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Coaly facies (coal, shaly coal and coaly mudstone) are proven petroleum source rocks in many sedimentary basins in New Zealand, Australia and Southeast Asia. Often these source rocks are mapped in the subsurface based on sparse well data and seismic amplitude character and their organic richness is estimated using average TOC (total organic content) values from well data. However, as the lateral distribution and organic richness of coaly facies are highly heterogeneous and the seismic amplitude response of facies is non-unique, delineation of coaly facies and TOC estimation away from wells are highly uncertain. To reduce this uncertainty, we characterised coaly facies in Cretaceous–Eocene intervals in Taranaki and Great South Basins using density, P-wave velocity, P-impedance and TOC data from 13 wells, supplemented with rock physics and TOC data of numerous coal samples collected from mines and outcrops around New Zealand. We then carried out a case study from the Paleocene–Eocene interval in the Maari 3D seismic survey area in offshore Taranaki Basin in which we develop a P-impedance model and delineate the coaly facies away from wells based on P-impedance character. Average P-impedance maps were analysed together with seismic amplitude character, structure maps and depositional environment maps to understand lateral and temporal variation of coaly facies. These maps were then converted to proxy-TOC maps using a relationship developed from the outcrop coal samples. The coaly facies show density <2.5 g/cc, P-wave velocity <4000 m/s, P-impedance <9000 m/s × g/cc and varied TOC character in the Cretaceous–Eocene interval.

In geothermal environments, the physical properties of rocks, such as porosity and alteration, are highly variable and control the reservoir's elastic and hydraulic properties. Elastic wave velocity in volcaniclastic rocks and their relation to fractures, pore shapes, and mineral alteration is mostly unknown. We measure ultrasonic Pand Swave speeds on volcanic rocks from the Ngatamariki Geothermal Reservoir, New Zealand. Data clustering of wave speed versus porosity and density allow us to classify lithotypes of variable propylitic and phyllic mineral alteration. Wave speeds increase first due to porosity reduction as a result of mineral alteration and welding and, second, due to the high elasticity of alteration minerals (epidote, chlorite, and carbonates). We model the rock porosity as composed of equant pores and oblate‐spheroidal microfractures following an elastic effective medium model. For our samples, microfracture porosities range from 4% in the volcaniclastic tuffs to less than 1% in the ignimbrites, which we validate with pycnometer porosity data under effective pressure. We quantify that within one geological volcaniclastic formation, wave speeds vary up to 47% due to rock alteration and welding, while the effect of microfractures and changes in effective stress on wave speeds is secondary (up to 11%) but not negligible. From the experimental and numerical results we show that equant pores remain open at reservoir conditions (2,000 m) and can retain considerable porosity (10%). Our analysis has implications for microfracture and pore network characterization in geothermal reservoirs, in particular, in vapor‐dominated systems where matrix porosity and permeability play a role. For our volcaniclastic rocks, propylitic and phyllic alteration as well as welding decrease porosity and increase elastic wave speeds; there is no correlation between rock alteration and sample depthMicrofracture porosity is quantitatively estimated from elastic wave effective media models and validated with experimental dataRock alteration and welding in volcaniclastic rocks of similar origin have a larger effect on seismic wave speeds than microfracture porosity alone

