**2. Principles of seismic interpretation of igneous rocks**

The seismic reflection method is a geophysical technique designed to observe the Earth's subsurface indirectly. This method is based on the recording of artificially generated seismic waves that travel into the Earth's geological formations. At the interface of rock bodies with different physical properties, the waves reflect and refract, producing seismic events with wave amplitudes proportional to the contrast in density and velocity of the rocks that bound the interface [30, 31]. Motion- or pressure-sensitive geophones and hydrophones receivers capture the reflected wavefield from the seismic source. A systematic arrangement of the seismic sources and receivers enables the construction of cross-sections that display images of the Earth's subsurface, with better quality at depths of <10 km [32].

Igneous rocks buried in sedimentary basins are often identified by the presence of anomalously high-amplitude reflections within seismic datasets (**Figures 1** and **2**). Characteristically, dense lavas of basaltic composition and mafic intrusions have compressional (P-wave) velocities >5000 ms−1, contrasting with softer sedimentary rocks which commonly have velocities <3000 ms−1 [4, 33, 34]. Despite the straight-forward concept underpinning the identification of igneous rocks based on their high-amplitude reflections, seismic techniques have limitations which leads to uncertainties in the interpretations. Such interpretations are dependent on the quality and resolution of seismic data, which are controlled by geophysical parameters such as wavefield scattering due to changes in rock densities and strata geometries, increasing energy attenuation with depth, and the size of the igneous bodies relative to the wavelength of the seismic signal [35, 36].

Distinguishing buried volcanoes from sedimentary strata can be problematic when the igneous rocks have similar physical properties and geometries as the enclosing host rocks. For example, it may be challenging to differentiate volcanoes from carbonate mounds, or sequences of bedded volcaniclastic and siliciclastic rocks [37, 38]. Secondary alteration processes including mineral changes induced

*Updates in Volcanology – Transdisciplinary Nature of Volcano Science*

sedimentary basins globally (e.g. [10–17]).

and degradation of diverse volcanic landforms [22–24].

Volcanic morphologies provide information about the primary and secondary processes that formed them [5–7], and can be used as analogues for understanding buried igneous systems [8, 9]. Our ability to identify buried volcanoes and igneous intrusions emplaced in the shallow (<10 km) layers of the crust has developed immensely over the past four decades in parallel with improvements in the quality and quantity of seismic reflection data. Today, interpretation of seismic reflection datasets indicates that buried volcanoes are characteristic elements of many

Seismic interpretation of buried volcanoes benefits from innovations made in the field of sedimentology, in which seismic datasets have been used to analyse in detail the architecture and stratigraphic signature of terrestrial and marine sedimentary systems [18–20]. As noted in [21] *"since the 1960s', attempts to make sense of the diversity of rocks, processes, stratigraphic models and deposition settings of volcanic successions have been aided by major advances in the field of sedimentology"*. Now, seismic reflection data offer unique opportunities to investigate the role of intrusions, host rocks, crustal structures, and relative sea-level variations in the construction

Modern seismic reflection datasets allow us to observe the entire architecture of volcanic systems, from the intrusive to the extrusive realms, with resolutions down

*Seismic reflection visualisation of a small cone-shaped volcano buried in the Taranaki Basin, New Zealand. (a) Shows an amplitude display of a seismic reflection profile across the volcano, coupled with time-slice RMS amplitude display of its plumbing system. (b) 3D opacity-rendered perspective view of the volcano shown in (a) and its shallow (<200 m) plumbing system, in which the low-amplitudes are set as transparent. Note the spatial relationship between the saucer-shaped intrusion and the central vent of the volcano. PrES is the* 

**72**

**Figure 1.**

*pre-eruptive surface and PoES is the post-eruptive surface.*

### **Figure 2.**

*(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 top of the sequence.*

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

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 within sedimentary basins.

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

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

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

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** 

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, altera-

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

**of emplacement of buried volcanic systems**

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

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

sedimentary basins [1, 4].

outcrop analogues [26].

tion, compaction, and faulting.

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