Napier Port Geology Electrical Properties Analysis

Comprehensive Geoelectrical and Stratigraphic Analysis of the Port of Napier: Tectonic Evolution, Subsurface Characterization, and Engineering Implications

The Port of Napier, situated on the eastern coastline of New Zealand’s North Island, represents a critical intersection of complex geological history, high-stakes tectonic activity, and advanced maritime engineering. Understanding the geology of this site requires an integrated approach that considers the deep-seated crustal dynamics of the Hikurangi subduction margin alongside the surficial processes that have shaped the Heretaunga Plains and the Napier urban area.1 This analysis provides an exhaustive examination of the geological units, tectonic influences, and specifically, the electrical properties of the subsurface materials, which are fundamental to modern infrastructure design, earthing systems, and corrosion mitigation in an aggressive marine environment.3

Regional Geodynamic Context and the Hikurangi Subduction Margin

New Zealand’s geological identity is inextricably linked to its position astride the boundary of the Pacific and Australian plates. The Port of Napier is located within the Hawke's Bay region, where the Pacific plate subducts beneath the Australian plate at the Hikurangi Trough, centered approximately 160 kilometers offshore from the city.2 This subduction process drives the regional tectonics, resulting in a landscape characterized by intense compressional folding, active faulting, and rapid vertical land movements.6

The Port of Napier is specifically positioned at the northern edge of the Heretaunga Basin, a structural syncline that has been subsiding for approximately 1.5 million years.2 This basin acts as a significant sediment trap for material eroded from the high-standing Ruahine and Kaweka ranges to the west. The interaction between tectonic subsidence and sea-level fluctuations has created a complex alternating sequence of marine and terrestrial deposits, which form the stratigraphic framework of the Port area.1

The Accretionary Borderland and Structural Fabric

The geology of the Port area is part of the broader Accretionary Borderland, a zone where sediments originally deposited on the ocean floor are scraped off the subducting plate and accreted to the overriding Australian plate.2 These rocks, which include Pliocene and Miocene mudstones and sandstones, form the "hydrogeological basement" upon which younger Quaternary sediments are deposited.1

The structural fabric of the region is defined by northeast-southwest trending folds and faults. These include the Awanui and Tukituki faults, which are steeply-dipping reverse faults reflecting the prevailing compressional regime.1 The Ruahine Mountains, which form the western boundary of the Hawke's Bay district, have undergone rapid uplift—on the order of 2,000 meters during the past one million years.2 This rapid uplift, coupled with high precipitation, yields large quantities of gravel and coarse sand derived from resistant Mesozoic greywacke rocks.2 These materials are transported by the Ngaruroro, Tukituki, and Tutaekuri Rivers to the coast, where they become the primary components of local beaches and the subsurface aquifer systems.2

Lithostratigraphy of the Napier-Ahuriri Area

The stratigraphic sequence at the Port of Napier and the adjacent Ahuriri district can be divided into three primary categories: the hydrogeological basement (Mangaheia Group), the Quaternary cover (Kidnappers and Heretaunga Formations), and the anthropogenic reclamation fill that constitutes the modern port facilities.3

Mangaheia Group: The Pliocene Foundation

The Mangaheia Group serves as the competent foundation for heavy infrastructure in the Napier region. At the Port, this group is represented primarily by the Scinde Island Formation, which comprises calcareous sandstone and shelly limestone.3 These rocks are famously exposed at Bluff Hill (Scinde Island), which rises immediately south of the port and provides the high ground upon which much of the city is built.3

The Scinde Island Formation records a period of shallow-marine deposition during the Late Pliocene, where carbonate-rich environments allowed for the accumulation of extensive shell beds.10 These deposits are often tightly cemented and exhibit significant structural integrity, making them the preferred founding layer for deep piles used in wharf construction.12 Beneath these Pliocene layers lie older Miocene mudstones and sandstones, which are distributed in fault-controlled strips.1