One of the leading hydrothermal alteration processes in volcanic environments is when rock‐forming minerals with high concentrations of iron, magnesium, and calcium react with CO2and water to form carbonate minerals. This is used to the advantage of geologic sequestration of anthropogenic CO2. Here we experimentally investigate how mineral carbonation processes alter the rock microstructure due to CO2‐water‐rock interactions. In order to characterize these changes, CO2‐water‐rock alteration in Auckland Volcanic Field young basalts (less than 0.3 Ma) is studied before and after a 140 day reaction period. We investigate how whole core basalts with similar geochemistry but different porosity, permeability, pore geometry, and volcanic glass content alter due to CO2‐water‐rock reactions. Ankerite and aluminosilicate minerals precipitate as secondary phases in the pore space. However, rock dissolution mechanisms are found to dominate this secondary mineral precipitation resulting in an increase in porosity and decrease in rigidity of all samples. The basalt with the highest initial porosity and volcanic glass volume shows the most secondary mineral precipitation. At the same time, this sample exhibits the greatest increase in porosity and permeability, and a decrease in rock rigidity post reaction. For the measured samples, we observe a correlation between volcanic glass volume and rock porosity increase due to rock‐fluid reactions. We believe this study can help understand the dynamic rock‐fluid interactions when monitoring field scale CO2sequestration projects in basalts. Basalt‐carbonic acid reactions precipitate ankerite mineralsPorosity increases and rock elasticity decreases due to these reactionsRock dissolution, linked to volcanic glass content, dominates over carbonate precipitation

Rocks commonly display a reduction in elastic modulus during an acoustic perturbation followed by a logarithmic recovery of the modulus to its original value. This nonlinear elastic behavior can also be induced by changes in temperature of the rock, although experimental data for this are sparse. In this paper, we utilize temperature perturbations to broaden our understanding of the causes and mechanisms of nonlinear elasticity in rocks. We perform laboratory experiments tracking the nonlinear response of a Berea sandstone under vacuum both during and following 10°C changes in temperature. Ultrasonic velocity decreases by up to 0.30 ± 0.02% during both temperature increases and decreases, before recovering in the following days. Continuous monitoring over a 2‐month period also reveals a longer‐term 0.78 ± 0.04% increase in velocity. To explain these observations, we modify a recent model for nonlinear elasticity based on the shearing and subsequent recreation of internal microscopic contacts. Application of this model to our data suggests that internal stresses from differential thermal expansion/contraction of mineral grains break contacts, inducing nonlinear weakening. The degree of weakening depends on the temperature gradient. Slow dynamics recovery in the ∼10 hr following a temperature change likely results from the recreation of broken contacts due to nanoscopic adhesive forces. In contrast, the long‐term velocity increase results from evaporation of bound water from clay minerals. In addition to furthering our understanding of nonlinear elasticity in rocks, our results imply that sudden changes of temperature in shallow crustal environments will induce nonlinear weakening that persists for hours to days. Rocks weaken when seismic waves travel through them, a phenomenon which may drive critically stressed rocks to failure. This weakening is not permanent, however, and the rock recovers its original strength in the following minutes, hours, or days. Such behavior can also occur when the temperature of the rock changes, although only a handful of laboratory studies investigating this behavior exist. In this study, we monitor the speed of an ultrasonic wave (a proxy for strength) through a sample of Berea sandstone during and after temperature changes of 10°C. We find that whenever the temperature increases or decreases, the wave speed decreases by up to 0.30% before recovering to the original value in the following hours. By modifying a recently developed model, we find that this behavior is caused by microscopic bonds in the rock breaking and reforming during and after a temperature change. Additionally, an overall 0.78% increase in wave speed over 2 months is caused by water molecules evaporating from clay minerals. The results of our study help us to understand why rocks show this unusual behavior, and imply that a change in temperature in the earth will weaken nearby rocks for a period of time. We monitor nonlinear reduction and recovery of ultrasonic velocity in Berea sandstone caused by 10°C changes in temperatureUltrasonic velocity drops by ∼0.3% for both temperature increases and decreases before recovering in the following daysOur results and modeling emphasize that the rate of temperature change controls the magnitude of nonlinear weakening We monitor nonlinear reduction and recovery of ultrasonic velocity in Berea sandstone caused by 10°C changes in temperature Ultrasonic velocity drops by ∼0.3% for both temperature increases and decreases before recovering in the following days Our results and modeling emphasize that the rate of temperature change controls the magnitude of nonlinear weakening

