A geophysicist surveys two sites at the same elevation. Site A sits over a dense mafic intrusion; Site B sits over a low-density sedimentary basin. After applying the full Bouguer correction, which site will show a higher Bouguer anomaly value?
ASite B — sedimentary basins trap denser fluids that increase the signal
BSite A — the denser-than-average mafic body produces a positive anomaly
CNeither — the Bouguer correction removes all subsurface density variation
DThey will be equal because the same reference density is subtracted at both sites
The Bouguer anomaly reveals subsurface density contrasts relative to the reference crustal density (~2,670 kg/m³). A mafic intrusion is denser than this reference, so the observed gravity exceeds the reference prediction, yielding a positive anomaly. A sedimentary basin is less dense than the reference, yielding a negative anomaly. The Bouguer correction only removes the effect of topography — it does not flatten out subsurface variation; it exposes it.
Question 2 Multiple Choice
Two geophysicists model the same residual Bouguer anomaly. Geophysicist A proposes a shallow, wide body with a moderate density contrast. Geophysicist B proposes a narrow, deep body with a very high density contrast. Both models fit the observed data equally well. What does this situation illustrate?
AOne of the models must be wrong — two different geometries cannot produce the same gravity field
BThe non-uniqueness of potential field inversion — multiple subsurface distributions can match the same surface data
CThat the Bouguer anomaly has not been computed correctly, since a unique solution should exist
DThat gravity surveys are unreliable and should be replaced by seismic methods
Non-uniqueness is a fundamental mathematical property of gravity (and magnetic) fields: a given surface field can be produced by infinitely many different subsurface distributions. This is not a measurement error or a modeling failure — it is inherent to potential fields. Resolving the ambiguity requires external constraints from geology, drilling, or other geophysical methods. Recognizing non-uniqueness is what separates rigorous interpretation from naive curve-fitting.
Question 3 True / False
The Bouguer anomaly directly measures the absolute density of rock beneath a gravity station.
TTrue
FFalse
Answer: False
The Bouguer anomaly measures the density *contrast* between the actual subsurface and the assumed reference density used in the Bouguer correction (typically 2,670 kg/m³). A positive Bouguer anomaly means the subsurface is denser than that reference; a negative anomaly means it is less dense. Absolute density cannot be determined from gravity alone without additional constraints — and even then, the non-uniqueness problem applies.
Question 4 True / False
A broad, smooth gravity anomaly over a large area is more likely caused by a deep source than a narrow, sharp anomaly of similar amplitude over a small area.
TTrue
FFalse
Answer: True
Anomaly wavelength (horizontal extent) scales with source depth. A shallow density contrast creates a short-wavelength anomaly — the gravity signal falls off rapidly with horizontal distance. A deep density contrast affects a broader area at the surface, producing a long-wavelength signal. This relationship between anomaly shape and source depth is a key interpretive tool, and it is the basis of regional-residual separation, which exploits wavelength differences to separate deep (regional) from shallow (residual) sources.
Question 5 Short Answer
Why can collecting higher-quality gravity measurements not, by itself, resolve the ambiguity about the depth and shape of a subsurface body?
Think about your answer, then reveal below.
Model answer: Because gravity non-uniqueness is a mathematical property of potential fields, not a measurement problem. Any surface gravity field can be reproduced by infinitely many different subsurface density distributions — making the measurement more precise does not eliminate this fundamental ambiguity. Resolving it requires independent geological or geophysical constraints (boreholes, seismic data, geological mapping) that discriminate among the infinite family of mathematically equivalent solutions.
Students often assume that better data yields unique answers. In potential field geophysics this is false: the inverse problem is fundamentally underdetermined regardless of data quality. More and denser measurements constrain the anomaly field better, but they still cannot uniquely determine the source geometry. This is why gravity interpretation is always combined with other methods, and why geophysicists must explicitly state what a gravity dataset constrains and what it cannot determine.