**3. Methods and concepts of seismic-reflection volcanology**

Interpretation of buried volcanic systems requires a multidisciplinary approach that combines insights from complementary disciplines such as sedimentology, stratigraphy, structural geology, and volcanology into a unified framework. During the last 40 years, our knowledge about the formation and evolution of sedimentary basins has improved mainly due to advances in the fields of seismic and sequence stratigraphy [46–48]. More recently, these stratigraphic approaches have been successfully applied to interpret the processes and products of igneous activity within sedimentary basins [1, 4].

Seismic-reflection volcanology is here defined as the study of buried volcanoes from seismic reflection datasets. This method is typically applied to investigate the nature and evolution of volcanic and igneous plumbing systems buried in sedimentary strata. Sedimentary basins that contain a significant amount of igneous rocks are informally referred to as "volcanic basins" [49–51]. The interpretation of volcanic basins usually begins by mapping the top and base of seismic units (sequences) that are potentially of volcanic origin using 2D regional lines. Mappable seismic facies units are then identified by their distinct aspects in, for example, reflection configuration, continuity, geometry, and interval velocity. A volcanological interpretation is then performed to determine the igneous facies and their intrusive and extrusive enclosing environments. If available, 3D datasets are subsequently interpreted to provide detailed images of the past volcanic surfaces and landforms now buried in the host basin, which is further analysed using the method of igneous seismic geomorphology [29] and volcanic architectural elements [52, 53]. Finally, a more accurate volcanological characterisation of buried igneous rocks can be achieved by correlating the seismic units with data from drillholes and outcrop analogues [26].

The methods used to characterise volcanic basins vary between interpreters and are dependent on the available dataset, scale, and purpose of the study. The following sections summarise these methods focusing on the interpretation of the spatiotemporal expression of buried volcanoes and reconstruction of the scenarios in which volcanic events occurred synchronously with basin sedimentation and erosion.

### **3.1 Reconstructing the geomorphic aspects, eruptive time, and environment of emplacement of buried volcanic systems**

Magma that reaches the Earth's surface can produce a variety of subaerial and subaqueous volcanic landforms. This diversity of volcanic landforms reflects a range of physical factors such as magma composition, discharge rate of effusion, degree of material fragmentation and dispersion, and tectonic and environment settings, in particular, the presence or absence of water where the eruptions occurred [54–57]. In detail, the volcanic landforms are likely the product of many competing processes such as steady versus dynamic mechanisms of fragmentation, fixed versus variable location of the eruptive centre, and single versus multiple eruption phases. Multiple variables can complicate the interpretation of the processes that shaped the geomorphic aspects of volcanoes [6], which is especially true for the characterisation of volcanoes buried in sedimentary strata. In addition to volcanic complexity and limitations of subsurface interpretation, the morphology of buried volcanoes is likely influenced by superimposed post-eruptive processes such as erosion, alteration, compaction, and faulting.

To understand the geological processes that shaped ancient volcanic landforms now buried in sedimentary strata, critical parameters such as the interval acoustic

*Updates in Volcanology – Transdisciplinary Nature of Volcano Science*

by metasomatism and weathering, cementation, compaction during progressive burial, substitution of interstitial pore fluids, and fracturing can also lower the impedance contrast between igneous and sedimentary rocks [39, 40]. In addition, steeply inclined bodies such as dykes and highly heterogeneous subvolcanic zones are often poorly resolved in seismic reflection datasets. These zones can contain numerous intrusive bodies emplaced with variable geometries and spatial relation-

*(a) 2D seismic section across the flank of a polygenetic volcano buried offshore Canterbury Basin, New Zealand. The highest-amplitude seismic reflector in this image marks the interface between the top of the volcanic structure and its overlying sedimentary rocks. (b) Cross-section across an outcropping sequence of lava flows of the Mangahouhounui Fm, Tongariro compound volcano, New Zealand, exposed by erosion. Note that in both seismic and outcropping examples, the relationship between the strata defines a succession of volcanic events bounded by unconformities, across which younger rocks are deposited at the* 

In light of these limitations, seismic interpretation of buried volcanoes can benefit from a fully integrated approach that includes information from drillhole data analysis and insights from modern volcano analogues [42, 43]. In recent years, particular attention has been given to the interpretation of 3D seismic volumes from which cross-sections can be displayed in any given orientation, allowing the visualisation of complex volcanic forms in great detail [44, 45]. This new integrated seismic method, from 2D regional scale to detailed 3D analysis and correlation with drillhole data and analogues, can provide robust interpretations of volcanoes buried

ships to their host strata, leading to loss of reflection coherency [41].

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**Figure 2.**

*top of the sequence.*

within sedimentary basins.

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. dimensions of >102 –104 meters) interpretations of volcanic morphologies [60].

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

### **Figure 3.**

*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).*

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**Figure 4.**

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

stratigraphic analysis calibrated with paleoenvironmental data obtained from microfossils from drillholes across the studied areas or correlative outcrops [62]. As standard procedure in the analysis of seismic datasets, 2D sections and 3D perspective views are often displayed with vertical exaggeration to enhance the stratal relationship of seismic reflections, which modify the visual geometric aspect of the

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

*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.*

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

buried volcanic landforms.

**3.2 Seismic volcanostratigraphy**

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

stratigraphic analysis calibrated with paleoenvironmental data obtained from microfossils from drillholes across the studied areas or correlative outcrops [62]. As standard procedure in the analysis of seismic datasets, 2D sections and 3D perspective views are often displayed with vertical exaggeration to enhance the stratal relationship of seismic reflections, which modify the visual geometric aspect of the buried volcanic landforms.