Stratigraphic Group

Typical Lithology

Age

Engineering Role

Heretaunga Formation

Unconsolidated silts, clays, gravels

Holocene

Reclaimed substrate, aquifer

Kidnappers Group

Pumiceous sand, silt, gravel

Pleistocene

Intermediate cover

Mangaheia Group

Limestone, calcareous sandstone

Pliocene

Competent founding rock

Tolaga Group

Mudstone, sandstone

Miocene

Deep basement

The Quaternary Sequence and Alluvial Deposition

Overlying the Pliocene basement are the Early to Middle Pleistocene deposits of the Kidnappers Group. These include conglomerates, pumiceous sandstones, and carbonaceous mudstones preserve the record of a dynamic landscape.1 These deposits represent alluvial fans and marginal-marine environments heavily influenced by the influx of volcanic material from the Taupo Volcanic Zone.6

The most recent geological unit is the Heretaunga Formation, which consists of Late Quaternary alluvial and marine sediments.1 This unit forms the vast Heretaunga Plains and is subdivided into various lithofacies, including river-deposited gravels and marine silts and clays.1 At the Port of Napier, these sediments are often buried beneath reclamation fill or have been heavily modified by the tectonic events of the 20th century.3

Reclamation History and Substrate Modification

The modern Port of Napier is constructed largely on reclaimed land situated immediately north of Bluff Hill.3 Historically, the area behind Napier was a swampy lagoon known as the Ahuriri Lagoon.2 Reclamation has occurred in several phases, beginning in the late 19th century with the construction of the breakwater between 1887 and 1890.2

The material used for reclamation is heterogeneous, consisting of dredged marine sediments, limestone rocks blasted from Bluff Hill, and more recently, recycled concrete and crushed limestone fill.15 This creates a subsurface environment with highly variable mechanical and electrical properties, necessitating detailed geotechnical investigations prior to any major construction.3

The 1931 Hawke's Bay Earthquake: A Geological Pivot Point

The most transformative geological event in Napier's recorded history is the 1931 Hawke's Bay Earthquake. Occurring at 10:47 am on February 3, 1931, this magnitude 7.8 (Ms) event caused the largest loss of life and most extensive damage in New Zealand's history.7 Beyond the immediate destruction, the earthquake fundamentally altered the coastal geomorphology and subsurface properties of the Port area.

Tectonic Uplift and Environmental Change

The earthquake was generated by slippage on a deeply buried reverse fault, estimated to be between 5 and 25 kilometers deep.7 This motion caused a sudden uplift of the land across a 90-kilometer-long dome.18 In the vicinity of Napier, the ground rose by approximately 1.8 to 2 meters.7

The most profound impact was the drastic raising of the Ahuriri Lagoon bed, which resulted in the draining of approximately 3,600 hectares (9,000 acres) of tidal flats and lagoon.7 Hundreds of fish and shellfish were left exposed on what suddenly became dry land.18 This newly created land allowed for the subsequent development of the Hawke's Bay Airport and various industrial and residential suburbs.7 At the Port, the navigation channel became shallower by the same 1.8 meters, requiring significant dredging and reconstruction of the harbor facilities.19

Subsidence and Variable Deformation

While the area north of Napier rose, the region to the south, including Hastings, experienced subsidence of up to one meter.2 This differential movement reflects the complex folding of the crust under the compressional stress of the Hikurangi margin. The earthquake also triggered extensive fissuring, slumping, and landslides throughout the Hawke's Bay district.18 In the Port Ahuriri area, ground fissures were particularly severe, complicating the stability of the shoreline and the integrity of existing structures.18

Principles of Electrical Resistivity in Geological Materials

A central focus of this analysis is the electrical property of the Napier Port geology. Electrical resistivity ($\rho$), measured in ohm-meters ($\Omega \cdot m$), is a material constant that characterizes the resistance of a cubic meter of earth to the flow of an electrical current.21 In geotechnical and environmental engineering, resistivity is a powerful proxy for several physical and chemical properties of the subsurface.

