We present a case study on the use of magnetic amplitude inversion in imaging volcanics that are buried in sedimentary basins and have strong remanent magnetization. The application arises in exploration of natural gas hosted in volcanic units. We show that combined with reliable approaches for magnetic anomaly separation, amplitude inversion can overcome the lack of information on magnetization direction and effectively identify the volcanic units at large depths.
Exploration for natural gas hosted in volcanics in sedimentary basins often makes use of magnetic method because of difficulties associated with seismic imaging under such conditions. However, the interpretation of magnetic data in such applications can be hampered severely by the small amplitude of anomalies and the presence of remanent magnetization. In such cases, the remanent magnetization causes the total magnetization to be rotated to an unknown direction. As a result, inversions of the total-field anomaly are no longer useful because the magnetization direction is a piece of basic information required for such algorithms. Yet, inversion-based interpretation is often necessary given the weak anomalies from volcanics. Themagnetic amplitude inversion technique developed formineral exploration (Shearer, 2005; Li et al., 2010) may be used as a viable tool to tackle this problem. This method inverts the amplitude of the magnetic anomaly vector, which is weakly dependent upon the source magnetization direction, and recovers a 3D distribution of magnitude of magnetization. We present a case study and show that such an approach is viable in exploration for natural gas hosted in volcanic units. However, it is important to be able to first extract the desired anomaly through a reliable separation technique. Figure 1 displays a set of total-field magnetic data over a sedimentary basin with buried volcanics. The ambient field at this location has an inclination of 62? and declination of -1?. The basement depth in the area ranges from 6000 to 8000 m and the volcanics are located at intermediate depths. There appear to be a number of magnetic anomalies surrounding a broad magnetic low in the southwest quadrant. Drilling indicates that most magnetic highs visible in the data map are localized magnetic units of relatively young ages. The regions of broad magnetic low near the center was not considered an area of interest. However, the only two wells that intersected volcanics are located precisely in that magnetic low. Drill logs and core sample measurements indicate that the volcanics unit is located at depths just below 2500 m, its average susceptibility is 0.0144 SI and the maximum strength of remanent magnetization is about 1.16 A/m.
A closer inspection suggests that the total-fieldmagnetic anomaly in Figure 1 consists of several distinct scales. The most obvious positive anomalies are of higher frequencies and localized. The broad anomalies have wavelengths on the order of tens of kilometers and they are clearly related to the basement.
The main impediment to the quantitative interpretation of magnetic data affected by remanent magnetization is the unknown magnetization direction.
