Detectability

Detectability

Pre-survey detectability, modelled and reproducible

Describe a buried target and your survey. Detectability models its gravity and magnetic response, compares it with instrument and field noise, and tells you whether the target is detectable, with the station spacing, profile length, assumptions and a Python script that reproduces every number.

Target

Cavity, boulder or buried object.

Methods
Survey

Field noise is what remains after corrections: drift, tares and terrain for gravity; diurnal variation, heading error and cultural noise for magnetics. Urban sites are far noisier than the defaults.

Your location is used only to compute the Earth's magnetic field at the site; it is not stored.

More survey settings

Contrasts are target minus host. Leave them empty to use typical values (Telford et al. 1990); site measurements are always better.

Recommended method

Gravity

Gravity

Clearly detectable
Peak anomaly
−114 µGal
Effective noise
11.2 µGal
Signal / noise
10.2
Station spacing
≤ 5 m
Profile length
≥ 153 m
Gravity anomaly along the profile: peak −114 µGal at 0 m; the shaded band is ±11.2 µGal of effective noise. Cross-section: sphere (top at 15 m, diameter 20 m) of air in limestone.
Modelled gravity profile across the target; the band is ±11.2 µGal effective noise.

Contrast: −2,550 kg/m³ · Scintrex CG-6: 5 µGal; field noise 10 µGal

Assumptions
  • Signal-to-noise thresholds: >= 5 clearly detectable, 2-5 marginal, < 2 not detectable.
  • Effective noise 11.2 µGal (instrument and field noise added in quadrature).
  • Station spacing allows at least 5 stations across the anomaly width (38.3 m).
  • Flat topography, homogeneous host and no regional trend; terrain and regional effects reduce the margin.
  • Scintrex CG-6: 5 µGal per reading (repeatability, standard deviation < 5 µGal; Scintrex CG-6 Autograv data sheet).

Magnetic

Not detectable
Peak anomaly
−0.463 nT
Effective noise
2 nT
Signal / noise
0.231
Needed for S/N 2
4 nT
Magnetic anomaly along the profile: peak −0.463 nT at −7.33 m; the shaded band is ±2 nT of effective noise. Cross-section: sphere (top at 15 m, diameter 20 m) of air in limestone.
Modelled magnetic profile across the target; the band is ±2 nT effective noise.

Contrast: −0.0003 SI · Overhauser: 0.022 nT; field noise 2 nT · Main field (IGRF-14): 48,584 nT, inclination 58.4°, declination 6.3°

Assumptions
  • Signal-to-noise thresholds: >= 5 clearly detectable, 2-5 marginal, < 2 not detectable.
  • Effective noise 2 nT (instrument and field noise added in quadrature).
  • Station spacing allows at least 5 stations across the anomaly width (26.1 m).
  • Flat topography, homogeneous host and no regional trend; terrain and regional effects reduce the margin.
  • Peak is the largest absolute value of the (often two-lobed) anomaly; no remanence unless given.
  • Overhauser: 0.022 nT per reading (sensitivity 0.022 nT/√Hz, i.e. at 1 Hz; GEM Systems GSM-19 v7 data sheet).
  • Sensor 0.5 m above ground; profile azimuth 0°; induced magnetisation only, no demagnetisation.