Mechanisms of Conduction

In the shallow subsurface typical of the Port environment, conduction is primarily driven by two contributions:

• Ionic (Electrolytic) Contribution: This involves the movement of free ions through the pore fluid.22 Since most mineral grains (like quartz or calcite) are insulators, the resistivity of the soil is heavily dependent on the salinity and volume of the water trapped in the pores.22
• Electronic Contribution: This involves the movement of free electrons through metallic or semi-conductive minerals.23 While less dominant in sedimentary rocks, the presence of disseminated sulfides or iron-rich minerals can occasionally influence readings.26

The Influence of Soil Composition and Structure

The bulk resistivity of a geological formation is governed by a range of factors:

• Porosity and Saturation: Higher porosity and higher moisture content generally lead to lower resistivity, as there is more room for the electrolytic pore fluid to carry current.22
• Pore Fluid Salinity: An increase in the concentration of dissolved ions (such as sodium and chloride in a marine environment) drastically reduces resistivity.29
• Clay Content and Surface Conductivity: Clay minerals, especially montmorillonite, possess an electrical double layer that allows for "surface conductivity" along the mineral-water interface.22 This means that clayey soils often have much lower resistivity than sands, even at similar saturation levels.24
• Compaction and Density: As soil is compacted, the connectivity between grains increases and the volume of pore space decreases. This can have a dual effect: it might increase resistivity by reducing the volume of pore fluid, or decrease it by improving grain-to-grain contact if the minerals are semi-conductive.22

Quantitative Resistivity Profiles of Napier Lithologies

Drawing from the Hawke's Bay 3D Aquifer Mapping Project (SkyTEM) and various site-specific geotechnical soundings, it is possible to delineate the typical resistivity ranges for the materials found at the Port of Napier.8

Typical Subsurface Resistivity Ranges

Lithological Category

Resistivity Range (Ω⋅m)

Predominant Factors

Salt Sea Water

0.15 – 0.25

High ionic concentration

Estuarine/Marine Silts

2 – 15

Saline pore water, high clay content

Saturated Marine Sands

30 – 60

Variable salinity, moderate porosity

Clean Alluvial Gravels

60 – 200+

High porosity, low clay content

Scinde Island Limestone

60 – 550

Cementation, shell density, fracturing

Mangaheia Sandstone

30 – 150

Consolidation, variable silt content

Reclamation Fill

10 – 1000

Extreme heterogeneity, anthropogenic origin

Second-Order Insight: The Leaching Effect and "Quick" Clays

A critical observation in the Napier area is the transition of marine clays following the 1931 uplift. Marine clays originally deposited in a saline environment typically exhibit resistivities between 1 and 10 $\Omega \cdot m$.29 However, once uplifted and exposed to freshwater infiltration (leaching), the salt content of the pore fluid decreases.

In the Ahuriri Lagoon area, stratigraphic sections are dominated by peat and clay.2 Research indicates that leached clays (which may develop "quick" properties) typically show resistivities in the 10 to 100 $\Omega \cdot m$ range.29 This transition is not merely an electrical curiosity; it signifies a fundamental change in the soil's mechanical stability. The 1931 earthquake may have "pre-conditioned" some of these marine deposits by raising them out of the saline environment, initiating a decade-long process of geoelectrical and geotechnical evolution.9

Advanced Geophysical Mapping: SkyTEM and 3D Modeling

To understand the 3D distribution of these units, the Hawke's Bay Regional Council commissioned the 3D Aquifer Mapping Project using SkyTEM technology.8 SkyTEM works by flying an electromagnetic loop slung below a helicopter, which collects data along flight lines spaced roughly 200 meters apart.31

Mapping the Heretaunga Basin

The resulting resistivity models provide a 3D view of the subsurface to depths of several hundred meters.31 In the Heretaunga Plains adjacent to the Port, the models clearly differentiate between:

• High Resistivity Features (Pink/Orange): Correlating with young gravels associated with the Ngaruroro, Tutaekuri, and Tukituki Rivers.31 These are interpreted as the primary freshwater aquifers.
• Low Resistivity Features (Green/Blue): Correlating with older marine deposits of mud and silt.31 These act as confining layers over the aquifers.