Understanding the elastic behavior of basalt is important to seismically monitor volcanoes, subsea basalts, and carbon sequestration in basalt. We estimate the elastic properties of basalt samples from the Snake River Plain, Idaho, at ultrasonic (0.8 MHz) and seismic (2–300 Hz) frequencies. To test the sensitivity of seismic waves to the fluid content in the pore structure, measurements are performed at three saturation conditions: saturated with liquid CO2, water, and dry. When CO2replaces water, the P‐wave velocity drops, on average, by 10%. Vesicles and cracks, observed in the rock microstructure, control the relaxation of pore‐fluid pressures in the rock as a wave propagates. The bulk and shear moduli of basalts saturated with liquid CO2are not frequency dependent, suggesting that fluid pore pressures are in equilibrium between 2 Hz and 0.8 MHz. However, when samples are water saturated, the bulk modulus of the rock is frequency dependent. Modeling with Gassmann's equations predicts the measured saturated rock bulk modulus for all fluids for frequencies below 20 Hz but underpredicts the water‐saturated basalt bulk modulus for frequencies greater than 20 Hz. The most likely reason is that the pore‐fluid pressures are unrelaxed. Instead, the ultrasonic frequency rock moduli are modeled with high‐frequency elastic theories of squirt flow and Kuster–Toksöz (KT). Although KT's model is based on idealized pore shapes, a combination of spheres (vesicles) and penny‐shaped cracks (fractures) interpreted and quantified from petrographical data predicts the ultrasonic dry and saturated rock moduli for the measured basalts. Monitoring the replacement of water by CO2 over time with seismic is possibleGassmann and Kuster‐Toksoz models predict measured rock elasticityCracks in water‐saturated basalt result in a frequency dependent bulk modulus

Magnetic surveys are used to identify and monitor hydrothermally altered regions on volcanoes. Commonly such magnetic data are interpreted on the premise that hydrothermal alteration consumes Fe‐Ti oxides in the host rocks, reducing their total magnetization. Here, we report a contrasting observation from Whakaari (White Island) volcano in New Zealand. We study the magnetic properties of 42 conduit‐filling and surficial lithologies that have undergone varying degrees of acid‐sulfate alteration. We find that while the induced magnetization of lavas decreases with hydrothermal alteration, some altered lavas have an order of magnitude higher remanent magnetization than fresh lavas. We discuss plausible mechanisms by which altered lavas can retain high remanent magnetization including the importance of magnetic mineralogy and grain size. Our results urge caution in correlating reduced magnetization with hydrothermally altered regions. Furthermore, they highlight the importance of measuring both the induced and remanent magnetization of samples used to interpret field‐scale data. Magnetic surveys are used to find regions in volcanoes where hot fluids alter the rocks and weaken them, locate ore bodies, and for geothermal exploration. These regions are expected to show low total magnetization on the surveys because the alteration process could consume iron‐bearing magnetic minerals in these rocks. Here, we report a different observation from our laboratory experiments on rocks from Whakaari (White Island) volcano in New Zealand. The total magnetization of rocks is the sum of magnetization from (a) minerals that temporarily become magnetized when an external magnetic field is applied (induced magnetization) and (b) minerals that stay magnetized even without an external magnetic field (remanent magnetization). We find that sometimes altered rocks, despite low induced magnetization, can have higher remanent magnetization than fresh rocks. We discuss the potential causes for this. Given the importance of magnetic surveys in volcanic slope stability assessment and mineral and geothermal exploration, we suggest exercising caution in assuming that hydrothermally altered regions would necessarily be associated with low total magnetization. Natural remanent magnetization dominates induced magnetization in volcanic rocks from WhakaariHydrothermally altered lavas can carry higher remanent magnetization than fresh lavasBoth induced and remanent magnetizations should be used to interpret field‐scale data Natural remanent magnetization dominates induced magnetization in volcanic rocks from Whakaari Hydrothermally altered lavas can carry higher remanent magnetization than fresh lavas Both induced and remanent magnetizations should be used to interpret field‐scale data

