Guide: Magnetometry

A confused archaeologist sitting in a DeLorean with the Flux Capacitor glowing brightly

Contents

1 Magnetometry in Archaeology

Magnetometry in Archaeology

Introduction to Magnetometry

Magnetometry is one of the most widely used geophysical techniques in archaeology. It involves the measurement of the Earth’s magnetic field and detecting variations in magnetic properties caused by human activity. By detecting anomalies in the magnetic field, archaeologists can identify buried structures, features, and artifacts, often without the need for excavation. This non-invasive technique is invaluable for locating large-scale features, such as ancient buildings, ditches, kilns, and even settlements.

Magnetometry surveys are particularly effective for detecting iron-rich materials and features that have been subject to heat, like hearths and kilns. As a non-destructive method, it allows for rapid surveying of large areas and contributes greatly to understanding the layout of archaeological sites.

How Magnetometry Works

Magnetometers measure the magnetic anomalies caused by buried objects that have different magnetic properties than the surrounding soil. These objects may be materials such as iron, clay, or burnt features, which can create detectable disturbances in the magnetic field.

Fluxgate Magnetometers: These are the most commonly used magnetometers in archaeology. They measure changes in the magnetic field by detecting the presence of ferrous materials (iron, steel, etc.). The readings from these devices are usually presented as a series of magnetic anomalies that are indicative of buried archaeological features.
Gradiometers: These devices measure the difference in the magnetic field between two sensors placed at a specific distance from each other. The advantage of gradiometers is that they are more sensitive to near-surface features, making them ideal for detecting shallow, buried features like ditches, pits, and walls.
Magnetometer Surveys: These surveys can be conducted using transects or grids, where the area is divided into a grid, and readings are taken at regular intervals. The data is then processed and analyzed to generate a magnetic anomaly map, which highlights areas of interest for further investigation.

Applications of Magnetometry in Archaeology

Locating Features: Magnetometry is particularly useful for detecting features that are magnetic or have been affected by heat, such as kilns, hearths, and furnaces. It’s also effective for locating buried walls, ditches, and pits.
Mapping Ancient Settlements: By identifying the location of structures and features buried beneath the soil, magnetometry helps archaeologists map out ancient settlements, fortifications, and ceremonial sites.
Monitoring Changes in the Landscape: Magnetometry can be used to track changes in the landscape over time, such as areas of soil disturbance caused by human activities. This is useful for understanding how ancient peoples modified their environments.
Surveying Large Areas: Magnetometry surveys can cover large areas quickly and efficiently, making it ideal for large-scale surveys of archaeological landscapes, such as those conducted at prehistoric monuments, forts, and village sites.

Advantages of Magnetometry

Non-invasive: Magnetometry is a non-destructive method that doesn’t require excavation, which means that archaeological features can be detected and mapped without disturbing the site.
High resolution: Magnetometry provides high-resolution data that can reveal a wide range of features, from large monuments to small artifacts buried beneath the surface.
Time-efficient: Magnetometry can cover large areas relatively quickly, providing archaeologists with useful data that guides further excavation efforts.
Ideal for detecting heat-affected features: Since it’s sensitive to magnetic anomalies caused by heat, magnetometry is especially effective at locating hearths, kilns, and other heat-related features.

Limitations of Magnetometry

Soil conditions: Magnetometry can be less effective in areas with very homogeneous or highly disturbed soils, where magnetic anomalies may be difficult to detect.
Depth limitations: The depth at which magnetometers can detect features depends on soil conditions, the type of material being detected, and the sensitivity of the equipment. Deeper features may be harder to identify.
Interference: Modern-day disturbances, such as metal debris, pipelines, or electromagnetic interference, can cause false positives or obscure actual archaeological features.

Interpretation of Magnetometry Data

Magnetometry data is typically presented as a magnetic anomaly map. These maps highlight areas where magnetic fields have been disturbed, and these anomalies are then analyzed to determine their archaeological significance. Interpreting these maps requires both experience with the method and an understanding of the local geological and environmental conditions.

The magnetic anomalies detected in a survey are categorized by their intensity, shape, and pattern. For example:

High-intensity anomalies may indicate large ferrous objects like furnaces or kilns.
Linear anomalies might represent ditches or walls.

The interpretation of these anomalies often involves ground truthing: confirming the features identified in the survey through targeted excavation or further testing.