Near the Port, these models reveal the deepening of the hydrogeological basement (Mangaheia Group) as one moves west along the coast.10 The interface between the resistive Scinde Island limestone of Bluff Hill and the more conductive marine silts of the old lagoon is a key structural boundary identified in these geoelectric cross-sections.10

Inversion and Interpretation Challenges

The interpretation of resistivity data requires "connecting the dots" between boreholes and SkyTEM soundings.31 One significant challenge in the Napier urban area is the "geological noise" introduced by urban infrastructure.33 Buried metallic pipes, earthing systems, and concrete pads can all provide false low-resistivity readings.17 Furthermore, there is no one-to-one relationship between resistivity and rock type; for example, a clay-rich gravel might exhibit the same resistivity as a clean sand, requiring the integration of lithological logs to reduce uncertainty.10

Engineering Geotechnics at the Port of Napier: Wharf 6 Development

The construction of the new Wharf 6 at Napier Port serves as a prime case study for how the site’s geology and electrical properties dictate engineering design.12 The project involved the construction of a 350-meter-long wharf supported by approximately 400 deep piles.12

Piling and Founding Conditions

Geotechnical investigations for Wharf 6 included land-based and barge-based machine boreholes reaching depths of up to 41 meters.3 These boreholes identified that the site is underlain by reclamation fill, which is in turn underlain by marine silts and sands.3 The piles for the wharf are typically 900mm to 1200mm in diameter and consist of bored reinforced concrete with permanent steel casings.12

The critical design requirement was to socket these piles into the underlying competent Mangaheia Group sandstone.12 The founding level for this sandstone varies significantly along the length of the wharf, becoming progressively deeper towards the western end.12 This variation is likely controlled by the pre-existing topography of the Scinde Island formation before it was buried by Quaternary sediments and reclamation fill.1

Ground Improvement: Cutter Soil Mixing (CSM)

To address the risk of liquefaction—a phenomenon where saturated silty soils lose their strength during earthquake shaking—the Port utilized the Cutter Soil Mixing (CSM) technique.13 CSM is a type of deep soil mixing that uses a two-wheel cutter head to grind the in-situ soil and mix it with a cementitious slurry.34

At Wharf 6, Wagstaff constructed 471 CSM panels to form lattice walls to a depth of 17 meters.13 These walls create a "closed lattice" that provides confinement and stiffness, limiting the buildup of pore water pressure in the untreated soil during a seismic event.16 This approach was specifically chosen for its high level of quality control and minimal vibration, which is essential given the proximity to sensitive port infrastructure and marine wildlife.15

Third-Order Insight: The Geoelectrical Impact of CSM

The introduction of 15,000 cubic meters of soil-cement mixture fundamentally alters the electrical environment of the wharf area.16 Fresh cement grout is highly alkaline and rich in ions, which initially lowers the resistivity of the treated zone.28 However, as the cement hydrates and forms calcium silicate hydrate (C-S-H) products, the porosity decreases and the unconfined compressive strength (UCS) increases—from nominal values to as high as 15 MPa.35

In the long term, these CSM panels act as resistive barriers in the otherwise conductive marine silt.34 This geoelectrical heterogeneity must be considered when designing earthing systems for the wharf, as the current will tend to follow the more conductive, untreated pathways between the CSM lattices.38

Corrosion and Cathodic Protection in the Port Environment

The Port of Napier presents a "very high" to "extreme" corrosivity environment due to its location and industrial activities.39 The proximity to the shoreline results in continuous exposure to marine chloride deposits (sea spray), while fertilizer storage facilities introduce airborne chemical contaminants.39

Soil Corrosivity and Resistivity Correlations

In the subsurface, the corrosion of steel piles and pipelines is heavily influenced by soil resistivity. Low resistivity (high conductivity) facilitates the flow of the electrochemical currents that cause metal to lose electrons and revert to an oxide (rust).4

Soil Resistivity (Ω⋅m)

Corrosivity Classification

Engineering Implication

< 10

Severe / Extremely Corrosive

Mandatory ICCP/SACP and robust coatings

10 – 50

Corrosive

Likely requires cathodic protection

50 – 100

Moderately Corrosive

Monitoring and protective coatings required

> 100

Slightly Corrosive

Standard protection may suffice

The marine silts and saline groundwater at the Port fall into the "severely corrosive" category.29 This requires the implementation of Cathodic Protection (CP) systems for all buried metallic assets.4

