GP-3105 GRAVITY & MAGNETIC - Universitas Pertamina

Program Studi Teknik Geofisika

Fakultas Teknologi Eksplorasi dan Produksi

Universitas Pertamina

Djedi S. Widarto

Dicky Ahmad Zaky

GP-3105 GRAVITY & MAGNETICMETODE GAYABERAT & MAGNETIK TA 2019/2020

LECTURE #03

GRAVITY METHOD▪ The Sources of Gravity Anomaly▪ The Characteristics of Gravity Anomaly ▪ Gravity Anomaly Separation

✓ Regional & Residual Anomaly✓ Moving Average✓ Spectral Analysis✓ Field Continuation

▪ There are 2 (two) anomalous sources that affect the gravity value, i.e. ✓ Regional anomaly due to regional/large dimension of geologic features → i.e. basement;

✓ Residual/local anomaly due to smaller geologic features → i.e. intrusion, basin/sub-basin, geological structure, etc.

𝑔𝐵𝑜𝑢𝑔𝑢𝑒𝑟 = 𝑔𝑟𝑒𝑔𝑖𝑜𝑛𝑎𝑙 + 𝑔𝑟𝑒𝑠𝑖𝑑𝑢𝑎𝑙

The Sources of Gravity Anomaly

The Sources of Gravity Anomaly

▪ A gravity anomaly is the difference between the observed acceleration of free fall, or gravity, on the Earth's surface, and the corresponding value predicted from a model of the Earth's gravity field.

▪ Regional anomaly is caused by a large-scale geologic features of anomalous source in spatial extent. It is a slowly varying function of position along the profile line;

▪ Residual/local anomaly is defined as those that change value rapidly along the profile line. The sources for these anomalies must be small in spatial extent, with respect to the length of the gravity profile and close to the Earth’s surface;

▪ A broader anomaly is associated with the deeper sources and assumed to contribute for regional gravity value; and

▪ A narrower/sharper anomaly is associated with the more shallow sources and to contribute to the residual/local gravity value ….

The Characteristics of Gravity Anomaly

▪ The deeper of anomalous sources, the amplitude of gravity anomaly becomes smaller and the area is widened ….

The Characteristics of Gravity Anomaly

Separating Regional – Residual Gravity Anomalies

In general, the methods are divided into three broad categories:

▪ Direct Estimates - These are estimates of the regional gravity anomaly determined from an independent data set. For example, if your gravity survey is conducted within the continential US, gravity observations collected at relatively large station spacings are available from the National Geophysical Data Center (NGDC) on CD-ROM. Using these observations, you can determine how the long-wavelength gravity field varies around your survey and then remove its contribution from your data.

▪ Graphical Estimates - These estimates are based on simply plotting the observations, sketching the interpreter's estimate of the regional gravity anomaly, and subtracting the regional gravity anomaly estimate from the raw observations to generate an estimate of the local gravity anomaly.

▪ Mathematical Estimates - This represents any of a wide variety of methods for determining the regional gravity contribution from the collected data through the use of mathematical procedures. Examples of how this can be done include: …….. on the next page!

Separating Regional – Residual Gravity Anomalies

▪ Mathematical Estimates - This represents any of a wide variety of methods for determining the regional gravity contribution from the collected data through the use of mathematical procedures. Examples of how this can be done include:

✓ Moving Averages - In this technique, an estimate of the regional gravity anomaly at some point along a profile is determined by averaging the recorded gravity values at several nearby points. Averaging gravity values over several observation points enhances the long-wavelength contributions to the recorded gravity field while suppressing the shorter-wavelength contributions.

✓ Function Fitting - In this technique, smoothly varying mathematical functions are fit to the data and used as estimates of the regional gravity anomaly. The simplest of any number of possible functions that could be fit to the data is a straight line.

✓ Filtering and Upward Continuation - These are more sophisticated mathematical techniques for determining the long-wavelength portion of a data set. Those interested in finding out more about these types of techniques can find descriptions of them in any introductory geophysical textbook.

▪ Regional anomaly is represented by a broader or long-wavelength gravity anomalous trend;

▪ Shorter wavelength residual anomaly isdue to shallower geologic features and overlay to the regional anomaly; and

▪ Residual anomaly should be separated from the regional for further analysis/ modeling and interpretation.

