4D MICROGRAVITY, TIME LAPSE GRAVITY

Case Study : Geothermal Fluid Flow

The 4D microgravity method, which is the development of the gravity method with its fourth dimension is the time. Characteristic of 4D microgravity is the conscientious repetition of microgravity measurements in the order of µGal, and conscientious height measurements in the order of mm. This method measures the value of changes in the earth's gravity (Sarkowi, 2005).

Processing of 4D microgravity is different from microgravity processing for exploration. The 4D microgravity has no correction because each measurement point will be reduced with the previous measurement, in this case the measurement that been conducted in year before. This reduction causes any correction will be mutually subtracted (F D. Haq, 2019).

The map can tells the density has been changes in some periods that could occurs because of the production and re-injection activity on the surface on geothermal field. On the Delta Gobs Map (Gobs Map 2002 – Gobs Map 2008) there is a negative value which is drawn in blue to orange, indicating a decrease in the value of gravity from 2002 to 2008 which means the fluid has decreased or moved. While positive values are depicted in red and purple this indicates an increase in the gravity value, so it is interpreted that there is an increase in the volume of geothermal fluid in the area.

The result of 4D microgravity measurement can be used to estimate mass changes that occurs beneath the surface. The mass changes estimation can be calculate by inversion method. The inversion method is a backward modelling method by providing inputs of the initial model and certain parameters (Grandis 2009). The calculation results of the mass changes estimation can using the Gaus’s Flux Method which is need the amount of cube volume to cover the anomaly.   

REFERENCES 

[1]. F D Haq dkk. 2019. Production and Reinjection Evaluation on The Basic of 3D Time-Lapse Microgravity Inversion. IOP Conference Series: Earth and Environmental Science. 

[2]. Grandis, Hendra. 2009. Introduction to Geophysical Inversion Modeling. Jakarta: Indonesian Geophysics Expert Association (HAGI). 

[3]. Saibi, Hakim. 2017. Microgravity and Its Applications in Geosciences. Intechopen. 

[4]. Sarkowi, Muh. 2005. Survey Gayaberat Mikro 4D Untuk Monitoring Dinamika Air Tanah. Jurnal Sains dan Teknologi, Desember 2005, Vol 11, No. 3.

Fauzi & Thomas