Cathodic Protection Systems at Napier Port

Cathodic protection works by making the protected metal surface the cathode of an electrochemical cell.4 There are two primary methods used:

• Sacrificial Anode Cathodic Protection (SACP): More active metals like zinc or aluminum are connected to the structure. These "sacrificial" anodes corrode instead of the steel.4
• Impressed Current Cathodic Protection (ICCP): An external DC power source is used to drive a current to the protected structure. This is ideal for large infrastructures like Wharf 6 and allows for more precise control in fluctuating environmental conditions.4

For atmospheric protection, the Port has utilized viscoelastic treatments like Goldseal IHS for its fertilizer storage buildings.39 These coatings prevent moisture, oxygen, and chlorides from reaching the steel substrate, effectively extending the asset's life by over 15 years even in "extreme" CX environments.39

Earthing System Design for Port Infrastructure

The design of a safe and effective earthing (grounding) system for the Port’s electrical facilities, such as the new Wharf 6 substation, is critically dependent on site-specific soil resistivity data.21 Without this data, only a "geometric" design can be performed, which may not meet safety performance requirements (e.g., a target resistance of less than 5 $\Omega$).21

The IEEE 81 Standard and Field Testing

The industry standard for measuring soil resistivity is IEEE Standard 81.17 At the Port of Napier, the Wenner 4-Point Array Test is the preferred method.21 This involves driving four stakes into the ground at equal intervals in a straight line.21 The meter forces a current between the outer two stakes and measures the voltage drop across the inner two.21

An important physical concept is that the spacing between the stakes is equivalent to the depth of the soil being tested.21 For instance, a 10-meter spacing measures the average resistivity from the surface down to a 10-meter depth.30 To develop a reliable model for a substation ground grid, measurements must be taken at multiple locations and multiple spacings to account for vertical stratification of the soil.23

Modeling and Mitigation of High-Voltage Risks

The goal of earthing design is to safely dissipate fault currents and lightning into the earth while minimizing "step" and "touch" potentials that could injure personnel.17

• Step Voltage: The potential difference between a person's feet when walking near an earthed structure during a fault.
• Touch Voltage: The potential difference between an earthed metallic structure and the ground where a person is standing.43

At the Port, the high conductivity of the marine silts helps in achieving low overall earth resistance. However, it also means that fault currents can travel longer distances through the ground.43 To protect workers, high-resistivity surface layers—such as crushed rock or asphalt—are often laid in substation yards to provide an insulation barrier.45 Typical crushed rock has a wet resistivity of around 3,000 $\Omega \cdot m$, significantly higher than the underlying native soil.45

Geochemical and Environmental Stability of Recycled Fill

Modern port construction increasingly utilizes recycled materials to improve sustainability.13 At the Port of Napier, this includes the use of recycled fill for foundations and the creation of artificial reefs using limestone boulders removed during wharf construction.15

Recycled Aggregate Concrete (RAC) in Marine Environments

Research into the performance of recycled aggregate concrete in harsh marine environments reveals that it is more susceptible to chloride ion invasion than natural aggregate concrete due to higher porosity.46 However, durability can be significantly enhanced through carbonation modification of the recycled coarse aggregate (RCA).46

The chemical reaction between $CO_2$ and the cement components in the recycled aggregate forms dense $CaCO_3$, which fills pores and microcracks.46 This treatment has been shown to improve the durability life of marine structures by approximately 28%.46 At the Port, the choice of fill and concrete for Wharf 6 and its associated substation foundations must account for these long-term chloride transport mechanisms.13

Ground-Water Interactions and Tectonic Pre-Conditioning

The 1931 uplift drained the Ahuriri Lagoon, transforming it from a saline tidal environment to a terrestrial one.18 This change initiated a long-term hydrogeological process where rainwater began to leach salt from the upper layers of the sediment.29 This leaching increases the electrical resistivity of the shallow soils while the deeper layers remain highly conductive.10

This "geoelectrical layering" is a critical consideration for both earthing design and corrosion protection. A surface-layer resistivity survey may show relatively high values (e.g., 50–100 $\Omega \cdot m$), masking the presence of "severely corrosive" saline silts just a few meters below.23