Acid‐sulphate alteration commonly changes the physicochemical properties of volcanic conduits. Although several conduit pressurization models suggest hydrothermal sealing, little experimental evidence exists from conduit‐filling rocks on the development of such a seal. Here we show that acid‐sulphate alteration affects conduit‐filling lavas and tuffs differently, with implications for their role in sealing the conduit. In lavas, alteration creates fluid pathways and decreases rock stiffness by dissolving primary minerals. In contrast, in the inherently porous tuffs, alteration reduces fluid pathways and increases rock stiffness by precipitating secondary minerals. Compaction of tuffs under subsurface pressures together with such alteration‐related sealing can form low porosity and low permeability zones within the conduit. Such zones could promote fluid‐pressure build‐up and predispose the volcano to explosive eruptions. We discuss our results in the context of observed seismicity and ground deformation and suggest using our elastic properties data to constrain geophysical inversions of acid‐sulphate altered volcanic conduits. In volcanoes, hot fluids circulate through the rocks and change their physical and chemical properties. Here we study how these changes occur in different rocks within the volcano's interior. We find that in lavas, rocks that generally have a low volume of pores, these fluids create new pathways to flow through by dissolving the rock‐forming minerals. In contrast, fluid pathways in tuffs, which are inherently highly porous rocks, are blocked by the precipitation of new minerals in the pore spaces. Compaction under the pressure of overlying rocks in the volcano's interior can further reduce the fluid pathways through tuffs. Such reduction in fluid pathways within the volcano can aid the build‐up of pressure, making them prone to explosive eruptions. In volcanic conduits, lavas undergo net dissolution and tuffs net secondary mineral precipitationThe elasticity of conduit‐filling lavas and tuffs also changes due to alterationInherently porous and permeable tuffs, when compacted and highly altered, can form seals within volcanic conduits In volcanic conduits, lavas undergo net dissolution and tuffs net secondary mineral precipitation The elasticity of conduit‐filling lavas and tuffs also changes due to alteration Inherently porous and permeable tuffs, when compacted and highly altered, can form seals within volcanic conduits

We perform laser ultrasonic measurements to investigate the spatial dependence of dynamic nonlinear weakening in rocks from the Alpine Fault, New Zealand. Rocks outside the damage zone display no nonlinear weakening. Within the damage zone (<30 m from the fault), cataclasites present a 3% reduction in shear modulus from wave amplitudes inducing 1–2 microstrain at atmospheric pressure. This nonlinear elasticity decreases with a characteristic pressure between 1 and 2.5 MPa. We show that rock weakening is therefore strongest in the near surface. However, this significant elastic nonlinearity in cataclasites at low strains confirms that rock weakening may play an important role in earthquake processes, such as fault weakening, triggering of slip, rupture propagation, and coseismic velocity decreases. Passing seismic waves can cause rocks to temporarily weaken, a phenomenon known as dynamic nonlinear rock weakening. Using rock samples from the Alpine Fault in New Zealand, we investigate how rock weakening varies with distance to the fault and depth in the Earth. We do this using lasers to generate and record ultrasonic waves in our rocks in the laboratory. Close to the fault (<30 m), fractured and granular cataclasite rocks show a 3% reduction in strength (shear modulus) during the passage of medium‐intensity waves (1–2 microstrain). As depth in the Earth increases, we find that the amount of weakening in the cataclasites decreases; rock weakening is strongest in the several hundred meters below the surface. For rocks further from the fault, we do not observe any weakening. The nonlinear changes in rock strength we observe, though they may appear small, are important for understanding processes such as the triggering of critically stressed faults by seismic waves and fault rupture propagation. We present the first laboratory measurements of dynamic nonlinear weakening in a suite of whole fault rocks under in situ pressure.A 3% reduction of shear modulus occurs in damage‐zone cataclasites at the surface but weakening reduces rapidly with depth.These results will contribute toward understanding the role of dynamic rock weakening in earthquake processes. We present the first laboratory measurements of dynamic nonlinear weakening in a suite of whole fault rocks under in situ pressure. A 3% reduction of shear modulus occurs in damage‐zone cataclasites at the surface but weakening reduces rapidly with depth. These results will contribute toward understanding the role of dynamic rock weakening in earthquake processes.

