**3.2 Seismic volcanostratigraphy**

*Updates in Volcanology – Transdisciplinary Nature of Volcano Science*

dimensions of >102

–104

velocity, and the amount of degradation and compaction of the buried igneous rocks have to be addressed [25]. The height of buried volcanoes is initially inferred from their present-day morphology (i.e. after erosion and compaction during burial) by multiplying the transit time of seismic waves within the volcano by an estimated acoustic velocity of the volcanic interval [58]. The degree of compaction can be estimated by seismic analysis that indicates differential compaction between the volcanic and hosts rocks [59]. Erosional features such as gullies and canyons are typically visible in seismic imagery and can help to evaluate the degree of preservation of the buried volcanic structure [27]. After determining these variables, the morphology of each buried volcanic edifice is approximated as a 3D geometric shape such as a cone, or a spherical cap to roughly estimate their volume. These estimations are "best-fit" approximations which do not affect the first-order (i.e.

To make sense of this seismic morphological information, the interpreter of volcanic basins typically construct volcanostratigraphic frameworks that help to explain the succession of igneous and sedimentary events occurred during the evolution of the basin (**Figure 3**). The age of the volcanic rocks in the subsurface is commonly determined by correlating seismic isochron horizons with biostratigraphic markers and radiometric dating of rocks penetrated by nearby drillholes. This approach gives time resolution in the order of 0.1 to 5 Myr, assuming that the seismic reflections provide a proxy of timelines [48, 61]. Interpretation of the environment in which the buried volcanoes erupted can be determined by seismic

*Amplitude display of seismic reflection profiles across the Vøring volcanic rifted margin, offshore Norway (a) and the Romney volcanic field, offshore New Zealand (b). Note that the internal and external configuration of seismic reflections determines the spatial relationship of distinctive seismic units, providing information about* 

*the succession of events that have formed these units. Data courtesy of TGS (a) and NZPAM (b).*

meters) interpretations of volcanic morphologies [60].

**76**

**Figure 3.**

Seismic volcanostratigraphy is a subset of the seismic stratigraphic method developed to analyse the geological evolution and environments of emplacement of igneous extrusive rocks using seismic reflection datasets [4]. This method consists of two main steps: (1) mapping of the top and base of volcanic sequences, and (2) seismic facies analysis, including characterisation of volcanic and enclosing sedimentary seismic facies units and their volcanological interpretation (**Figure 3**).

The application of seismic volcanostratigraphy relies on the identification of changes in basin depositional trends, placing stratigraphic boundaries at the contacts between volcanic units that are genetically related [1, 29, 63]. In nonvolcanic basins, such trends represent the dispersal and accommodation of material in specific stacking patterns of progradation, retrogradation and aggradation. These depositional trends reflect oscillations of the base level that result in erosion and accumulation of sediments within the basin, which is typically controlled by the balance between variables such as tectonics, eustasy, and climate [64, 65].

Igneous activity can strongly impact the depositional trends of sedimentary basins, which requires adaption when using conventional stratigraphic concepts and nomenclature for stratigraphic interpretation of volcanic sequences (**Figure 4**). For example, the stratal trends of non-volcanic basins are typically described according to variations in the position of the shoreline through time [66]; while in volcanic

### **Figure 4.**

*Simplified representation of the main stratal patterns, volcanic architecture, and depositional settings of cone-shaped volcanoes buried in sedimentary strata. The arrows indicate the patterns of material dispersal in specific stacking patterns of progradation, retrogradation and aggradation. The geometric configuration of strata reflects the interplay between volcanic and sedimentary processes experienced during the evolution of the basin. Note that the eruptive centre is the focal point that determines the spatial relationships between proximal to ultradistal depositional settings, which can be used as a model to predict how volcanic and sedimentary lithofacies may be distributed within and around the volcano. SIS: syn-intrusive surface. PrES: pre-eruptive surface. PoES: post-eruptive surface. PoDS: post-degradational surface. PoBs: post-burial surface. See [53] for detailed information of these volcano stratigraphic surfaces.*

systems, the focal point for discussing stratal trends is the eruptive centre [8, 52]. This is because the addition of material sourced by eruptions and isostatic adjustments of the crust caused by magma emplaced in the subsurface can overprint normal basin processes such as sediment supply and the available accommodation space [67, 68]. As a consequence, igneous activity can have a major control on the basis stratal trends, possibly impacting the architecture and evolution of the basin over thousands of square kilometres and for millions of years (**Figures 3** and **5**).