Separating Regional – Residual Gravity Anomalies

Graphical Estimation

Separating Regional – Residual Gravity Anomalies

Graphical Estimation: A Gravity Anomaly Map

▪ Gravity anomaly map is represented by a set of contour line. A contour line represents the same gravity value (isogal);

▪ Circular dashed-contour lines are representing residual gravity anomaly. Contour line at Point A has a different of 0.2 mGal to the regional gravity anomaly (red line), while Point B has 0.4 mGaldifferent …

𝑔𝑟𝑒𝑔 𝑖 =𝑔 𝑖 − 𝑛 +⋯+ 𝑔 𝑖 + ⋯𝑔(𝑖 + 𝑛)

𝑁

𝑁 = Width of the Window𝑛 = (𝑁 − 1)/2

Separating Regional – Residual Gravity Anomalies

Moving Average – 01

▪ By averaging the value of gravity anomaly;

▪ The result of this averaging process is regional anomaly;

▪ Residual anomaly is then obtained by subtracting observed gravity value to the regional anomaly ...

where: 𝑛 =𝑁−1

2, N = number of data to be processed, odd number !

Exp. Moving Average at window 7 x 7:

𝑔𝑟𝑒𝑔 =1

49

𝑁=1

49

𝑔𝑜𝑏𝑠(𝑁)

∆𝑔𝑟𝑒𝑔 𝑖, 𝑗 =∆𝑔 𝑖 − 𝑛, 𝑗 − 𝑛 +⋯+ ∆𝑔 𝑖, 𝑗 + ⋯+ ∆𝑔(𝑖 + 𝑛, 𝑗 + 𝑛)

𝑁

∆𝑔𝑟𝑒𝑠 𝑖, 𝑗 = ∆𝑔𝑜𝑏𝑠(𝑖, 𝑗) − ∆𝑔𝑟𝑒𝑔(𝑖, 𝑗)

∆𝑔𝑟𝑒𝑠 𝑖, 𝑗 = 𝑔 𝑟𝑒𝑠𝑖𝑑𝑢𝑎𝑙 ∆𝑔𝑟𝑒𝑔 𝑖, 𝑗 = 𝑔 𝑟𝑒𝑔𝑖𝑜𝑛𝑎𝑙

∆𝑔𝑜𝑏𝑠 𝑖, 𝑗 = 𝑔 𝑜𝑏𝑠𝑒𝑟𝑣𝑒𝑑

Separating Regional – Residual Gravity Anomalies

Moving Average – 02

▪ Depth and spatial distribution of anomaly sources are associated with frequency or wavelength of gravity data:

✓ Low frequency/long wavelength → deeper and regional anomaly;

✓ High frequency/short wavelength → shallow and local anomaly;

✓ The depth of anomalous body at the subsurface can be obtained by spectral analysis of potential fields (gravity and magnetic) ………

Separating Regional – Residual Gravity Anomalies

Spectral Analysis – 01

Frequency / wavelength vs Anomalous Body

Separating Regional – Residual Gravity Anomalies

Spectral Analysis – 02

▪ By calculating 2-D FFT (Fast Fourier Transform) and radially averaged spectra; there are several types:

✓ Relationship between slope of power spectrum vs depth;

✓ Filtering: low-pass, high-pass, band-pass;

✓ Regional – residual separation;

✓ Pseudo-depth slicing …

Separating Regional – Residual Gravity Anomalies

Spectral Analysis – 03

0.0 0.2 0.4 0.6 0.8 1.0 1.2

wavenumber (cycle/km)

-15

-10

-5

0

5

10

log

po

we

r

deep

intermediate

shallow

noise

Radially averaged spectra and segmentation by linear lines to separate type of anomalous sources

▪ The depth of anomalous source is determined by:

▪ ℎ =−𝑠

4𝜋

▪ where h is depth and s is slope or gradient log (energy) of spectrum;

▪ In general, spectral analysis is carried out using application softwares, i.e. Geosoft Oasis Montaj, GoCad, ....

Separating Regional – Residual Gravity Anomalies

Spectral Analysis – 04

Separating Regional – Residual Gravity Anomalies

Spectral Analysis – An Example

Separating Regional – Residual Gravity Anomalies

Spectral Analysis – An Example

Separating Regional – Residual Gravity Anomalies Spectral Analysis – An Example

Separating Regional – Residual Gravity Anomalies Spectral Analysis – An Example

▪ The principle of continuation is mathematical projection of potential field data (gravity and magnetic) from a datum to another datum, upward or downward;

▪ The process of continuation is carried out by simulating Bouguer anomaly to an elevation below or above mean sea level;

▪ Upward continuation is relatively a projection of potential field data to a free space;

▪ Upward continuation is applied to filter the short wavelength, reducing the amplitude and noises ….

Separating Regional – Residual Gravity Anomalies

Field Continuation

▪ Upward continuation of Bouguer gravity anomaly for several elevation, i.e. 500 m (a); 2000 m (b); 4000 m (c); 6000 m (d); 8000 m (e); 10,000 m (f).

▪ Please pay your attention, see the changes that occur to the gravity anomalies pattern for each map ….

Separating Regional – Residual Gravity Anomalies

Field Continuation

Thank you,See you for the next lecture ....