# Remote Sensing and Geophysics

History I · Archaeology · https://tryals.app/learn/history-i/remote-sensing-and-geophysics

## Seeing Without Digging

Excavation destroys; **remote sensing** does not. It reads buried sites through soil, canopy, and pasture, turning survey into archaeology's most productive method.

**Aerial photography** exploits three distinct surface signatures:

| Signature | How it works |
| --- | --- |
| **Cropmarks** | Drier soil over walls stunts crops; deeper soil over ditches keeps them green. |
| **Soilmarks** | Plowing scatters different-colored buried material. |
| **Shadow marks** | Low evening sun reveals subtle earthworks. |

**LiDAR** (laser pulses from aircraft) strips vegetation away using ground-returning pulses, revealing missed landscapes like Maya causeways under jungle canopy.

## Ground Geophysics

| Method | Reads | Best For |
| --- | --- | --- |
| **Magnetometry** | Magnetic contrasts | Fast wide coverage of ditches, pits, kilns, hearths |
| **Resistivity** | Electrical resistance | Stone walls vs damp ditches |
| **GPR** | Reflected radar, timed | Depth and plan, 3D slices |

**Magnetometry** is the workhorse: burnt features like kilns and hearths shine because fire re-aligns soil magnetism. **GPR** is slower, uniquely giving depth by cutting horizontal floor-by-floor slices.

**Pitfall:** A picture is not an interpretation. Geophysics shows anomalies. A rectangular anomaly might be a Roman building, a sheepfold, or geology. The instrument tells you where to dig; only excavation tells you what it is.

## Practice questions

7 of this lesson's 12 practice questions, with answers. The full set is in the app.

### 1. Order a modern non-destructive investigation from first step to last.

**Answer:**

1. Study existing aerial photographs and LiDAR coverage
2. Walk the field to collect surface finds
3. Run a magnetometer survey for a fast, wide plan
4. Follow up promising anomalies with GPR for depth
5. Place one small trench to test the interpretation

**Why:** Cheap and wide first, expensive and narrow later, trowel last. By the time a trench opens, it is aimed at a question, not a hope.

Page: https://tryals.app/practice/history-i/remote-sensing-and-geophysics/order-a-modern-non-destructive-investigation-from-first-step-to-last

### 2. A parched cropmark and a lush green one appear side by side in one field. Why do buried features produce opposite signals in the same drought?

A. Because the camera’s exposure varies across the frame
B. Because green marks indicate modern disturbance and parched marks ancient features
C. Because a buried ditch holds damp fill and stone leaves it dry
D. Because the two marks were photographed in different seasons

**Answer:** C. Because a buried ditch holds damp fill and stone leaves it dry

**Why:** The crop is a moisture gauge. A ditch stores water and a wall steals it, so one feature makes the crop thrive and the other makes it fail, in the same field on the same day.

Page: https://tryals.app/practice/history-i/remote-sensing-and-geophysics/a-parched-cropmark-and-a-lush-green-one-appear-side-by-side-in-one

### 3. Why has LiDAR transformed archaeology in forested regions?

A. It maps whole landscapes rather than single sites
B. It reveals the age of the structures it finds
C. Software can remove vegetation to model bare earth
D. Some laser pulses reach the ground through gaps in the canopy

**Answer:** A. It maps whole landscapes rather than single sites; C. Software can remove vegetation to model bare earth; D. Some laser pulses reach the ground through gaps in the canopy

**Why:** Canopy penetration, digital stripping, and landscape-scale coverage revealed Maya and Angkorian cities under jungle. Dating, as ever, remains the trench’s job.

Page: https://tryals.app/practice/history-i/remote-sensing-and-geophysics/why-has-lidar-transformed-archaeology-in-forested-regions

### 4. Sort each signature: does it mark a buried **ditch/pit** or a buried **wall**?

**Answer:**

- Buried ditch or pit: Crop stays green and tall during drought, Low electrical resistance (damp fill), Strong magnetic response from silted organic fill
- Buried wall or road: Crop parches early and grows stunted, High electrical resistance (dry stone), Hard reflective surface in a GPR slice

**Why:** Ditches are damp, magnetic, and generous to roots; walls are dry, resistant, and hostile. Every geophysical method reads the same opposition through a different physics.

Page: https://tryals.app/practice/history-i/remote-sensing-and-geophysics/sort-each-signature-does-it-mark-a-buried-ditch-pit-or-a-buried-wall

### 5. A clear rectangular anomaly in a magnetometry survey proves a Roman building lies buried there.

**Answer:** False

**Why:** Geophysics finds _anomalies_, not identities or dates: that rectangle could be a Roman villa, a modern sheepfold, or geology. The instrument aims the trench; only excavation names what it found.

Page: https://tryals.app/practice/history-i/remote-sensing-and-geophysics/a-clear-rectangular-anomaly-in-a-magnetometry-survey-proves-a-roman

### 6. In drought a crop grows taller and greener in a narrow strip. What is buried, and why does it produce that specific signal?

A. A ditch or pit, whose deeper fill holds more moisture
B. A wall, whose stone retains heat and accelerates growth
C. A buried floor, whose flat surface holds a shallow reservoir
D. A road surface, which channels rainwater to the plants above it

**Answer:** A. A ditch or pit, whose deeper fill holds more moisture

**Why:** Deeper soil holds more water, so negative features show as lush positive cropmarks while walls starve the crop and produce stunted parch marks. Reading which way the mark runs tells you whether the feature was cut into the ground or built upon it, before any spade is lifted.

Page: https://tryals.app/practice/history-i/remote-sensing-and-geophysics/in-drought-a-crop-grows-taller-and-greener-in-a-narrow-strip-what-is

### 7. Match each technique to the physical property it measures.

**Answer:**

- Magnetometry → Local contrasts in soil magnetism
- Resistivity → Resistance to an electrical current
- GPR → Timing of reflected radar pulses
- LiDAR → Return time of laser pulses from the ground

**Why:** Four instruments, four physics — magnetism, current, radar echo, laser echo — each turning an invisible contrast into a map.

Page: https://tryals.app/practice/history-i/remote-sensing-and-geophysics/match-each-technique-to-the-physical-property-it-measures