Volcanic activity often causes sudden changes in basin stratal patterns, which make it relatively straight-forward to identify the large-scale unconformities that mark the boundaries of entire volcanic sequences [45]. A typical volcanic sequence initiates with a progradational or aggradational trend marked by truncations and downlaps onto the pre-eruptive surface, and it ends with a retrogradation trend visible by onlap terminations on the top of the post-eruptive surface [4, 52]. Internal unconformities and trends within the volcanic sequence are more subtle than large regional unconformities, and may only be identified in high-quality 3D datasets (**Figure 2**). The identification of volcanic stratal patterns can be complicated due to rapid and in some cases cyclical switches from constructional to degradational stages of polygenetic volcanoes, making it challenging to map the lateral extension of volcanic unconformities [69, 70].

In some circumstances, the reduced seismic quality below thick volcanic sequences can difficult the identification of the pre-eruptive surface [35]. Similarly, the post-eruptive surface is not always marked by onlap of overlying strata onto a volcanic structure, which depends on the interplay between the rate of material sourced by eruptions versus the rate at which the volcano has been buried by sediments sourced from other parts of the basin [52]. In other words, onlap onto an active volcanic edifice can occur if the rate of burial overcomes the rate and volume of erupted material, which may be expected during the later stages of long-lived volcanoes, especially if the eruptions do not form layers thick enough to be resolved in seismic data (**Figures 4** and **5**). Additional stratigraphic markers such as the synintrusive, post-degradational and post-burial surfaces help to constrain the impacts of igneous activity in the host basin into a spatio-temporal framework [53].

### **3.3 Igneous seismic facies units**

Buried volcanic systems often show distinctive seismic facies units that result from the interaction of igneous activity and its surrounding sedimentary host rocks and

### **Figure 5.**

*Processes that control the stratigraphic signature and architecture of sedimentary basins impacted by igneous activity. The interplay of competing autogenic (i.e. from within the system) and allogenic (i.e. from outside of the system) mechanisms defines the depositional trends of volcanic basins. Adapted from [66].*

**79**

**Figure 6.**

*Seismic Geomorphology, Architecture and Stratigraphy of Volcanoes Buried in Sedimentary Basins*

environments. Seismic facies analysis consists of mapping of 3D units and 2D profiles whose seismic parameters differ from those of adjacent units [71], followed by a volcanological interpretation of the mapped seismic facies units [3]. Discrete seismic reflection packages often correspond to depositional units that are genetically related and bounded by seismic discontinuities (**Figure 2**). Variations in igneous seismic facies represent changes in the volcanic processes and environments that enclose the buried volcanoes (**Figure 6**). These seismic facies units can be interpreted in terms of volcanic eruptions, magma emplacement mechanisms, and sedimentation patterns

Seismic attribute analysis such as coherency, amplitude, frequency, and attenuation (or a combination of these) can be used to enhance the contrasts between variations in the physical properties of the buried igneous rocks units and their

*(a) Amplitude display of a seismic reflection profile across Vulcan composite volcano, offshore Deepwater Taranaki Basin, New Zealand, illustrating a variety of intrusive, extrusive and sedimentary seismic facies. The age and lithofacies and their correspondent seismic facies are calibrated with information from the Romney-1 petroleum exploration well, located 50 km north of Vulcan volcano. Approximate ages of the chronostratigraphic surfaces are shown in the back circles. Note how igneous and limestone rocks tend to form the highest amplitude events in this cross-section. The low reflectivity seismic facies below the volcanic edifice are often present in subvolcanic zones. (b) Pseudo-relief and amplitude displays (c and d) seismic profiles across Vulcan volcano. These seismic attributes highlight the differences between igneous and sedimentary rocks. The increase in the frequency of the seismic signal (10–30 Hz) highlights the internal structure of the volcano. (e) Spectral-decomposition display of a seismic reflection profile across Vulcan volcano illustrating the idealised facies architecture of large polygenetic volcanoes. The schematic facies diagram is adapted from [67].* 

*X- corresponds to the average diameter and height of composite volcanoes, based on [72].*

developed during the evolution of the host sedimentary basin [4, 22, 73].

*DOI: http://dx.doi.org/10.5772/intechopen.95282*

*Seismic Geomorphology, Architecture and Stratigraphy of Volcanoes Buried in Sedimentary Basins DOI: http://dx.doi.org/10.5772/intechopen.95282*

environments. Seismic facies analysis consists of mapping of 3D units and 2D profiles whose seismic parameters differ from those of adjacent units [71], followed by a volcanological interpretation of the mapped seismic facies units [3]. Discrete seismic reflection packages often correspond to depositional units that are genetically related and bounded by seismic discontinuities (**Figure 2**). Variations in igneous seismic facies represent changes in the volcanic processes and environments that enclose the buried volcanoes (**Figure 6**). These seismic facies units can be interpreted in terms of volcanic eruptions, magma emplacement mechanisms, and sedimentation patterns developed during the evolution of the host sedimentary basin [4, 22, 73].

