient changes by comparing the time or depth value between adjacent points in 3D seismic data. It is a good indicator of slope changes in a reference horizon. The banks of submarine channel are highlighted in dip maps because they are generally steeper than the dip of adjacent strata (Fig. 2.5).

Amplitude maps highlight lateral variations in acoustic impedance that are related to changes in the rock physical properties and fluid content of the horizon (Brown, 2004). Root-Mean Square (RMS) amplitude maps highlight high-amplitude reflections on seismic data in comparison with low- amplitude ‘background’ strata. They can be extracted along a specific horizon or within a defined time window. Because the basal lags of submarine channels are commonly coarse-grained and generate high-amplitude reflections, submarine channels can be clearly identified on amplitude maps due to their high amplitude, sinuous reflections (Fig. 2.6).

Variance maps are calculated based on discontinuities by comparing how similar are adjacent waveforms in a picked horizon, in contrast with coherence map that calculates continuities of seismic data. They are generally used for imaging lateral extent of geological features. The discontinuities shown in the variance map may be stratigraphic (e.g. channels) or fault discontinuities. For example, channel banks are erosional truncations on seismic data, they can be well defined on variance maps based on the higher variance value of channel banks (Fig. 2.7). The variance map is generally extracted from a variance volume that computed based on a reference horizon. This reference horizon can be a time slice from seismic volume or a flattened interpreted horizon.

Isochron maps highlight the variations in thickness (in TWTT) between two reference horizons. They provide detailed information on the thickness and lateral extent of sedimentary units. For example, isochron maps were used to quantify variations in sediment dispersal patterns in the studied channel systems, and to assess the role of sediment supply from tributaries in the sediment distribution within the channel system.

2.6 Seismic-data interpretation

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Figure 2.6.Root-Mean Square (RMS) amplitude map highlighting submarine channels with higher- amplitude reflections compared to lower-amplitude slope strata. High-amplitude reflections are also shown surrounding salt diapirs.

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picking and/or autotracking of relevant reflections, combined with an analysis of seismic facies based on the geometry, continuity and amplitude of reflections. The main principles of such an analysis follow concepts in Mitchum et al. (1977) (Fig. 2.8).

Four types of seismic facies associated with submarine channels were interpreted based on their seismic amplitude, reflection continuity, reflection geometry, and termination patterns against adjacent strata (Fig. 2.9). The interpretation of seismic facies was based on widely accepted criteria for deep-water settings (e.g. Posamentier and Kolla, 2003; Mayall et al., 2006; Catterall et al., 2010; Deptuck et al., 2003; Janocko et al., 2013).

Seismic Facies 1 is characterised by high-amplitude, continuous to discontinuous seismic reflections (Fig. 2.9A), and is usually confined within V- or U-shaped erosional surfaces (Fig. 2.9B– E). This facies is interpreted as comprising the basal lags of submarine channels (e.g. Deptuck et al., 2003; Mayall et al., 2006; Catterall et al., 2010). It corresponds to coarse-grained sediments accumulated at the base of submarine channels (e.g. McHargue and Webb, 1986; Mayall et al., 2006).

Seismic Facies 2 consists of low- to high-amplitude, subparallel and continuous seismic reflections (Fig. 2.9A). This facies may have different origins: channel deposits accumulated in a channel-abandonment stage (e.g. Deptuck et al., 2003; Catterall et al., 2010; Janocko et al., 2013), or overbank deposits, some of which show a wedge-shaped geometry (Fig. 2.9B–E). These latter are interpreted as levee deposits (e.g. Posamentier and Kolla, 2003; Deptuck et al., 2003; Catterall et al., 2010; Janocko et al., 2013).

Seismic Facies 3 is composed of variable-amplitude, chaotic, discontinuous seismic reflections (Fig. 2.9A). This facies represents mass-transport deposits (MTDs) and is attributed to processes such as slides, slumps and debris flows (e.g. Posamentier and Kolla, 2003; Mayall et al., 2006; Janocko et al., 2013). Blocks with high-amplitude, parallel reflections are observed in the MTDs (Figs. 2.9B and C).

Seismic Facies 4 is composed of low-amplitude, parallel, continuous seismic reflections interpreted as background hemipelagic sediment (Fig. 2.9).

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Figure 2.8. Stratal relationships within depositional sequences (from Mitchum, 1977). Erosional truncation highlights strata along a stratigraphic unconformity. Toplap highlights strata against an overlying surface. Top concordance represents strata that do not terminate against an upper boundary. Onlap shows horizontal strata terminate against an inclined surface. Downlap indicates strata terminate against an inclined surface. Base concordance shows strata do not terminate against a lower boundary.

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Figure 2.9. A) Description and interpretation of seismic facies observed in the channel system. B, C, D, E) Uninterpreted and interpreted seismic sections highlighting the seismic facies interpreted in the channel system.

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2.7 3D seismic volume used in the thesis

The interpreted 3D seismic volume is located in the northern part of the Espírito Santo Basin (Fig. 2.10). It covers an area of 1600 km2.

The seismic data were acquired by a dual airgun array and a 6 x 5700 m array of streamers. The survey has a bin spacing of 12.5 m by 12.5 m and a 2 ms vertical sampling interval, later resampled at 4 ms together with the application of an anti-aliasing filter. Data processing included resampling, spherical divergence corrections, and zero-phase conversions undertaken prior to stacking, 3D prestack time migration using the Stolt algorithm, and one-pass 3D migration. The polarity of data is SEG normal i.e., positive amplitude reflections (red) on the seismic profiles represent an increase in acoustic impedance.

The vertical resolution of the data is ~10 m at the depth of analysis in this study, based on a dominant frequency of 40 Hz and a P-wave velocity of 1600 m/s for near-seafloor strata.

Petrel provided by Schlumberger is the software used for interpretation of seismic data in this thesis.

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Figure 2.10. Regional map of the SE Brazilian margin (Bathymetric data from GeoMapApp, http://www.geomapapp.org; Amante and Eakins, 2009), showing the location of Espírito Santo Basin and the studied 3D seismic volume in the Espírito Santo Basin.

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Chapter 3

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