The transpressional Alpine Fault in New Zealand has created a thick shear zone with associated highly anisotropic rocks. Low seismic velocity zones and high seismic reflectivity are recorded in the Alpine Fault Zone, but no study has explored the underlying physical rock parameters of the shallow crust that control these observations. Protomylonites are the volumetrically dominant lithology of the fault zone. Here we combine experimental measurements of P‐wave speeds with numerical models of elastic wave anisotropy of protomylonite samples to explore how the fault zone can be seismically imaged. Numerical models that account for the porosity‐free real samples' fabric elastic tensors from electron backscatter diffraction (EBSD) are calculated by MTEX and a finite element model (FEM), while microfractures are modeled with differential effective medium (DEM) theory. At effective pressures representative of the Alpine Fault brittle zone, experimental wave speeds are lower than those predicted by MTEX/FEM. A possible DEM model suggests that a combination of random and aligned microfractures with aspect ratios increasing with pressure can explain the experimental wave speeds for pressures <70 MPa. Such microporosity in the form of foliation‐ and mica basal plane‐parallel microfractures and grain boundaries is validated with synchrotron X‐ray microtomography and transmission electron microscopy (TEM) images. Finally, by modeling anisotropy of seismic reflection coefficients with angle of incidence, we demonstrate that the high reflectivity and low‐velocity zone (LVZ) observed at the Alpine Fault can only be explained if this microporosity is accounted for throughout the brittle fault zone, even at depths of 7–10 km. By combining elastic wave models and laboratory data of Alpine Fault protomylonites, we show the effects of mineral anisotropy and fracturesMicrostructures imaging and numerical modeling confirm open grain boundaries and (micro)fractures at all depths within the brittle regimeThese combined structural features explain field observations of high seismic reflectivity and low velocities within the Alpine Fault

Quantifying the amount and alignment of microfractures is important to understand the geomechanics, fluid flow, and seismic imaging of fault zones. At the Alpine Fault, New Zealand, the preferred alignment of minerals, foliation, and fractures results in elastic wave anisotropy. We have designed a unique laser‐ultrasonic laboratory setup to study Alpine Fault rock samples at upper crustal conditions. Combined with differential effective medium modeling, we distinguish microfracture porosity and orientation from mineral alignment, as a function of distance to the principal slip zone (PSZ). Nearest to the PSZ, the cataclasite has the lowest Pwave anisotropy with the most (randomly oriented) fractures. Next, the ultramylonite exhibits the greatest Pwave anisotropy (∼45%) with 40% of its fractures aligned with foliation. Further from the PSZ, Pwave anisotropy is 14–19% on average, due to 20–30% of the fractures being oriented in the same direction as mineral alignment. The movement on faults causes fractures and alignment of minerals in the adjacent rocks. This causes seismic waves to travel fastest parallel to these features, a phenomenon known as anisotropy. Quantifying the amount and orientation of the fractures that contribute to this anisotropy is important for understanding fault zones and the processes which could influence the nature of earthquakes. However, this requires a method that can separate the effects of the fractures from those of the minerals. We have designed a system that uses laser ultrasonics to measure the speed of waves through rocks under pressure at spatially dense locations. We combine these measurements with numerical modeling, allowing us to accurately quantify the amount and orientation of fractures in rocks from the Alpine Fault, New Zealand. We find that the total amount of fractures increases toward the fault. Additionally, the amount of fractures aligned with the fault plane (up to 40%) increases toward the fault until the fractures become almost completely randomly oriented for the cataclasite in the core of the fault. We present a new method combining laser ultrasonics with DEM modeling to separate effects of microfractures from mineral foliationMicrofracture porosity is generally small at the Alpine Fault but increases toward the principal slip zoneUp to 40% of fractures are aligned with the foliation in the schist and mylonites while fractures are randomly oriented in the cataclasite