Mathematical Modeling of Electrical Soundings

To interpret resistivity data collected at the Port, engineers use 1D, 2D, or 3D inversion models.44 A 1D model assumes a uniform layer of thickness ($h$) and resistivity ($\rho_1$) overlying a second layer ($\rho_2$).44

For a more accurate representation of the Port's heterogeneous reclamation fill, 2D Electrical Resistivity Imaging (ERI) is used. ERI involves an array of dozens of electrodes and thousands of measurements, allowing for the visualization of "anomalous zones" like buried seawalls, pockets of saline water, or variations in the depth to the Scinde Island limestone.44

Vertical Electrical Sounding (VES) Analysis

VES is particularly useful for identifying the depth of the water table or the thickness of the reclamation layer.38 The relationship between apparent resistivity and depth is plotted on a log-log scale. If the curve decreases with depth, it indicates a transition from resistive topsoil/fill to conductive saturated silts.38

The resistivity of the subsurface ($\rho$) can be modeled using the formula:

$$\rho = \frac{\Delta V}{I} \cdot K$$

where $\Delta V$ is the measured voltage, $I$ is the injected current, and $K$ is a geometric factor based on the electrode arrangement (e.g., $K = 2 \pi a$ for the Wenner method).38

Emerging Technologies: Induced Polarization (IP) Tomography

While DC resistivity measures the ease of current flow, Induced Polarization (IP) measures the ground's ability to temporarily store an electric charge.25 After the current is shut off, the voltage decays over time, a phenomenon known as the IP effect.25

Applications in Geotechnical Discrimination

In a sedimentary environment like Hawke's Bay, IP is a powerful tool for lithologic discrimination.50 It is highly sensitive to the presence of clays and the microstructure of the porous system.25 For instance:

• High Chargeability: Often indicates the presence of disseminated sulfides or significant clay content.26
• Low Chargeability: Typical of clean, high-resistivity gravels or sands.26

At the Port of Napier, IP tomography can be used concurrently with resistivity to identify "hidden" clay zones within the reclamation fill or the underlying marine sequence, providing a more nuanced map of the subsurface for pile design and liquefaction assessment.50

Synthesis and Engineering Conclusions

The geoelectrical and stratigraphic analysis of the Port of Napier reveals a site of extreme geological complexity. The interaction between the subsiding Heretaunga Basin, the rapid uplift of the surrounding ranges, and the catastrophic 1931 earthquake has created a subsurface characterized by high heterogeneity and aggressive chemical properties.1

Key Findings and Recommendations

• Founding Conditions: Heavy infrastructure at the Port must be socketed into the Pliocene Mangaheia Group sandstone/limestone, found at depths of 20 to 40 meters. The overlying Heretaunga silts and reclamation fill are mechanically weak and liquefiable.3
• Corrosivity Risk: The combination of saline groundwater and low soil resistivity (< 10 $\Omega \cdot m$) creates a "severely corrosive" environment. All metallic infrastructure, especially Wharf 6 piles, requires sophisticated Cathodic Protection (ICCP/SACP) and advanced viscoelastic coatings.4
• Electrical Stratification: The 1931 uplift and subsequent freshwater leaching have created a layered geoelectrical profile. Surface measurements must be supplemented with deep Wenner soundings or SkyTEM data to accurately model the corrosive and earthing environment.21
• Ground Improvement Resilience: The use of Cutter Soil Mixing (CSM) has provided seismic resilience for Wharf 6. These cement-stabilized lattices also serve as geoelectrical anomalies that influence the behavior of earthing and cathodic protection currents.13
• Future Outlook: Continued 3D aquifer and geoelectric mapping is essential for managing the Port's groundwater and structural stability, particularly as sea-level rise may alter the saline-freshwater interface in the coming decades.8

In conclusion, the Port of Napier is not merely a piece of infrastructure but a "living" geological system. The integration of traditional geotechnical boring with modern geophysical resistivity mapping is the only way to ensure the long-term safety and performance of this vital maritime gateway for the Hawke's Bay region.13

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