Seismic attribute analysis such as coherency, amplitude, frequency, and attenuation (or a combination of these) can be used to enhance the contrasts between variations in the physical properties of the buried igneous rocks units and their

### **Figure 6.**

*Updates in Volcanology – Transdisciplinary Nature of Volcano Science*

square kilometres and for millions of years (**Figures 3** and **5**).

of volcanic unconformities [69, 70].

**3.3 Igneous seismic facies units**

systems, the focal point for discussing stratal trends is the eruptive centre [8, 52]. This is because the addition of material sourced by eruptions and isostatic adjustments of the crust caused by magma emplaced in the subsurface can overprint normal basin processes such as sediment supply and the available accommodation space [67, 68]. As a consequence, igneous activity can have a major control on the basis stratal trends, possibly impacting the architecture and evolution of the basin over thousands of

Volcanic activity often causes sudden changes in basin stratal patterns, which make it relatively straight-forward to identify the large-scale unconformities that mark the boundaries of entire volcanic sequences [45]. A typical volcanic sequence initiates with a progradational or aggradational trend marked by truncations and downlaps onto the pre-eruptive surface, and it ends with a retrogradation trend visible by onlap terminations on the top of the post-eruptive surface [4, 52]. Internal unconformities and trends within the volcanic sequence are more subtle than large regional unconformities, and may only be identified in high-quality 3D datasets (**Figure 2**). The identification of volcanic stratal patterns can be complicated due to rapid and in some cases cyclical switches from constructional to degradational stages of polygenetic volcanoes, making it challenging to map the lateral extension

In some circumstances, the reduced seismic quality below thick volcanic sequences can difficult the identification of the pre-eruptive surface [35]. Similarly, the post-eruptive surface is not always marked by onlap of overlying strata onto a volcanic structure, which depends on the interplay between the rate of material sourced by eruptions versus the rate at which the volcano has been buried by sediments sourced from other parts of the basin [52]. In other words, onlap onto an active volcanic edifice can occur if the rate of burial overcomes the rate and volume of erupted material, which may be expected during the later stages of long-lived volcanoes, especially if the eruptions do not form layers thick enough to be resolved in seismic data (**Figures 4** and **5**). Additional stratigraphic markers such as the synintrusive, post-degradational and post-burial surfaces help to constrain the impacts

of igneous activity in the host basin into a spatio-temporal framework [53].

Buried volcanic systems often show distinctive seismic facies units that result from the interaction of igneous activity and its surrounding sedimentary host rocks and

*Processes that control the stratigraphic signature and architecture of sedimentary basins impacted by igneous activity. The interplay of competing autogenic (i.e. from within the system) and allogenic (i.e. from outside of* 

*the system) mechanisms defines the depositional trends of volcanic basins. Adapted from [66].*

**78**

**Figure 5.**

*(a) Amplitude display of a seismic reflection profile across Vulcan composite volcano, offshore Deepwater Taranaki Basin, New Zealand, illustrating a variety of intrusive, extrusive and sedimentary seismic facies. The age and lithofacies and their correspondent seismic facies are calibrated with information from the Romney-1 petroleum exploration well, located 50 km north of Vulcan volcano. Approximate ages of the chronostratigraphic surfaces are shown in the back circles. Note how igneous and limestone rocks tend to form the highest amplitude events in this cross-section. The low reflectivity seismic facies below the volcanic edifice are often present in subvolcanic zones. (b) Pseudo-relief and amplitude displays (c and d) seismic profiles across Vulcan volcano. These seismic attributes highlight the differences between igneous and sedimentary rocks. The increase in the frequency of the seismic signal (10–30 Hz) highlights the internal structure of the volcano. (e) Spectral-decomposition display of a seismic reflection profile across Vulcan volcano illustrating the idealised facies architecture of large polygenetic volcanoes. The schematic facies diagram is adapted from [67]. X- corresponds to the average diameter and height of composite volcanoes, based on [72].*

enclosing sedimentary strata (**Figure 7**) [30]. More recently, the use of machine learning techniques and artificial neural networks have been applied to delineate igneous seismic facies [74]. Description of igneous seismic units can be used to interpret volcanic landforms and different parts of volcanic systems. For example, cone-type volcanoes such as cinder cones and stratovolcanoes typically display a pair of inward- and outward-dipping reflections that mark the location of a central crater and peripheral flanks. Optimal characterisation of buried volcanoes can be obtained by analysing the igneous seismic facies as part of a genetically related network in different scales of observation, which consist in mapping intrusive and extrusive igneous seismic units into a unified interpretation framework [29, 52, 73].
