**Gravity magnetic reprocessing**

Critical in ensuring that geophysical surveys of different vintages, whether land, shipborne or airborne, are suitable to be interpreted. All of these gravity magnetic reprocessing compilations involve the application of QC processes including:

- Digitizing legacy analog contour maps
- Data validation and application of Bouguer, terrain, and IGRF corrections
- Grid interpolation algorithms
- Enhanced horizontal and vertical derivatives
- Cross line and micro-leveling and merging magnetic surveys
- Generation of grids and vector data in industry standard open source GIS formats

Derivatives of gravity and magnetic data are critical for the QC of surveys and to begin interpretation. We use only our Eslan developed gravity magnetic reprocessing software which allows us to apply specialist algorithms that are not available in third-party software.

Enhancement operations such as gridding and visualization are an integral part of advanced **gravity magnetic reprocessing**, and require care to avoid distortions of data.

**Network Adjustment**

**gravity magnetic reprocessing**progressively changes base levels, gradients, and curvatures of line segments to minimize the misfit of flight line and tie line measurements across the network of intersection points. Leveling errors are primarily manifest as sharp field variations along the axes of the flight lines.

**Micro-leveling**

The standard network adjustment leaves residual errors. Micro-leveling is a grid-based gravity magnetic reprocessing designed to attenuate these residual level errors between adjacent lines. These features are typically parallel to the flight line direction and having a spacing related to the flight line spacing. Micro-leveling aims to attenuate noise.

#### De-Culturing of Magnetic Field Data

We use automated statistical search algorithms in our gravity magnetic reprocessing to scan data for possible cultural anomalies. Removal of cultural features is applied to the primary data and we append key channels to the data so that changes can be tracked through the gravity magnetic reprocessing.

After leveling, gridding interpolates the data from the measurement locations to nodes of a regular mesh, creating a new and fundamentally different data structure, critical to all subsequent processing, enhancement, display, and interpretation.

If *the data is noisy, then the notion of a continuous magnetic field may break down* and result in significant gridding artifacts. Such gridding artifacts can be attenuated by re-gridding at a coarser cell-size, or by using a redesigned gridding operator to produce a minimum curvature grid. However, limitations in preserving the sharpest gradients through the gridding process mean that depths estimated from inversion of grid data may be overestimated.

#### IGRF correction

Across surveys of tens or hundreds of kilometers variations in the earth’s primary field contribute to the measured field variations, and a global model of the earth’s magnetic field is needed to be subtracted from the measured data. The IGRF is not a separation of field components from specific sources, but the long wavelength IGRF terms represent mostly core components and broad, lower crustal magnetizations. We maintain a database of IGRF defined and published by the International Union of Geodesy and Geophysics (IUGG).

#### Reduction to Pole (RTP)

The external field of a magnetic body varies substantially as a function of the local geomagnetic inclination. The simplest relationship between the magnetic field and source magnetizations occurs at the geomagnetic poles where the field orientation is vertical and induced anomalies due to steeply dipping bodies are essentially centered above the bodies. At inclinations of less than 30°, the RTP transform is unstable and the severity depends on data quality.

#### Multi-Scale Analysis

The bulk of the information in both gravity and magnetic data are contained in the derivatives of the field. Multi-scale analysis is useful when the main field contributions are caused by sources of different depths and extents. Our multi-scale horizontal and vertical derivative approach is *needed in gravity magnetic reprocessing to achieve a stable transformation* that results from the stable behavior of the vertical integral transformation of the field with the properties of high order derivative transformations (Fedi M. and Florio G., 2001).